Frequently Asked Questions about AspectJCopyright (c) 1997-2001 Xerox Corporation,
2002 Palo Alto Research Center, Incorporated,
2003-2006 Contributors. All rights reserved.
Last updated November 3, 2006
For a list of recently-updated FAQ entries, see OverviewWhat is AspectJ?
AspectJ(tm) is a simple and practical extension to the
Java(tm) programming
language that adds to Java aspect-oriented programming (AOP)
capabilities. AOP allows developers to reap the benefits of
modularity for concerns that cut across the natural units of
modularity. In object-oriented programs like Java, the natural unit
of modularity is the class. In AspectJ, aspects modularize concerns that
affect more than one class.
You compile your program using the AspectJ compiler
(perhaps using the supported development environments)
and then run it,
supplying a small (< 100K) runtime library.
The AspectJ technologies include
a compiler (ajc),
a debugger (ajdb),
a documentation generator (ajdoc),
and integration with
Eclipse, Sun-ONE/Netbeans, GNU Emacs/XEmacs,
JBuilder, and Ant.
What are the benefits of using AspectJ?AspectJ can be used to improve the modularity of software
systems.
Using ordinary Java, it can be difficult to modularize design
concerns such as
system-wide error-handlingcontract enforcementdistribution concernsfeature variationscontext-sensitive behaviorpersistencetestingThe code for these concerns tends to be spread out across the
system. Because these concerns won't stay inside of any one module
boundary, we say that they crosscut the
system's modularity.
AspectJ adds constructs to Java that enable the modular
implementation of crosscutting concerns. This ability is
particularly valuable because crosscutting concerns tend to be both
complex and poorly localized, making them hard to deal with.
Can AspectJ work with any Java program?AspectJ has been designed as a compatible
extension to Java. By compatible, we mean
upward compatibleAll legal Java programs are legal AspectJ
programs.
platform
compatible
All legal AspectJ programs run on standard Java
virtual machines.
tool
compatible
Existing tools can be extended to work with
AspectJ.
programmer compatibleProgramming in AspectJ feels natural to Java
programmers.
The AspectJ tools run on any Java 2 Platform compatible
platform. The AspectJ compiler produces classes that run
on any Java 1.1 (or later) compatible platform.
How is AspectJ licensed?Since AspectJ 1.9.7, source code and documentation is available under the
Eclipse Public License v 2.0.
AspectJ 1.5.2 through 1.9.6 source code and documentation is available under the
Eclipse Public License v 1.0.
AspectJ 1.1 through 1.5.1 source code and documentation is available under the
Common Public License 1.0.
The AspectJ 1.0 tools are open-source software available under the
Mozilla Public License 1.1.
That documentation is available under a separate license
that precludes for-profit or commercial
redistribution.
The runtime jar aspectjrt.jar and its distribution are also covered by the
Eclipse Public License.
For answers to common licensing questions, see the
Eclipse Public License FAQ.
What is the AspectJ Project?AspectJ is based on over ten years of research at
Xerox Palo Alto Research Center
as funded by Xerox, a U.S. Government grant (NISTATP), and a
DARPA contract.
It has evolved through open-source releases
to a strong user community and now operates as an
open source project at
http://eclipse.org/aspectj
The AspectJ team works closely with the community
to ensure AspectJ continues to evolve as an effective
aspect-oriented programming language and tool set.
The latest release is 1.2
which can be downloaded from the
AspectJ project page,
including sources as described
.
Development is focused on supporting applications,
improving quality and performance,
enhancing integration with IDE's,
and building the next generations of the language.
Quick Start
What Java versions does AspectJ require and support?
The AspectJ compiler produces programs for any released version of the
Java platform (jdk1.1 and later). When running, your program classes must
be able to reach classes in the
small (< 100K) runtime library (aspectjrt.jar) from the distribution.
The tools themselves require J2SE 1.3 or later to run,
but the compiler can produce classes for any 1.1-compliant
version of the Java platform.
How do I download and install AspectJ?From AspectJ's
web page
, download the AspectJ distribution.
The jar file is installed by executing
java -jar jar file nameDo not try to extract the
jar file contents and then attempt to execute
java org.aspectj.tools.Main. (A
NoClassDefFoundError exception will be
thrown.) The AspectJ distribution is not designed to be installed
this way. Use the java -jar form shown above.
To uninstall, remove the files the installer wrote in your
file system. In most cases, you can delete the top-level install
directory (and all contained files), after you remove any
new or updated files you want to keep. On Windows, no
registry settings were added or changed, so nothing needs to be
undone. Do not install over prior versions, which might have
different files. Delete the prior version first.
How should I start using AspectJ?Many users adopt AspectJ in stages, first using it
to understand and validate their systems (relying on it only in
development) and then using it to implement crosscutting concerns
in production systems. AspectJ has been designed to make each
step discrete and beneficial.
In order of increasing reliance, you may use AspectJ:
In the development
process
Use AspectJ to trace or log
interesting information. You can do this by adding
simple AspectJ code that performs logging or tracing.
This kind of addition may be removed ("unplugged") for
the final build since it does not implement a design
requirement; the functionality of the system is unaffected by
the aspect.
As an ancillary part of your
system
Use AspectJ to more completely and
accurately test the system.
Add sophisticated code that can check contracts,
provide debugging support, or implement test strategies.
Like pure development aspects, this code may also be
unplugged from production builds. However, the same code
can often be helpful in diagnosing failures in deployed
production systems, so you may design the functionality
to be deployed but disabled, and enable it when debugging.
As an essential part of your
system
Use AspectJ to modularize
crosscutting concerns in your system by design.
This uses AspectJ to implement logic integral to a system
and is delivered in production builds.
This adoption sequence works well in practice and has been
followed by many projects.
How does AspectJ integrate with existing Java development
tools?
AspectJ products are designed to make it easy to integrate
AspectJ into an existing development process.
Each release includes
Ant tasks for building programs,
the AspectJ Development Environment (AJDE) for writing
aspects inside popular IDE's, and
command-line tools for compiling and documenting Java and AspectJ code.
AspectJ provides replacements for standard Java tools:
ajc, the AspectJ compiler,
runs on any Java 2 compatible platform, and produces classes
that run on any Java 1.1 (or later) compatible platform.
ajdoc produces API documentation like
javadoc, with additional crosscutting links. For example,
it shows advice affecting
a particular method or all code affected by a given aspect.
At present, ajdoc is only supported in AspectJ 1.0.
For debugging, AspectJ supports JSR-45, which provides a mechanism for
debugging .class files that have multiple source files.
Debugger clients and VM's are beginning to support this;
see Sun's J2SE 1.4.1 VM and jdb debugger
and recent versions of JBuilder.
The AspectJ Development Environment (AJDE)
enables programmers to view and navigate the crosscutting structures
in their programs, integrated with existing support in
popular Java IDE's for viewing and navigating object-oriented
structures. For many programmers this provides a deeper understanding
of how aspects work to modularize their concerns and permits them
to extend some of their development practices without
having to abandon their existing tools.
AJDE is a set of API's providing the basis for the following
development tool integrations:
Eclipse (version 2.0)
in the Eclipse AspectJ Development Tools project
http://eclipse.org/ajdt
Emacs (GNU version 20.3)
and XEmacs (version 21.1 on Unix and 21.4 on Windows),
in the SourceForge AspectJ for Emacs project
http://aspectj4emacs.sourceforge.net
JBuilder (versions 4 through 7) from Borland
in the SourceForge AspectJ for JBuilder project
http://aspectj4jbuildr.sourceforge.net
Netbeans up to 3.4
(and Sun Microsystems' Forte for Java (versions 2 and 3), Sun/One)
in the SourceForge AspectJ for NetBeans project
http://aspectj4netbean.sourceforge.net
The common functionality of AJDE is also available in
the stand-alone source code browser ajbrowser,
included in the tools distribution.
Finally, as mentioned above,
AspectJ also supports building with Ant by providing
task interfaces to the ajc and ajdoc tools.
Typical AspectJ programsAre aspects always optional or non-functional parts of
a program?
No. Although AspectJ can be used in a way that allows AspectJ
code to be removed for the final build, aspect-oriented code is not
always optional or non-functional. Consider
what AOP really does: it makes the modules in a program correspond
to modules in the design. In any given design, some modules are
optional, and some are not.
The examples directory included in the AspectJ distribution
contains some examples of the use aspects that are not optional.
Without aspects,
beanPoint objects would not be JavaBeans.introductionPoint objects would not be cloneable, comparable or
serializable.
spacewarNothing would be displayed.telecomNo calls would be billed.
What is the difference between development and production aspects?
Production aspects are delivered with the finished product,
while development aspects are used during the development process.
Often production aspects are also used during development.
What are some common development aspects?
Aspects for logging, tracing, debugging, profiling
or performance monitoring, or testing.
What are some common production aspects?
Aspects for performance monitoring and diagnostic systems,
display updating or notifications generally, security,
context passing, and error handling.
Basic AOP and AspectJ ConceptsWhat are scattering, tangling, and crosscutting?
"Scattering" is when similar code is distributed throughout many
program modules. This differs from a component being used by
many other components since
it involves the risk of misuse at each point and of inconsistencies
across all points. Changes to the implementation may require
finding and editing all affected code.
"Tangling" is when two or more concerns are implemented in
the same body of code or component, making it more difficult to understand.
Changes to one implementation may cause unintended changes
to other tangled concerns.
"Crosscutting" is how to characterize a concern than spans
multiple units of OO modularity - classes and objects. Crosscutting
concerns resist modularization using normal OO constructs, but
aspect-oriented programs can modularize crosscutting concerns.
What are join points?Join points are well-defined points in the execution of a
program. Not every execution point is a join point: only those
points that can be used in a disciplined and principled manner are.
So, in AspectJ, the execution of a method call is a join point, but
"the execution of the expression at line 37 in file Foo.java" is
not.
The rationale for restricting join points is similar to the
rationale for restricting access to memory (pointers) or
restricting control flow expressions (goto) in
Java: programs are easier to understand, maintain and extend
without the full power of the feature.
AspectJ join points include reading or writing a field; calling
or executing an exception handler, method or constructor.
What is a pointcut?
A pointcut picks out
join points
. These join points are described by the pointcut
declaration. Pointcuts can be defined in classes or in aspects,
and can be named or be anonymous.
What is advice?Advice is code that executes at each
join point picked out by a
pointcut. There are three
kinds of advice: before advice, around advice and after advice. As
their names suggest, before advice runs before the join point
executes; around advice executes before and after the join point;
and after advice executes after the join point. The power of
advice comes from the advice being able to access values in the
execution context of a pointcut.
What are inter-type declarations?AspectJ enables you to declare members and supertypes of another class
in an aspect, subject to Java's type-safety and access rules. These are
visible to other classes only if you declare them as accessible.
You can also declare compile-time errors and warnings based on pointcuts.
What is an aspect?Aspects are a new class-like language element that has been
added to Java by AspectJ. Aspects are how developers encapsulate
concerns that cut across classes, the natural unit of modularity in
Java.
Aspects are similar to classes because...
aspects have type
aspects can extend classes and other aspects
aspects can be abstract or concrete
non-abstract aspects can be instantiated
aspects can have static and non-static state and
behavior
aspects can have fields, methods, and types
as members
the members of non-privileged aspects follow the
same accessibility rules as those of classes
Aspects are different than classes because...
aspects can additionally include as members pointcuts,
advice, and inter-type declarations;
aspects can be qualified by specifying the
context in which the non-static state is available
aspects can't be used interchangeably with
classes
aspects don't have constructors or finalizers,
and they cannot be created with the new operator;
they are automatically available as needed.
privileged aspects can access private members of
other types
Why AOP?Are crosscutting concerns induced by flaws in parts of the
system design, programming language, operating system, etc. Or is
there something more fundamental going on?
AOP's fundamental assumption is that in any sufficiently
complex system, there will inherently be some crosscutting
concerns.
So, while there are some cases where you could re-factor a
system to make a concern no longer be crosscutting, the AOP idea
is that there are many cases where that is not possible, or where
doing so would damage the code in other ways.
Does it really make sense to define aspects in terms of
crosscutting?
Yes.The short summary is that it is right to define AOP in terms of
crosscutting, because well-written AOP programs have clear
crosscutting structure. It would be a mistake to define AOP in
terms of "cleaning up tangling and scattering", because that isn't
particular to AOP, and past programming language innovations also
do that, as will future developments.
(Slides for a long talk on this topic were once available at
http://www.cs.ubc.ca/~gregor/vinst-2-17-01.zip.)
Is AOP restricted to domain-specific
applications?
No. Some implementations of AOP are domain-specific, but
AspectJ was specifically designed to be general-purpose.
Why do I need AOP if I can use interceptors
(or JVMPI or ref
lection)?
There are many mechanisms people use now to implement
some crosscutting concerns. But they don't have a way to express
the actual structure of the program so you (and your tools)
can reason about it. Using a language enables you to express the
crosscutting in first-class constructs. You can not only avoid the
maintenance problems and structural requirements of some other
mechanisms, but also combine forms of crosscutting so that all
the mechanisms for a particular concern are one piece of code.
Related Technology
How does AspectJ compare to other new forms of programming?
There are many recent proposals for programming languages that
provide control over crosscutting concerns. Aspect-oriented
programming is an overall framework into which many of these
approaches fit. AspectJ is one particular instance of AOP,
distinguished by the fact that it was designed from the ground up
to be compatible with Java.
For more alternatives for aspect-oriented programming, see
http://aosd.net.
How do you compare the features of AspectJ with
reflective systems?
Reflective and aspect-oriented languages have an important
similarity: both provide programming support for dealing with
crosscutting concerns. In this sense reflective systems proved
that independent programming of crosscutting concerns is
possible.
But the control that reflection provides tends to be low-level
and extremely powerful. In contrast, AspectJ provides more
carefully controlled power, drawing on the rules learned from
object-oriented development to encourage a clean and understandable
program structure.
How do AspectJ features compare with those of mixin-based
inheritance?
Some features of AspectJ, such as introduction, are related to
mixin-based inheritance. But, in order to
support crosscutting, a core goal for AspectJ, AspectJ goes beyond
mixin-based inheritance.
Firstly, an aspect imposes behavior on a class, rather than a
class requesting behavior from an aspect. An aspect can modify a
class without needing to edit that class. This property is
sometimes called reverse inheritance.
Secondly, a single aspect can affect multiple classes in
different ways. A single paint aspect can add different paint
methods to all the classes that know how to paint, unlike mixin
classes.
So mixin-based inheritance doesn't have the reverse inheritance
property, and mixins affect every class that mixes them in the same.
If I want to do something like SubjectObserverProtocol, I need two
mixins, SubjectPartofSubjectObserverProtocol and ObserverPartof...
In AspectJ, both halves of the protocol can be captured in a single
aspect.
How does AspectJ compare with more dynamic AOP?
Some AOP techniques are presented as "dynamic" because the weaving
occurs when classes are loaded, because aspects can be configured
in a separate XML file before launch, or because some advice
depends on runtime reflection. They are said to be more flexible
than AspectJ.
This is a misconception. First, the AspectJ 1.1 weaver has always
supported weaving at compile-time or class-load-time. Weaving at
compile-time reduces application launch and running time, and it helps
IDE's offer support for tracking down weaving errors and understanding
the impact of aspects on a system.
On the other hand, weaving at load-time simplifies build and deployment.
Before AspectJ 1.2, the user had to write a class loader that used the
weaver API to weave at load time; since 1.2, AspectJ comes with a
command-line launcher to support weaving at class-load-time without
any other changes to a build configuration. In AspectJ 5, we expect
to get a similar level of support as AspectWerkz, and to exploit
the class bytecode weaving hook available in Java 5 VM's.
Second, AspectJ programs, like Java programs generally, can be
written to support any level of XML configuration or to depend on
runtime reflection. There are some benefits to using AspectJ;
e.g., the proceed() form within around advice simplifies a lot of
the work that otherwise would go into writing a generalized
interceptor, without introducing many of the runtime errors that can
result from interceptors.
For AspectJ examples of configurable or reflection-dependent programs,
see the sample code linked off the AspectJ documentation page
or the examples discussed on the mailing list, e.g.,
Incremental and runtime weaving support?.
What is the relationship between AOP and
XP (extreme programming AKA agile methods)?
From a question on the user list:
> Anyone know the connections between AOP and Extreme Programming?
> I am really confused. It seems AOP is a programming paradigm, which
> is the next level of abstraction of OOP. Extreme Programming, however,
> this is a lightweight software development process. One of the common
> motivations of AOP and XP is designed to adopt to the requirement
> changes, so that it can save the cost of software development.
This is Raymond Lee's answer:
You're not really that confused. AOP and XP are orthogonal concepts,
although AOP can be used to help accomplish XP goals.
One of the goals of XP is to respond to changing requirements.
Another is to reduce the overall cost of development. These are
not necessarily the same thing.
One of the principles of XP that contribute to meeting those goals
is to maintain clean, simple designs. One of the criteria for clean,
simple designs is to factor out duplication from the code. Benefits
of removing duplication include the code being easier to understand,
better modularity of the design, lower costs of code changes, less
chance of conflicting changes when practicing collective code
ownership, etc.
Different types of duplication lend themselves to being addressed by
different design paradigms and language features. Duplicate snippets
of code can be factored out into methods. Duplicate methods can be
factored out to common classes, or pushed up to base classes.
Duplicate patterns of methods and their use can be factored out to
mechanisms of classes and methods (i.e. instantiations of design
patterns).
AOP addresses a type of duplication that is very difficult to handle
in the other common paradigms, namely cross-cutting concerns. By
factoring out duplicate cross-cutting code into aspects, the target
code becomes simpler and cleaner, and the cross-cutting code becomes
more centralized and modular.
So, AOP as a paradigm, and the associated tools, gives an XPer, or
anyone wanting to remove duplication from the code base, a powerful
way to remove a form of duplication not easily addressed until now.
Will you support C#?Not at this time. Although the resemblances between C# and Java
means it would probably be a fairly straightforward matter to take
the AspectJ language design and produce AspectC#, our current focus
is only on supporting effective uses of AspectJ.
Deciding to adopt AspectJ
Is it safe to use AspectJ in my product plans?
You may use AspectJ in your product or project with little
risk. Several factors play a role in reducing the risk of adopting
this new technology:
AspectJ is an addition to
Java, and can be introduced into a project
in a way that limits risk.
See for
some suggestions on how to do this.
The AspectJ compiler accepts standard Java as
input and produces standard Java bytecode as output.
In 1.0, an optional mode produces standard Java source code
which may then be compiled with any compliant Java compiler
(e.g. Sun's javac compiler
or IBM's jikes compiler).
In 1.1, an optional mode accepts standard Java bytecode
from any compliant Java compiler
and weaves in the aspects to produce new bytecode.
AspectJ is available under a non-proprietary, open source license, the
Eclipse Public License v 2.0.
AspectJ will continue to evolve and be available, regardless
of the fate of any particular organization involved with
AspectJ.
Removing AspectJ from your program is not
difficult, although you will lose the flexibility and
economy that AspectJ provided.
A number of significant open-source projects and industry
products use AspectJ successfully. One list is kept on
the AOSD FAQ, and more appear on the mailing
lists (search for, e.g., "AspectJ in real world", as
described in ).
What is the effect of using AspectJ on the source code
size of programs?
Using aspects reduces, as a side effect, the number of source
lines in a program. However, the major benefit of using aspects
comes from improving the modularity of a
program, not because the program is smaller. Aspects gather into a
module concerns that would otherwise be scattered across or
duplicated in multiple classes.
Does AspectJ add any performance overhead?
The issue of performance overhead is an important one. It is
also quite subtle, since knowing what to measure is at least as
important as knowing how to measure it, and neither is always
apparent.
We aim for the performance of our implementation of AspectJ to
be on par with the same functionality hand-coded in Java. Anything
significantly less should be considered a bug.
There is currently no benchmark suite for AOP languages in
general or for AspectJ in particular. It is probably too early to
develop such a suite because AspectJ needs more maturation of the
language and the coding styles first. Coding styles really drive
the development of the benchmark suites since they suggest what is
important to measure.
Though we cannot show it without a benchmark suite, we believe
that code generated by AspectJ has negligible performance overhead.
Inter-type member and parent introductions should have very little
overhead, and advice should only have some indirection which
could be optimized away by modern VM's.
The ajc compiler will use static typing information
to only insert the advice and dynamic pointcut tests that are absolutely necessary.
Unless you use 'thisJoinPoint' or 'if', the main dynamic checks will be
'instanceof' checks which are generally quite fast.
These checks will only be inserted when they can not be inferred from
the static type information.
When measuring performance, write AspectJ code
fragments and compare them to the performance of the
corresponding code written without AspectJ. For example, don't
compare a method with before/after advice that grabs a lock to just
the method. That would be comparing apples and oranges. Also be
sure to watch out for JIT effects that come from empty method
bodies and the like. Our experience is that they can be quite
misleading in understanding what you've measured.
I've heard that AspectJ leads to modularity violations. Does it?
Well I haven't yet seen a language in which you can't write bad code!
But seriously, most AspectJ users find that just like when they learned
OO, it takes a while to really get the hang of it. They tend to start
in the usual way, by copying canonical examples and experimenting with
variations on them.
But users also find that rather than being dangerous, AspectJ helps them
write code that is more clear and has better encapsulation -- once they
understand the kind of modularity AspectJ supports. There are several
good papers that talk about this (see below), but here's a basic point
to keep in mind: when properly used, AspectJ makes it possible program
in a modular way, something that would otherwise be spread throughout
the code. Consider the following code, adapted from the AspectJ tutorial:
aspect PublicErrorLogging {
Log log = new Log();
pointcut publicInterface(Object o):
call(public * com.xerox.*.*(..)) && target(o);
after(Object o) throwing (Error e): publicInterface(o) {
log.write(o, e);
}
}
The effect of this code is to ensure that whenever any public method of
an interface or class in the com.xerox package
throws an error, that error is logged before being thrown to its caller.
Of course in the alternative implementation a large number of methods
have a try/catch around their body.
The AspectJ implementation of this crosscutting concern is clearly
modular, whereas the other implementation is not. As a result, if you
want to change it, its easier in the AspectJ implementation. For
example, if you also want to pass the name of the method, or its
arguments to log.write, you only have to edit
one place in the AspectJ code.
This is just a short example, but I hope it shows how what happens
with AOP and AspectJ is that the usual benefits of modularity are
achieved for crosscutting concerns, and that leads to better code,
not more dangerous code.
One paper someone else just reminded me of that talks some more
about this is:
http://www.cs.ubc.ca/~kdvolder/Workshops/OOPSLA2001/submissions/12-nordberg.pdf
Why does AspectJ permit aspects to access and add members of another type?
Isn't that violating OO encapsulation?
In the spirit of Smalltalk, we have decided to give more power
to the language in order to let the user community experiment and
discover what is right. To date this has proven to be a successful
strategy because it has permitted the construction of many useful
aspects that crosscut the internal state of an object, and as such
need access the its private members. However, we are not
discounting that some sort of restrictions are useful, rather, we
are seeking input from the community in order to decide on what
these restrictions should be.
In that light, our position on encapsulation is :
we respect Java's visibility ruleswe also provide open-classes, a mature OO technologywe provide "privileged" access if you really need it.
Introducing parents or members to classes is a well-studied OO technique
known as open classes.
Open classes have been used in many languages prior to AspectJ,
including CLOS, Python, Smalltalk, Objective-C, and others.
Building from Java, introduction in AspectJ provides better
name hygiene and access control than prior languages.
Introduced code obeys all of Java's normal accessibility rules
for its lexical location in the aspect that it is introduced from.
Such code can not even see, much less access, private members of
the class it is introduced into. Further, introductions can be
declared private to the aspect, so they are not visible to
other clients of the class.
Privileged aspects do permit access to private members of another
class. They are a response to the very few cases where developers
genuinely need such access (typically for testing purposes where it
access is necessary), but it would be more risky to open access by
putting the aspect in the same package, adding test code, or changing
access in the target class. We recommend using privileged aspects
only as necessary, and believe that marking them "privileged" makes
any potential misuse apparent.
Can I use AspectJ with J2EE?
Consider the component types in J2EE:
Servlet: AspectJ works well within servlets
JSP: It is possible to use AspectJ to affect code in JSPs by precompiling
them into Java sources and compiling these with ajc. This can be used, e.g., to
customize displays by turning on and off custom JSP taglibs. The mapping from a
given jsp source to java package and class name is not standardized, which means
doing this imposes dependencies on specific container versions.
EJB: AspectJ supports a wide variety of aspects for EJBs. It can be used for
logging, tracing, debugging, error handling by layers, correlated method-level
interception (e.g., chargebacks), metering, fine-grained transactions, etc.
Indeed, it can be used to enforce adherence to coding restrictions within an
EJB (e.g., not using java.io, creating a class loader, or listening on
sockets) using declare error.
The basic limitations are that there is no built-in support for writing J2EE
analogs for AspectJ extensions to Java, like distributed aspects, distributed
cflow, or managing state between invocations. These don't prevent one from using
AspectJ to do useful intra-container implementation, nor need they prevent one
from building distributed support, state management, and inter-component
implementations that leverage AspectJ. It just takes some work. In more detail:
All AspectJ implementations may define "code the implementation controls".
The AspectJ 1.0 implementation defines this as the files passed to the compiler
(AspectJ 1.1 will also support bytecode weaving).
Some advice on EJB operations will generate methods that confuse ejb compilers.
To avoid this problem, you can use the -XaddSafePrefix flag when compiling with ajc.
EJB components may be invoked remotely, and containers may passivate and
pool EJB's. Servlets have similar limitations, and in both cases the
lifespan of the defining class loader is implementation-dependent
(though it must span the operation of a particular request).
Being limited by lifecycle and namespace, the AspectJ 1.0 implementation
supports aspects that operate through non-remote invocations during the lifetime
of the namespace for a particular
deployment unit compiled in its entirety by the ajc compiler.
This means AspectJ supports common aspects only within a single local runtime
namespace (usually implemented as a class loader hierarchy).
Further, AspectJ recognizes language-level join points (object initialization,
method calls, etc.), not their EJB analogs (ejb find or create methods...).
These lead to the following consequences:
Issingleton aspects (the default) are limited to the lifetime of
the defining class loader, which in some implementations may not span
multiple invocations of the same application or EJB component.
EJB lifecycles are different from object lifecycles, so perthis
and pertarget aspects will make little sense. They do not work
in the current implementation, which uses synchronized methods
to ensure a correct association in threaded environments
(EJB's may not have synchronized methods).
Percflow or percflowbelow aspects are restricted to a chain of
non-remote invocations. While EJB 2.0 permits declaring an interface
local, this information is not available to the AspectJ compiler today.
For same reasons as stated above fore perthis, these will not work even
in the EJB container.
Evaluation of cflow or cflowbelow pointcuts will be valid only
with respect to a chain of non-remote invocations.
In addition, any AspectJ code should respect EJB operations:
The EJB container accesses EJB component fields directly, i.e.,
in code outside the control of the compiler. There is no join point for
these accesses, and hence no way to write a pointcut to advise that access.
The EJB container may pool EJB components, so any initialization
join points may run once per component constructed, not once per
component initialized for purposes of a client call.
The EJB container is permitted to change class loaders, even
between invocations of a particular EJB component (by passivating and
activating with a new class loader). In this case, instances of singleton
aspects will not operate over multiple invocations of the component, or that
static initialization join point recur for a given class as it is re-loaded.
This behavior depends on the container implementation.
Can I use AspectJ with Generic Java?We plan to support Generics when Java 1.5 is available.
But at this time, unfortunately not. The two compilers are just not
at all compatible. In an ideal world, there would be a wonderful
Open Source extensible compiler framework for Java that both GJ and
AspectJ would be built on top of, and they would seamlessly
interoperate along with all other extensions to Java that you might
be interested in, but that's not the case (yet?).
However, on 09 October 2000, the Java Community Process
approved a proposal to add generic types to Java that is largely
based on GJ (JSR 14). A draft specification was submitted for
public review, which closed on 01 August 2001, and a
prototype implementation has been released by Sun.
We are committed to moving very rapidly to add support for
generic types in AspectJ when generic types become part of the Java
language specification. Everyone on the AspectJ team is looking
forward to this, because we too would really like to be able to
write code that includes both aspects and generic types.
Can I use AspectJ with J2ME?The J2ME platform has several different components.
The diagram below shows how the different profiles
build on top of the two configurations CDC (Connected Device
Configuration) and CLDC (Connected Limited Device Configuration):
--------------
| Personal |
-------------- --------
| Foundation | | MIDP |
------------------ ------------------
| CDC | | CLDC |
------------------------------------------
| Java |
------------------------------------------
Which configuration you have dictates the restrictions when
running AspectJ compiled programs.
If you're running with a profile which sits on top of CDC then
there are not, as far as we are aware, any restrictions when
running AspectJ compiled code on this flavour of J2ME.
If you're running with a profile sitting on top of CLDC 1.1
you are currently unable to use the thisJoinPoint,
thisJoinPointStaticPart and
thisEnclosingJoinPointStaticPart variables, the
cflow and cflowbelow
pointcuts and the percflow and
percflowbelow perClauses.
Finally, if you're running with a profile which sits on top
of CLDC 1.0 you have all the restrictions of CLDC 1.1. There may
be further restrictions due to the lack of types corresponding
to the primitive types (e.g. Integer.TYPE), however, at the
time of writing we have been unable to do any extensive testing
on this.
Note that the aspectj runtime jar is now (as of AspectJ5) quite
large but only a small subset is required for executing code
in J2ME environments. We plan to ship a second aspectjrt.jar
built for the J2ME environment at some point.
For more discussion and to raise any issues you have with
AspectJ and J2ME, refer to
bugzilla entry 92933.
Are you working to put AOP into Java?
It seems that every AOP toolset currently uses proprietary mechanisms
to describe point-cuts, etc.
We are working on standardization, but it's
a question of timing/ripeness (imagine going from thousands of users
to millions). (See .) We believe
AspectJ addresses this question in the best way possible now:
It's open-source. Rather than being proprietary or controlled by a
vendor, it's available for anybody to use and build upon, forever.
AspectJ is not a set of mechanisms, it's a language. It is currently
implemented using certain techniques, but there's nothing that prevents
it from being implemented with other techniques. That means users can
adopt the language with confidence that implementations will get better.
There is no engineering need to change Java. The AspectJ language uses
the join point model already in Java, so there is no need to extend the
programming model. Our implementation produces valid Java bytecode, which
runs in any compliant J2SE VM and supports standard debuggers for those VM's
that support JSR-45 (debugging support for multi-language/multi-file sources).
This is a huge benefit to Sun since Sun must be extremely cautious
about extensions to the language or VM; before adopting AOP, Sun should
demand the kind of actual-proof that AspectJ implementations offer.
On the issue of "proprietary mechanisms to describe pointcuts, etc.": Any AOP
has to have some language to describe pointcuts and the like ("pointcuts"
of course being the AspectJ term). Users would like to have one language
(to avoid having to learn or transform between many languages) and the
choice of multiple implementations (tailored for a configuration, subject
to competitive pressure, etc.). That's what AspectJ offers.
That said, we believe the AspectJ extensions to Java could form the basis
for bringing AOP to Java; when that happens, there will be engineering
opportunities to make the implementation and tool support better.
What kind of support is available?
The mailing lists provide the primary support for everyone
in the community
(See ).
To request commercial support, tutorials, or presentations,
use the developer mailing list,
aspectj-dev@eclipse.org.
To find out about known issues, see the
AspectJ Programming Guide Appendix, "Implementation Notes"
and the AspectJ bugs in the database at
http://bugs.eclipse.org/bugs
(using the product AspectJ). Here are direct links to
view open compiler bugs,
view all Aspectj bugs (open or closed), or
add new bugs.
What mailing lists are there?
The AspectJ users mailing list
(aspectj-users@eclipse.org)
provides an informal network of AspectJ language users who
can answer usage questions about AspectJ programs
and the AspectJ tools.
This is the place to ask how to code something in AspectJ
or how to write Ant or shell scripts to invoke the tools.
The AspectJ developers mailing list
(aspectj-dev@eclipse.org)
provides an informal network of AspectJ technology experts who
aim to understand the technology behind AspectJ.
The committers to the AspectJ project use this list
for open technical and planning discussions.
Developers can answer questions about what's possible and about
integrating AspectJ technology with other technologies.
For both mailing lists, only subscribed members may post messages.
To subscribe, visit the
AspectJ web site.
There you can also subscribe to
aspectj-announce@eclipse.org,
a low-traffic list containing only announcements
about significant AspectJ events and product releases.
To get on a similar list for aspect-oriented software
development generally, see
http://aosd.net.
Using the AspectJ compiler
Do I have to use the AspectJ compiler?
The AspectJ compiler or weaver is required at some point, but
many people can use AspectJ without changing their build or
deployment process significantly. For aspects that are not
required to compile, you can use the AspectJ binary weaver, run
at build-time or class-load-time. You can write aspects using
the original code style (which must be compiled with the AspectJ
compiler) or using the annotation style new in AspectJ 5 (which
may be compiled with Javac or the AspectJ compiler).
For more information, see
.
What files do I need to include when compiling AspectJ programs?
You need to specify to the compiler the files that
contain your aspects and the files that contain the
types affected by your aspects.
See .
The AspectJ compiler will not search the source path for types
that may be affected (unlike Javac and Jikes).
In AspectJ 1.0, ajc requires all code to be in source form;
in AspectJ 1.1, Java and AspectJ code may be in either source
or binary form.
In some cases you should compile your entire system all at once.
If this is too slow, then you can try to make reasonable divisions
between sets of source files whose aspects do not interact to
achieve a shorter compile cycle (particularly for development
aspects). If you have aspects that apply to different modules,
you can try compiling them into a binary form and using them
to weave each module. However, if you get any problems
or if you wish to run tests or do a release, you should recompile
the entire system.
For more information, see the
Development Environment Guide
Reference for ajc.
I have to list many files in the command line to
compile with ajc. Is there any other way to
provide the file names to ajc?
Yes, use the argfile option to ajc. List source
files in a line-delimited text file and direct ajc to that
file using -argfile or @:
ajc @sources.lst
ajc -argfile sources.lst
Another way in AspectJ 1.1 is to use the
-sourceroots options, which reads all
source files in a given set of directories:
ajc -sourceroots "src;testsrc"
For more information, see the
Development Environment Guide
Reference for ajc.
What Java virtual machine (JVM) do I use to run the
AspectJ compiler?
Use the latest, greatest, fastest JVM you can get your hands on
for your platform. The compiler's performance is dependent on the
performance of the JVM it is running on, so the faster a JVM you
can find to run it on, the shorter your compile times will be. At a
minimum you need to use a Java 2 or later JVM to run the compiler
(J2SE 1.3 for AspectJ 1.1).
We realize that this constraint can be a problem for users who
don't currently have a Java 2 JVM available. We're sorry for the
inconvenience, but we had to make the hard decision that the
advantages of being able to rely on Java 2 were worth the cost of
losing a number of developers who are working on platforms without
Java 2 support. Here is a list of starting places where you might
find support for your system.
Java 2
Platform, Standard Edition
developerWorks : Java technology : Tools and products - Developer kits
developerWorks : Open Source - Jikes Project
Java
Platform Ports
The requirement of Java 2 support is only for
running the AspectJ compiler. The AspectJ
compiler can be used to build programs that will run on Java 1.1
(or probably even on Java 1.0) systems. This means that it can
build programs that will run on Macintosh, FreeBSD, and applets
that will run in Internet Explorer and Netscape Navigator that are
still not yet Java 2 compliant.
How can I use ajc to compile
programs for a JVM that is different from the one used to run it?
ajc can be used to develop programs that are
targeted at the Java 1.1 platform, even though the
ajc compiler won't run on that platform. Here's
an example of using ajc in this sort of
cross-compilation mode (assuming a Windows platform with all the
default installation directories):
ajc -target 1.1 -bootclasspath c:\jdk1.1.7\lib\classes.zip \
-classpath c:\aspectj1.0\lib\aspectjrt.jar -extdirs "" \
-argfile jdk11system.lst
This same technique can be used if you want to run
ajc on a JDK 1.3 JVM (highly recommended) but
need to generate code for JDK 1.2. That would look something
like:
ajc -bootclasspath c:\jdk1.2\jre\lib\rt.jar \
-classpath c:\aspectj1.0\lib\aspectjrt.jar \
-extdirs c:\jdk1.2\jre\lib\ext
-argfile jdk12system.lst
Does the ajc compiler support
the assert keyword in Java 1.4?
Yes. As with Javac,
use the -source 1.4 option as described
in the
Development Environment Guide
Reference for ajc.
Does the ajc compiler support
generics and the other new language features of Java 5?
Yes. As with Javac,
use the -1.5 option as described
in the
Development Environment Guide
Reference for ajc.
Will aspects work with different versions of the compiler/weaver and runtime?
Yes. Both ajc and
aspectjrt.jar should work with versions
of aspect code and libraries back to AspectJ 1.2.1.
Any aspects should be deployed
with the same version of aspectjrt.jar
they were compiled with. For more information, see the
Development Environment Guide
Reference for ajc
and
Deployment notes section on
Version compatibility.
Are there any issues using AspectJ with the Microsoft
JVM?
Since AspectJ requires Java 2 or later, it will not run on the
Microsoft JVM, which does not support Java 2.
Does ajc rely
on javac for generating Java bytecode
(.class) files?
No. Some previous versions of AspectJ had this requirement.
In AspectJ 1.0, javac can still be used as
ajc back end by using the
-usejavac flag. You can also run ajc
in preprocessor mode to generate Java source
(.java) files to be compiled using
javac or another java compiler.
Neither option is supported in AspectJ 1.1.
I noticed the AspectJ compiler doesn't use a parser generator. Why is that?
In AspectJ 1.0,
the PARSER for ajc is written by hand. This choice was made with full
awareness of the generator tools out there. (Jim had for example used
the excellent javacc tool for building the parser for JPython (now Jython)).
One of the reasons that AspectJ uses a hand-written parser is that using
javacc taught Jim about the LL-k design for parsers (pioneered by antlr).
As opposed to the state-machine parsers produced by yacc, these parsers are
very readable and writable by humans.
Antlr and javacc did not really suit the project:
Antlr's support for unicode in the lexer is still immature and this makes
using it with Java challenging. This was an even bigger issue 3 years ago
when we started on the Java implementation of ajc.
While javacc is freely available, it is not Open Source. Depending on a
closed-source tool to build an Open Source compiler would reduce some
of the transparency and control of open-source.
There were also several things that were easier to implement with
a hand-written parser than with any of the exiting tools.
Semi-keywords -- it's important to us that
"every legal Java program is also a legal AspectJ program."
This wouldn't be true if we made 'before' and 'call' full keywords in
AspectJ. It is easier to support these sorts of semi-keywords with a
hand-written parser. (Note: ajc-1.0.x handles 'aspect' and 'pointcut'
slightly specially which can break a few unusual pure Java programs.
This is a compiler limitation that will be fixed in a future release.)
Deprecated syntax warnings -- the syntax of AspectJ
changed many times from version 0.2 to the 1.0 release. It was easier
to provide helpful warning messages for these changes with our
hand-written parser.
Grammar modularity -- We like being able to have
AspectJParser extend JavaParser.
Part of the grammar for AspectJ is extremely hard for existing tools to
capture. This is the type pattern syntax, i.e. "com.xerox..*.*(..)".
The sort of case that gives standard parser generators fits is something
like "*1.f(..)" which no one would ever write, but which must be
supported for a consistent language.
In AspectJ 1.1, the parser was written as it is for the underlying
Eclipse compiler,
with some hand-coding of the sort that avoids adding keywords to
the language.
How does incremental mode work?
In incremental mode, ajc minimizes the files that need
to be recompiled after another file has changed. In Java,
only the changed files need to be recompiled, but in AspectJ,
other files might also need to be recompiled or re-woven.
Depending on what is modified, we may need to re-weave
code. Â If you change a pointcut and save it, we currently have
to check everywhere in case a new match is occurring or an old
match is no longer correct. Â However, if you simply change
the body of an advice in an aspect, there is (usually) no need
to reweave as the affected classes call the advice and the
advice (by design) maintains its name in the recompiled
aspect. If you make a change to a class (as opposed to an aspect) and
save it, we usually can get away with merely having to
compile that class then weave the existing aspects with it -
rather than doing a full recompile of the entire system.
There are a lot of possible optimizations to the
algorithms we use, by performing more complete analysis of
the change made to a file that will enable us to know more
accurately whether we need to reweave and if we do then what
we need to reweave - we just haven't gotten around to
implementing them yet. Integrating AspectJ into your development environmentHow do I know which aspects affect a class when looking
at that class's source code?
When you are working with the IDE support, you can get an
understanding of which aspects affect any class.
This enables AspectJ programmers to get the benefits of
modularizing crosscutting concerns while still having immediate
access to what aspects affect a class.
See for more
information on which Java development environments are
supported.)
When you are looking at documentation for AspectJ 1.0 programs,
ajdoc will provide links from aspects and
advice to the affected code, but it provides less information
than the IDE support because it only parses declarations.
When you are compiling your program, pointcuts that are
statically-determinable can be used in declare statements
to identify the code picked out by the pointcut.
(A pointcut is statically determinable if it only uses
the pointcut designators
within,
withincode,
execution,
call,
get,
set,
initialiation, and
staticinitialiation.)
The compiler will list the static code points which will be
affected by any advice specifying the same pointcut.
For example, the following will print a warning
whereever some code in class Bar gets a field value from Foo:
declare warning: get(* Foo.*) && within(Bar)
: "reading Foo state from Bar";
When you are running your program,
you can trace advice as it executes. This
enables you to identify advice on join points picked out
dynamically, which cannot be reflected precisely by IDE support.
For a related tracing question,
see What kind of IDE support is available for developing
AspectJ programs?
See What plans are there to support my IDE?
The AspectJ team directly provided components for JBuilder, Forte,
and Emacs and supported the open-source AspectJ plugin project
at http://eclipse.org/ajdt
which uses the AJDE API support for IDE's.
Supporting new IDE's is a matter of building on the AJDE API's,
mostly likely adopting one of the existing open-source IDE
extensions as a design template.
Here are the IDE's where we know people have expressed interest,
so interested developer may want to join with others in their
developer communities to build the integration.
IDEA/IntelliJ has an enthusiastic community and
the developers are working on an extensibility API
- http://intellij.comjEdit comes from a very active open-source community.
Oracle JDeveloper is supported at
https://jdeveloperaop.dev.java.net/.
Some have suggested Codeguide from Omnicore
http://www.omnicore.com/
For questions on AJDE, join the developer's list
aspectj-dev@eclipse.org.
For questions on the current IDE integrations, contact those projects.
Can I port AJDE support to my development environment?Yes. The core AJDE API is extensible and the source code is
available for download. Start by studying the sources
for the existing IDE support linked off the AspectJ site
http://eclipse.org/aspectj.
I want the aspects for development builds but
remove them for production builds. How can I set up the build
system so they are unpluggable? And so I use javac
in my production build?
If you are using development-time-only aspects - aspects that only
exist when you are developing the code, not when you ship it -
you can use implement a hybrid build process by listing
the production source files into a javac-compliant argfile,
and the development source files in another ajc argfiles:
-- file "production.lst":
One.java
two/Three.java
...
-- file "tracing.lst":
trace/Library.java
Trace.java
-- file "development.lst":
@production.lst
@tracing.lst
Then your development build can use ajc:
ajc @development.lst
And your development build can use
ajc or javac
or jikes:
jikes @production.lst
We compile module jars and then assemble them. Can we continue this with AspectJ?
Aspects apply to everything in a namespace, as if everything is
compiled together.
Sometimes you can break the build down into separate steps without breaking
this model, but we haven't stated exactly where it could break
because it depends on the interactions between all types.
You can try the approaches below, but remember to rebuild
everything in one go if there are problems.
The simplest scenario is when the aspects apply to all modules
and the modules compile without the aspects. In that case,
weaving in the aspects is just the final assembly step for
the build.
Next is the case where the aspects make changes to a common
library that are visible to other clients, which themselves
are otherwise unaffected by the aspects. In this case, the
common library can be built using ajc, and used on the
classpath for the module builds:
Combining these last two,
there's the case where a common set of aspects should
affect two or more modules that are in a dependency relationship
to one another. It should work to reuse the aspects
in binary form for each compile, in dependency order:
If two modules are visibly affected by aspects and
mutually-dependent, the only thing to do is compile
them together.
It's safest to assume that all aspects can affect all
types in a namespace; using build boundaries to effect
crosscutting limits causes a dangerous dependency on
the build process and might cause problems.
We use modules and would like to use incremental compilation.
Is that possible?
Just incrementally-compile the whole system.
Specify to ajc the modules as multiple source roots
(or input jars if you are weaving libraries).
In Eclipse's AJDT, you can create a top-level project with symbolic
links out to the sources:
Then everything is part of one huge incremental compile. Also, you
can close this master project and work the others using the Java
compiler or AJDT.
The links make incremental development possible without affecting
the modularized Ant builds. (Our practice runs along those lines.)
Programming notes and tipsIs it possible to change methods by introducing keywords (like
synchronized), adding parameters,
or changing the "throws" clause?
AspectJ does not enable you to change the signature of a method,
but you can (by express declaration) work around some
limits imposed by the signature. You can convert a checked exception to
unchecked using declare soft, privileged aspects
have access to private methods, and you can use a percflow aspect to
ferry additional state to a callee without changing intervening
signatures. For more details, see
The AspectJ Programming Guide.
In the case of synchronized,
we have what we consider a better solution that uses
around advice instead of introduction. This solution is described
in
this thread (no longer available)
on the AspectJ users list, with some
additional comments (no longer available)
.
I don't understand what join points exist. How can I see them?
You can trace them using using an aspect.
For example, you can start logging at a particular method call and
see what join points occur after the call and before it returns.
Here's some code Jim Hugunin wrote to trace join points
and posted to the users list. To reuse the aspect,
define a subaspect and implement the pointcuts, for example:
aspect JoinPointSampleAspect extends aj.TraceJoinPoints {
protected pointcut entry() :
execution(static void JoinPointSample.main(String[]));
protected pointcut exit() :
call(static void JoinPointSampleAspect.exit());
public static void main (String[] args) {
JoinPointSample.main(args);
JoinPointSampleAspect.exit();
}
public static void exit() {}
}
class JoinPointSample {
public static void main(String[] args) {}
}
Here's the aspect:
TraceJoinPoints tjp = TraceJoinPoints.aspectOf();
* if (null != tjp) tjp.message("Hello, World!");
*/
public void message(String s) {
out.println("" + prepareMessage(s) + "");
}
public void message(String sink, String s) {
if (null == sink) {
message(s);
} else {
out.println("" + prepareMessage(s) + "");
}
}
protected String prepareMessage(String s) { return s; } // XXX implement
//--------- end of added
PrintStream out;
int logs = 0;
protected void makeLogStream() {
try {
out = new PrintStream(new FileOutputStream("log" + logs++ + ".xml"));
} catch (IOException ioe) {
out = System.err;
}
}
protected void closeLogStream() {
out.close();
}
int depth = 0;
boolean terminal = false;
protected void logEnter(JoinPoint.StaticPart jp) {
if (terminal) out.println(">");
indent(depth);
out.print("<" + jp.getKind());
writeSig(jp);
writePos(jp);
depth += 1;
terminal = true;
}
void writeSig(JoinPoint.StaticPart jp) {
out.print(" sig=");
out.print(quoteXml(jp.getSignature().toShortString()));
}
void writePos(JoinPoint.StaticPart jp) {
SourceLocation loc = jp.getSourceLocation();
if (loc == null) return;
out.print(" pos=");
out.print(quoteXml(loc.getFileName() +
":" + loc.getLine() +
":" + loc.getColumn()));
}
String quoteXml(String s) {
return "\"" + s.replace('<', '_').replace('>', '_') + "\"";
}
protected void logExit(JoinPoint.StaticPart jp) {
depth -= 1;
if (terminal) {
out.println("/>");
} else {
indent(depth);
out.println("" + jp.getKind() + ">");
}
terminal = false;
}
void indent(int i) {
while (i-- > 0) out.print(" ");
}
}
]]>Note that if you are using AspectJ 1.0,
the line starting with declare precedence
would be removed, and the aspect declaration would look like
aspect TraceMyJoinPoints dominates *.
What is the difference between call and execution join points?
Briefly, there are two interesting times when a constructor or method is
run. Those times are when it is called, and when it actually
executes.
The main difference is that a call join point happens outside of
the target object (for non-static methods) or class (for static methods
and constructors), and that an execution join point happens inside
the object or class. This means that the within
and withincode pointcuts pick them out
differently: A call join point is picked out within the caller,
while an execution join point is picked
out where it is actually defined.
A call join point is the ``outermost'' join point for a particular
call. Once a call join point proceeds, then a number of different
things happen. For non-static methods, for example, method
dispatch happens, which will cause one method execution join point
-- perhaps more, if there are super calls. For constructors, the
super constructor is called, and fields are initialized, and then
various constructor execution join points will occur.
A call join point matches only the ``external'' calls of a method
or constructor, based on a signature, and it does not pick out
calls made with super, or
this constructor calls.
Here's more detail:
Consider method execution in Java as (1) the initial call from
this object to some method on the target object with a
particular signature; and (2) the execution of the actual code
in the particular method dispatched in the target object.
The call join point starts with the initial call and ends
when control returns to the call (by return or perhaps
thrown exception). The execution join point starts with
the method body and ends when the body completes (again
by return or throwing an exception), so the execution join
point always happens within the bounds of the corresponding
call join point. You can see this if you use the
join-point tracing aspect in see .
As you would expect, the context differs
in advice on pointcuts picking out execution and call join
points; for call, this refers to the caller, whereas
for execution this refers to the called
(executing) object.
There are some subtle interactions with other AspectJ semantics.
First, the meaning of the signature in the
execution() and call()
pointcut designators (PCD's) differ: the call type depends upon
the type of the reference making the call, while the execution
type depends on the enclosing class.
Second, you may choose one over another if you cannot bring all
your sources within the code the compiler controls
(described in the appendix
to the Programming Guide).
For example, to trace calls into a
method from classes which are outside the code the compiler controls
at compile time, then using execution() will work
while using call()may not. Finally, since
super invocations are not considered method calls,
to trace super.foo() would require using
execution.
Because of differences in the way AspectJ 1.0 and 1.1
are implemented, in 1.0
you should use the call()
pointcut designator unless you have a good reason to use
execution(); in AspectJ 1.1, the
reverse is true.
What is the difference between cflow and cflowbelow?
Both pick out all the join points in the control flow of
the specified join points.
They differ only in that the cflowbelow()
pointcut designator does not pick out the join points
specified, while cflow() does.
How do I say that I want the topmost entrypoint in a
recursive call? How about the most-recent prior entrypoint?
This is best seen by way of example.
Given a recursive call to int factorial(int)
you can print the arguments for
(a) the current and most-recent recursive call
or (b) the current and original recursive call:
aspect LogFactorial {
pointcut f(int i) : call(int factorial(int)) && args(i);
// most-recent
before(int i, final int j) : f(i) && cflowbelow(f(j)) {
System.err.println(i + "-" + j);
}
// original
before(int i, final int j) : f(i)
&& cflowbelow(cflow(f(j)) && !cflowbelow(f(int))) {
System.err.println(i + "@" + j);
}
}
What is the difference between constructor call,
constructor execution, initialization, and static
initialization join points?
Static initialization pertains to initialization of
a class or interface type. Constructor call and execution
are akin to method call, and initialization generalizes this and
picks out the first constructor called.
Their relations are best
demonstrated by tracing the join points. Below is the class
Test which implements an interface and extends a class
along with a trace of the join points below and including
the constructor call obtained using
TraceJointPoints.java
from .
For a program compiled with AspectJ 1.0,
the result is this:
]]>
Ordinarily, using a call pointcut designator
is best because the call join point surrounds the others, but in
the case of constructors there is no target object for
the call (because it has not been constructed yet), so you
might prefer to use the initialization
pointcut designator.
How do I work with an object right when it is created?
You can advise some form of constructor join point.
Constructors are tricky in Java, and that's exposed in AspectJ.
Here are some rules of thumb:
If you want the join point on the "outside" of object creation,
use after returning from call to the constructor:
after() returning (Foo newlyCreatedObject): call(Foo.new(..)) { ... }
You might be tempted to use "this" or "target" to expose the new object, but remember
that if you're on the "outside" of object creation, the object itself might not be
created yet... it only exists "on the way out", when you return the object.
If you want the join point inside a particular constructor, use:
after(Foo newlyCreatedObject) returning: this(newlyCreatedObject) && execution(Foo.new(..)) { ... }
Remember, though, that if you use "before" advice here, the body of the constructor
will not have run, and so the object may be somewhat uninitialized.
In the rare case that there are all sorts of constructors for the object that call
each other with this(...) and you want exactly one join point
for each initialization of Foo, regardless of the path of
constructors it takes, then use:
after(Foo f) returning: this(f) && initialization(Foo.new(..)) { ... }
I want advice to run at two join points, but it doesn't run at all. What gives?
This usually reflects both a conceptual error and a programming mistake.
Most likely you want to do something like "run the advice for all
public and private calls," and the code looks something like this:
within(com.xerox.printing..*) && call(public * *(..)) && call(private * *(..))
But a pointcut is evaluated at *each* join point.
The expression above would never pick out any call join point,
because no method signature has both public and private access.
In a pointcut, pc1() && pc2() means both
must be true at a given join point for advice to run at that join point.
The correct pointcut would use || as follows:
within(com.xerox.printing..*) && (call(public * *(..)) || call(private * *(..)))
Then the advice will run at the join point.
How do I refer to a static field when my advice crosscuts multiple classes?
There is no way in advice to refer to the type of the
code executing in a static context except by specification.
This makes it impossible to refer to static members using
runtime information.
However, AspectJ can determine the class for something
in the join point context, which you can use as a per-class key.
Then you can actually declare an instance field to contain
the per-class value (see the next question). This comes at
the cost of an extra reference, but the field can be final.
I would like to reuse a type pattern, e.g., to
write advice that is limited to a certain set of classes.
Do I have to retype it each time?
No. You can declare that all the types implement
an interface you define, and then use the interface type in
your program. For example:
/**
* Example of using an interface to represent a type pattern.
* sub-aspects use declare parents to add to traced types, e.g.,
* declare parents: com.mycompany.whatever..* implements Marked;
*/
abstract aspect MarkerExample {
/** marker interface for types that we want to trace */
interface Marked {}
/** calls to an instance of Marked not from an instance of Marked */
pointcut dynamicCallsIn(): call(* *(..)) && target(Marked) && !this(Marked);
/** calls to methods defined by a subtype of Marked
* that don't come from the body of a subtype of Marked
*/
pointcut staticCallsIn(): call(* Marked+.*(..)) && !within(Marked+);
/** print dynamic calls */
before(): dynamicCallsIn() { System.out.println("before " + thisJoinPoint); }
}
aspect MyMarker extends MarkerExample {
declare parents: com.mycompany.whatever..* implements Marked;
}
Where do I find example programs and how-to's?There are a number of places to find sample code
and instructions for using AspectJ with other programming tools.
The AspectJ release includes examples in its
doc directory.
There is a community repository of sample code and tutorials
in the AspectJ CVS tree
docs module sandbox directory.
These are extracted and published (online only)
here
.
The teaching directory of the
docs module contains public materials
the AspectJ committers use for presentations, some of
which include example code. To access CVS, see
.
The archives for the user and developer mailing lists
contain many good examples. To search the archives, see
.
This code can vary in quality.
Code that we publish or include with AspectJ is generally
correct. However, code found in our CVS tree might not have
been tested thoroughly, and code from the mailing lists might
be untested or use older versions of the language.
Are aspect libraries available?Some libraries are distributed in the release under the
examples folder in the distribution.
These are "libraries" in the sense that they are reusable,
but they are delivered in source form.
Similarly, some of the sample code is reusable; for that,
see .
If you develop such a library and want to make it available to
other users, feel to send it to the users mailing list
aspectj-users@eclipse.org.
In AspectJ 1.1, ajc supports binary aspects, so
you can distribute aspect libraries without distributing the
source. For more information, see the
-aspectpath
option in the
Reference for ajc.
How does ajc interact with the
serialVersionUID?
The current version of ajc can change the
serialVersionUID of generated
.class files as a result of weaving in advice.
This is an important fact that developers using both aspects and
serialization should be aware of. It is likely that a future
version of the compiler will be better behaved regarding the
serialVersionUID.
However, changes to the serialVersionUID
attribute are typically only important when using serialization for
the long-term persistence of objects. Using standard Java
serialization for long-term persistence has a number of drawbacks
and many developers already use alternative solutions. For one
possibly standard solution, see
Long-Term Persistence for JavaBeans Specification
.
How can I use AspectJ with applets?
Just include the aspectjrt.jar as a required archive.
For example, here is the HTML code for an HTML editor
applet that contains some debugging aspects:
]]>
The above markup has worked reliably with the Java Plugin
(included in the JRE 1.4.x) in IE 6, Mozilla 1.1 (Win32),
and Mozilla 1.0.1 (Red Hat Linux 8.0).
The following link describes how to configure Mozilla/Netscape
6.x/7.x to use the Java Plugin from a JRE/SDK installation:
http://java.sun.com/j2se/1.4.1/manual_install_linux.html.
(Thanks to Chris Bartling for this answer.)
How can I specify types for advice that captures primitives, void, etc.?
In some cases, AspectJ allows conversion from values of primitive types to Object,
so that highly polymorphic advice may be written. This works if an advice parameter
or the return type for around is typed to Object. So:
class Test {
static int i;
public static void main(String[] args) {
i = 37;
}
}
aspect TraceSet {
before(Object val): set(* Test.*) && args(val) {
System.err.println(val);
System.err.println(val.class);
}
}
will print out
37
java.lang.Integer
For more information, see the Programming Guide
semantics section "Context Exposure"
.
How do I detect which version I am running?The ajc
compiler emits the version when passed the
-version flag as an argument.
To programmatically
detect the version of the AspectJ runtime while running
under Java 1.4 or later, get the version from the package:
Package lang = org.aspectj.lang.JoinPoint.class.getPackage();
String version = lang.getImplementationVersion();
When running under Java 1.3 or earlier, read the manifest
directly. For example code, see the source for
AjBuildManager.checkRtJar(AjBuildConfig)
in the org.aspectj.ajdt.internal.core.builder
package of the org.aspectj.ajdt.core module,
available as described in
.
Note that the version of AspectJ for the tools in
aspectjtools.jar is in
org.aspectj.bridge.Version.
How do I write synchronized advice?The only modifier advice can take is strictfp.
However, you can enclose the body of the advice in a synchronized
clause:
before() : pc() {
synchronized (this) {
// advice code here
}
}
It should not be necessary to synchronize a percflow aspect,
but you might do this for perthis, pertarget, or issingleton (default)
aspects. To serialize advice in multiple aspects, synchronize on a
lock object available (only) to the aspects.
Common ProblemsWhen I run, I get a StackOverflowError
(or a long stack trace or no output whatsoever)
Most likely this is a case of infinite recursion,
where advice is advising itself. It presents as a
StackOverflowError
or silence as the VM exhausts itself in the recursion.
Of course, infinite recursion is possible in Java:
public class Main {
public static void main(String[] args) {
try {
main(args);
} finally {
main(args);
}
}
}
If you compile and run this program, and it will fail silently, trying
to process the finally clause even after throwing the StackOverflowError.
Here's a similar AspectJ program where the recursion is
not so obvious:
aspect A {
after(): call(* *(..)) { System.out.println("after " + thisJoinPoint); }
}
This re-invokes itself because it advises any call.
It invokes itself even after an exception is thrown, since
after advice, like a finally clause, runs even
after exceptions are thrown. You can fix this by following two practices:
In AspectJ 1.1, the String concatenation operator (+) is
advised in its StringBuffer form, so if your advise uses
String + in a way that is picked out by your pointcut,
you will get infinite recursion.
(1) Use after returning to advise normal completions
or after throwing to advise abrupt completions.
If you use after or after throwing,
write the advice with the same care you would a finally clause,
understanding that it may run after some failure.
(2) Avoid writing advice that advises itself. One simple way to
do so is to exclude the code within the current aspect:
aspect A {
after() returning: !within(A) && call(* *(..)) {
System.out.println("after " + thisJoinPoint);
}
}
A better way is often to re-write the pointcut.
If the advice is advising itself accidentally, that's a sign that
the pointcut is not saying what you mean.
aspect A {
pointcut withinTargetClasses() : within(A+) || within(B+);
after() returning: withinTargetClasses() && call(* *(..)) {
System.out.println("after " + thisJoinPoint);
}
}
I've declared a field on every class in
my package; how do I use it in advice?
aspect A {
boolean com.xerox..*.dirtyFlag;
after (Object target) returning
: target(target) && call(* com.xerox..*.set*(..)) {
target.dirtyFlag = true; // compile fails here
}
}
You need a type to refer to any member, field or method.
It's generally better to introduce onto an interface and
declare classes to implement the interface, which permits you
to use the interface type in advice formals.
aspect A {
interface TrackingSets {}
boolean TrackingSets.dirtyFlag;
declare parents : com.xerox..* implements TrackingSets;
after (TrackingSets target) returning
: target(target) && call(* com.xerox..*.set*(..)) {
target.dirtyFlag = true;
}
}
The AspectJ compiler aborts with an OutOfMemoryError when
compiling many classes. How can I fix this?
ajc can use more memory than a javac
compile of the corresponding pure-java sources when aspects
are added to the mix. You'll need to increase the memory
available.
The command ajc is actually a script that
launches a Java virtual machine with the correct classpath. You
should make a copy of this script, rename it, and then edit it.
Change the -Xmx option, size of memory allocation pool (heap). You
might try -Xmx128M or even
-Xmx256M.
When running under Ant, give Ant more memory or
use the fork option together with
the Xmaxmem option.
When running under an IDE, look to the documentation
for the IDE to determine how to increase available memory.
In either case, doing incremental compilations can hold on to
more memory than a one-shot compile process, as the compiler
trades space for time in recompiles.
Why do I get a message that my class is already defined?
Most commonly, a source file was specified twice on the command line
(e.g., directly and by a *.java entry in a .lst file).
However, sometimes you have defined a class in two files in the
same package, and you need to rename the class or change its
scope. You should get this message from any Java compiler.
ajc recompiles all files every time.
How can I make it recompile only the files that have changed?
ajc 1.0 does not support incremental
compilation, but since 1.1 ajc does when passed the
-incremental option. It may still recompile
files that have not changed, if they could be affected by aspects
in particular ways, but the files compiled should be fewer
and result in faster compiles.
Further, the 1.1 release supports binary weaving, so you
need not recompile if you already have .class files.
ajc is using the wrong JVM. How do I
fix it?
The easiest way to fix this is to re-install
ajc (using the same .class or
.exe file that you originally downloaded) and
this time make sure to tell it to use the desired JDK (typically
the JDK versions 1.2 or 1.3 from Sun).
If you are familiar with DOS batch files or shell programming,
you could also fix this by simply editing the
bin\ajc.bat or bin/ajc
script.
My IDE is trying to parse AspectJ files which makes my project unusable.
What can I do?
When working with an unsupported IDE that objects to the syntax of
AspectJ source files (and, e.g., automatically gathers them
in a source tree as Java files based on the .java extension),
you can use the .aj extension for your AspectJ files.
The ajc compiler accepts both .java and .aj files, and you can
set up your build scripts to include the correct list of
source files. (You will have to find another editor for
editing AspectJ files; you can use the ajbrowser to view
edit your AspectJ files and navigate the crosscutting structure.)
I used to be able to compile my program in my IDE, but when I
use AJDE, I run out of memory (or it goes really slow).
The ajc compiler does more analysis than (e.g.,) javac,
and AJDE may in some IDE's hold a copy of the structure tree until the
next tree is available from the compile process. Both mean that you may
need extra memory to compile the same program. However, increasing
available memory to the point that you are swapping to disk can
slow the process considerably.
If you are having problems and would like to find the optimal memory
allocation, iteratively decrease the amount of memory available until
AJDE or ajc signals out-of-memory errors, and then increase that
amount by 5-10%.
To increase memory for the ajc compiler, see .
For your IDE, do something similar or follow the provider's instructions.
For example, to increase memory in JBuilder, edit the
jbuilderX/bin/jbuilder.config
file to have an entry like:
vmparam -Xmx384m
If it turns out that your project is too big to use with AJDE, your IDE
may nonetheless support external commands or Ant build processes, which
run outside the IDE memory space. For a JBuilder Ant plugin, some
people have directed us to .
When I run, I get a NoAspectBoundException or a
ClassNotFound message for NoAspectBoundException.
This happens when an aspect is not associated with an object
that is being advised. We have seen this happen two ways:
You get a ClassNotFound message for
NoAspectBoundException when loading a
class affected by aspects if aspectjrt.jar
classes are not on the runtime classpath.
To fix this, put the classes on the classpath.
You can get a NoAspectBoundException when
there is a cycle in aspect initialization or static
initialization, most commonly when an aspect advises
its own initializer. To fix this, first find the class that
fails to load by running java in debug mode or looking
at the NoAspectBoundException trace,
and then fix the offending (probably unintended) dependency.
Most often, it comes from a pointcut like
staticinitialization(com.company..*)
or within(com.company..*), which
can include any aspects in the same subpackages.
You can avoid advising most join points associated with
the aspect TheAspect
by adding && !within(TheAspect)
to your pointcut.
My stack traces don't make sense. What gives?
In 1.0, unless you are using the ajdb debugger,
stack traces may
have synthetic methods in the stack, and the line numbers may
not track your source code. The
Development Environment Guide
discusses how to interpret stack at the end of the
Reference for ajc.
In 1.1, line numbers should work correctly.
The only difference from a normal stack might be the addition
of extra stack frames for call-backs.
My advice is not running (or running twice), and I don't know why.
When advice is not running,
there is probably a problem in the pointcut.
Sometimes users specify pointcuts that
do not mean what they intend -
most often when they misspell a type name. Run the compiler in
-Xlint mode, which will flag some likely mistakes,
like the type name.
If that does not work, and your pointcut is staticly-determinable,
use a declare statement to identify affected code. (For more
information, see .)
If that does not work and your pointcut is dynamically determined,
see if your join points are executing at all by using
TraceJoinPoints.java from .
When advice is running more than it should, either
(1) your advice is in an abstract aspect and the pointcut picks
out the same join point for more than one concrete instantiation
of the aspect, or
(2) your pointcut picks out more join points than you intend.
In the case of advice in abstract aspects, the advice will run once
for each concrete instance of the aspect.
If the pointcut for that advice picks out the same join point for two
concrete aspects, then the correct behavior is for the advice to run
the advice twice at that join point.
To see if your pointcut picks out the join points you intend, you
can use IDE support, logging, or declare-warnings.
If you are using IDE support, you should be able to trace back from
the pointcut or advice to the join points which can be statically
determined to be affected.
Without IDE support, you can write
declare-warning statements to identify code affected by staticly-
determinable pointcuts.
To identify advised dynamic join points,
you can try using TraceJoinPoints.java as above,
or update the advice to print the source location of the join point.
Doing any of these should show if the advice applies to code that
you did not expect.
If you've done this and convinced yourself it's not working,
it may be a bug. See .
My advice runs for each overridden method!
Most likely you are advising the method execution join
point and specifying the defining signature.
Since all overriding methods share this signature,
the advice runs for each method executed.
(This happens, e.g., when one method invokes the same method
in the superclass using super.{method}(..)).
This is the correct behavior.
To avoid this, use the call(..) pointcut
designator, or use !cflow(..) to pick
out only the initial method-execution.
I don't understand when thisEnclosingJoinPointStaticPart is available.
thisEnclosingJoinPointStaticPart is a special
variable available in the context of advice to refer to the
join point, if any, lexically enclosing the current join point:
thisEnclosingJoinPointStaticPartOne of these...will be tEJSP for each of these:
constructor-execution, method-execution,
advice execution, initialization,
pre-initialization, static initialization
constructor-call, method-call, handler,
field-set, field-get
Expressions in the body of handlers have the same
thisEnclosingJoinPointStaticPart
as the handler itself.
I declared a member on a class with package access, but other classes in the package cannot see it.
When declaring parents on other types from an aspect, package access only
applies to code the implementation controls. For AspectJ 1.0, that is the set of files
passed to the compiler. That means other classes not compiled with the aspect will not
be able to access the aspect-declared members even if they are in the same package.
The only way for classes outside the control of the implementation to access aspect-declared
members is to declare them public.
I declared a member on a interface, but javac does not see it.
You have to compile all the top-level implementating
classes of the interface using ajc.
From an email by Jim Hugunin on the requirements for AspectJ 1.1 to
implement members declared by an aspect on an interface:
If you introduce non-static fields or non-abstract methods on an interface
from an aspect, then all of the top-most implementors of that interface must
be woven by that same aspect.
(A class C is a top-most implementor of an interface I if C implements I
and the superclass of C does not implement I.)
ajc 1.0 complains that it can't find
javac. What's wrong?
ajc 1.0 does not try to locate
javac in your path: it uses the
javac classes directly. In JDK 1.2 and 1.3 these
classes are found in tools.jar (in the
lib directory of the JDK distribution), which
must be on your classpath to make
ajc work with javac.
Inspect the java command that launches ajc to make sure that
tools.jar is on the classpath for ajc;
the -classpath option only applies to the sources compiled.
I'm running under 1.4, but ajdoc asks for 1.3
(or throws IllegalAccessError for HtmlWriter.configuration)
The 1.0 implementation of ajdoc uses
specific javadoc classes in the J2SE 1.3 tools.jar.
We are working on addressing this limitation, but in the interim
it is best to run ajdoc under 1.3.
When running from the command-line scripts, edit the scripts directly
to put the 1.3 tools.jar first on the classpath. (The installer does
not know about this limitation of ajdoc.)
When running from Ant, users often have tools.jar in ${ant.classpath}
(to make javac, et al work). That makes it impossible to run the ajdoc
taskdef (which does not currently support forking), so you'll need to
run a separate ant process, either from the command-line or via Ant's
exec task (the Ant task will propagate the classpath).
If the wrong tools.jar is not on the ant classpath, then it should work
to put the 1.3 tools.jar in the taskdef classpath.
I set up different files to my compiles to change what
the aspects see, but now I don't
understand how the aspects are working.
It is a bad practice to use the compilation unit
to control crosscutting. Aspects and pointcuts especially
should be written to specify crosscutting precisely.
Aspects will behave the same when you add files if
you initially included all files affected by your aspects.
If you use the compilation unit, then your code will behave
differently in AspectJ implementations that do not limit
themselves to specified files.
I'm reading the code generated by ajc 1.0
in -preprocess mode, and it seems like it would not
work (or "like it works this way").
The generated code can be difficult for a human to read and
understand. The compiler uses implementation techniques which might
not be apparent. To determine if the code is behaving correctly, you
should write and run a program that attempts to provoke the error you
suspect. Similarly, you should not rely on invariants you infer from
the generated code (especially naming conventions for generated members).
Please rely only on the semantics stated in the appendix of the
AspectJ Programming Guide.
I've heard AspectJ can generate or inject code into my code.
Is this true?
This is a misconception spawned from the early implementation.
AspectJ does not "inject" or "generate" code. In AspectJ the
pointcut constructs allow the programmer to identify join points,
and the advice constructs define additional code to run at those
join points.
So the semantic model of advice is like the semantic model of a
method -- it says "when any of these things happen, do this".
People who worked with earlier versions of AspectJ, in which ajc
was very explicitly a pre-processor, sometimes thought of AspectJ
as injecting code. But that was an artifact of the implementation,
not the underlying language semantics.
This distinction is important for two reasons. One is that thinking
about it this way will make more sense at the implementation continues
to evolve towards load-time or runtime weaving. The other is that
it makes it much easier to understand the semantics of advice on
cflow pointcuts.
Why can't AspectJ pick out local variables (or array elements or ...)?
Users have sometimes wanted AspectJ to pick out
many more join points, including
method-local field accessarray-element accessloop iterationmethod parameter evaluation
Most of these have turned out not to make sense,
for a variety of reasons:
it is not a commonly-understood unit for Java programmersthere are very few use-cases for advice on the join pointa seemingly-insignificant change to the underlying program
causes a change in the join pointpointcuts can't really distinguish the join point in questionthe join point would differ too much for different
implementations of AspectJ, or would only be implementable
in one way
We prefer to be very conservative in the join point model for the language,
so a new join point would have to be useful, sensible, and implementable.
The most promising of the new join points proposed are for exception
throws clauses and for synchronized blocks.
Why doesn't AspectJ pick out reflective calls?
The pointcut call(void run())
won't pick out a call using reflection, like
((Method)run).invoke(null, args).
The pointcut
execution(void run()) will
work. The call pointcut doesn't work because
Method.invoke(..) is the Java method-call,
and AspectJ cannot delve into the Java reflection library to
implement call semantics. To advise a reflective call
(e.g., because the compiler does not control the code for the
method execution), test the context for invoke(..).
Here's a pointcut that tests only if the method name is
correct:
aspect A {
pointcut runReflectiveCall(Method run) : target(run) &&
call(Object Method.invoke(..)) && if("run".equals(run.getName()));
before() : runReflectiveCall(Method) {
System.out.println("before reflective call " + thisJoinPoint);
}
}
What are the bugs now most affecting users?The bugs affecting the semantics of the language
are marked with the "info" keyword. Find them with
the query
http://bugs.eclipse.org/bugs/buglist.cgi?product=AspectJ&keywords=info
What extra memory is required at runtime?
When running classes produced by the AspectJ weaver or compiler,
there are no significant hidden uses of memory. As would be expected,
each aspect is instantiated. The per-object aspects (like
pertarget or perthis)
in some implementations
use a map to link aspects and the associated object. When using
cflow-related pointcuts, a ThreadLocal
is used to track control flow for each affected thread.
Of course, the size and code in an aspect can require memory.
Aside from normal Java practices, take care with join point references.
When referencing the static part of a join point (e.g.,
thisJoinPointStaticPart), only one object is
created. However, if you reference the join point itself
(e.g., thisJoinPoint), then one
JoinPoint object will be created for each
join point running advice.
Aspect instances will be garbage collected just like regular objects
after there are no more strong references to them. For the default
aspect instantiation model, issingleton, the aspect
class retains a reference to the singleton instance, in order to
implement static {AspectClass} aspectOf(), so
singleton instances will not be garbage collected until the class is.
For long-running or memory-critical programs, consider using weak
references in singleton aspects for state that should be garbage collected.
Finally, when using load-time weaving, the weaver can require
memory in its own right. Because the class loader never can
know when it is done loading classes, the weaver can hold on
to the aspects required to weave for some time. There are
strategies for minimizing this (with different trade-off's),
so the time and memory required for load-time weaving will
vary as load-time weaving evolves.
I get a VerifyError when running CGLIB generated code that has been woven by
AspectJ. Why is this?
When weaving after advice into any piece of code, the AspectJ strategy is to make all
exit points from that code jump to a single exit point that executes the advice
before returning. There is a verifier rule in the JVM specification that specifies
that all routes to a jump destination must have the same height stack when they get there,
regardless of the route taken to get there through the bytecode. The CGLIB generated code has different
stack heights at the various exit points. This is not a problem with the CGLIB generated code,
it is perfectly valid - it is just unusual and the AspectJ weaving strategy causes the
verify error to trigger when it makes all exits jump to a single destination.
AspectJ could cope with this and instead implement after advice by calling the
advice and returning at each exit point. However, it is unlikely that the user
actually meant to weave the CGLIB generated code in the first place - and so usually
the right thing to do is to exclude CGLIB generate code from the weaving process by
appropriate use of the exclude element in the aop.xml. A typical clause in the aop.xml might
look as follows:
<weaver>
<exclude within="*CGLIB*" />
</weaver>
AspectJ 1.1 and eclipse.orgWhy did the AspectJ project move to eclipse.org?
From the message sent to users:
AspectJ has come a long way -- the language has
stabilized; there are a rapidly growing number of
commercial users; the 1.1 release is imminent and will
include byte-code weaving and incremental compilation;
and the tool support is now well integrated with several
major IDEs.
This growth of the community and the technology means
that the original research and prototype development of
AspectJ is complete. As such it is time for ongoing
development and support of AspectJ to move outside of
PARC. This has already started to happen; the Eclipse
AJDT plug-in and the several books in preparation are
examples.
To encourage the growth of the AspectJ technology and
community, PARC is transferring AspectJ to an
openly-developed eclipse.org project. This project will
include documentation, web site, mailing lists, bug
database, and sources for the compiler. The
command-line AspectJ compiler is still the primary tool
produced by this project, in addition to APIs that support
integration with a variety of IDEs. The Eclipse plug-in will
remain at eclipse.org, while the NetBeans, JBuilder and
Emacs support will move to SourceForge.net projects.
We look forward to your involvement with and
contribution to those projects.
We see Eclipse as an excellent new home for core
AspectJ technology development -- it is an active
community of Open Source development and innovation
in the Java space. Once development moves to
Eclipse.org, others will be able to contribute more easily.
Do I have to download Eclipse to use AspectJ?
No. The AspectJ tools download is completely self-contained
and does not require that you work in Eclipse.
For information on IDE support, see
.
What are the relationships between AspectJ, JDT,
Eclipse, AJDT, and IDE support generally?
Eclipse is a software platform.
JDT is an eclipse project to support Java development.
JDT has a Java compiler.
AspectJ 1.1 is built on Eclipse/JDT's Java compiler
but is distributed standalone and can run standalone.
With the AspectJ distribution, you can compile and run
AspectJ programs and use the AspectJ structure browser.
AJDT is an eclipse project to integrate AspectJ
into Eclipse/JDT so you can use Eclipse to develop
AspectJ programs. AJDT aims to support the full Eclipse
experience - searching, compiler-error tasks, etc.
AJDT will use the AspectJ Development Environment (AJDE)
API's for creating IDE integrations, as well as hooking
in to the model underlying the Java compiler.
Similarly, Sourceforge has projects integrating
AspectJ into other development environments
using the AJDE API's:
AspectJ for Emacs,
AspectJ for JBuilder, and
AspectJ for NetBeans.
This is the right level of separation/integration.
AspectJ is available standalone, leverages an existing open-source
compliant Java compiler, and supports external projects
doing IDE integrations in Eclipse, Emacs, JBuilder, and NetBeans
through a common API, AJDE.
AspectJ 5 and Java 5
What are the new features of AspectJ 5?
All the new features are documented in the
AspectJ 5 Developer's Notebook
and the
AspectJ Development Environment Guide.
To summarize:
Java 5 support: as an extension to Java, AspectJ supports
all the new language features of Java 5, including generics
(parameterized types), autoboxing, covariant return types,
enhanced for-loops, enums, varargs, and of course
annotations.
Java 5 extensions: the AspectJ language has been extended
to make use of Java 5 language features.
Generic aspects: an abstract aspect can be declared
with a generic type parameter which can be used
in pointcuts and when declaring members on the aspect
(but not when declaring members on other types).
Annotations: pointcuts can now pick out join points
based on the associated annotations, annotation
values can be bound in the same way that other
context variables are bound at the join point,
and annotations may be declared on other types in
an aspect.
Annotation-style aspects: AspectJ 5 integrates AspectWerkz-style
aspects declared in annotations. This permits aspects to
be written and compiled in pure-java code and woven using
build-time or load-time weaving with the AspectJ weaver.
(The original AspectJ language aspects are distinguished
as "code-style" aspects.)
AspectWerkz load-time weaving: Load-time weaving is
greatly improved for all versions of Java, and now supports
an XML configuration file which can declare concrete aspects.
This means developers can deploy binary abstract aspects
that deployers configure using only XML.
pertypewithin instantiation model: aspects may now be instantiated
on a per-class basis.
Reflection and runtime support: AspectJ 5 supports reflection
on aspects using the Aspect class, and also support runtime
evaluation of pointcuts using a pointcut parser.
Should I use code- or annotation-style aspects?
To use AspectJ, you can use the original code-style aspects
or the annotation-style aspects new in AspectJ 5.
The original code-style is a small extension of the Java language
designed to express crosscutting as clearly as possible
in ways familiar to most Java programmers.
To use the original code-style aspects,
compile them with the AspectJ compiler or weave
pre-compiled binary aspects using the AspectJ binary (.class)
weaver, either at build-time or at class-load-time.
Code-style aspects have excellent IDE support, allowing
you to navigate to and from affected source code.
Annotation-style
aspects are written (not surprisingly) using annotations.
They use the subset of the AspectJ language that works
when aspects are woven after the code is compiled.
The source files are compiled with Javac, which simply saves the
annotations in the .class files. The resulting .class files
must be woven using
the AspectJ weaver, which reads the annotations from the
.class file and uses them to define aspects.
Annotation-style aspects have the benefit of being compilable
by Javac, but you can't use the full AspectJ language,
and you don't enjoy the same level of IDE support
for viewing crosscutting structure.
What's new about the load-time weaving support in AspectJ 5?
While the AspectJ weaver could be used at load-time in previous
releases, the AspectJ 5 release supports much better integration
with the Java 5 VM and the BEA JRocket JVM. It also supports
an XML file for configuration that allows deployers to declare
concrete aspects using only XML. This means aspect developers
can write abstract aspects, and deployers need only configure
aop.xml and run using the AspectJ weaver in Java 5.
For example, to run Java 5 VM with load-time weaving,
To declare a concrete aspect, add a a
concrete-aspect XML entity to META-INF/aop.xml.
This example extends a tracing aspect to apply to
every type in the application:
]]>
For more information, see the
AspectJ Development Environment Guide.
Understanding AspectJ TechnologyDo I need to know how the compiler or weaver works?
Writing AspectJ programs only requires understanding the
Programming Guide.
However, current implementations do not control everything in
a system, so AspectJ program semantics may be limited to code
the implementation controls. For our implementation, these
limitations are stated in
Programming Guide Appendix: Implementation Notes.
Aside from understanding the use and limitations of the
implementation, there is no need to understand the underlying
technology when writing AspectJ programs.
The technology that implements AspectJ interests
some academic researchers and some developers
who want new features or new ways to weave.
These extensions are not discussed in the documentation.
Some are being developed already,
others are on the drawing board (or perhaps were left off
long ago), and still others haven't been considered.
If you are interested in a certain extension,
check the bug database for feature requests
and the mailing list archives for any past discussions.
Then email the list to see if it's been considered.
For more information, see
.
How does the compiler/weaver work? Are there any white papers?
There are currently no documents describing this process in detail.
You can compile programs and inspect the generated source or bytecode,
or view the source code (see ).
We hope to write papers on the bytecode weaving model used in
AspectJ-1.1 if we can find the time.
Erik Hilsdale and Jim Hugunin did draft a paper for AOSD 2004,
now available on Jim's web site:
http://hugunin.net/papers.html
Jim summarized advice weaving in the AspectJ 1.1 implementation in the
following mailing-list reply:
Each piece of advice in an aspect is associated with a pointcut.
This pointcut is stored in an attribute on the methods
corresponding to each piece of advice.
Before weaving, all of these pieces of advice are gathered
into one large list.
Each .class file is woven independently.
A .class file is woven by the following steps:
Collect all of the joinpoint shadows in the .class file.
For every dynamic joinpoint in the AspectJ language model,
there is a corresponding static shadow of that joinpoint
in the bytecode.
For example, every method call joinpoint has an INVOKE
bytecode as its static shadow. Some joinpoints
(such as initialization) have much more
complicated static shadows.
Each piece of advice is matched to each static shadow.
There are three results possible from this match.
Never matches,
in which case nothing is done to the shadow
Always matches,
in which case the advice is woven into this joinpoint shadow
Sometimes matches,
in which case the advice is woven into the shadow
along with the minimal dynamic tests to determine
if any particular joinpoint in the actual running
program matches the advice.
The simplest example of sometimes matches is
when the pointcut uses if(test()).
If any advice matched any static shadows in the .class file,
then the transformed .class file is written out,
otherwise it is left unchanged.
See BcelClassWeaver and
BcelShadow in the
org.aspectj.weaver.bcel package
for the two primary classes involved in this process.
Note: This explanation ignores the implementations of inter-type
declarations completely.
It also ignores performance optimizations such as fast-match
or pipelining that speed up the process.
How do I get load-time weaving to work in my chosen application server?
You have two choices based on how wide you want the weaving to take effect: application-server wide and application-specific weaving.
You choose between the two by loading aspect artifacts--aspects, associated types, and aop.xml--through the right classloader.
The aop.xml must be in the META-INF directory on the classpath for the chosen classloader. In either case, you modify the
startup script to specify the -javaagent:path-to/aspectjweaver.jar option to the Java virtual machine. Note that it is not
essential that all the artifacts be placed in a single jar.
For application-server wide weaving, you make aspect artifacts accessible to the server's classloader. Typically, you
achieve such access by putting these artifacts in the server's lib directory. For example, for Tomcat, you will place
the aspect artifacts in the TOMCAT_HOME/lib directory.For application-specific weaving, you make aspect artifacts accessible to application classloader by bundling
them along with application's classes. For example, for a web application, you will place the aspect artifacts in
the MY_APP/WEB-INF/lib and/or MY_APP/WEB-INF/classes directory.
We recommend that you start with application-specific weaving.
Note that you have an additional option if your application is based on the Spring framework. If you deploy in one of
the supported web servers or application servers, you can avoid modifications to the startup script. Please
see http://static.springframework.org/spring/docs/2.5.x/reference/aop.html#aop-aj-ltw-spring for more details.
Does AspectJ use reflection at runtime?
The only time that reflection is used during run-time is when the special
thisJoinPoint object is used to discover reflective information about the
join point. If you don't use thisJoinPoint then no reflection will be used.
What about load-time weaving? Can I weave aspects at runtime?
Since the 1.1 release, AspectJ can weave binary aspects
into classes in bytecode form. Hooked up to a class loader,
this can weave class bytecodes after they are read in,
before the
class is defined by the VM. (This means load-time weaving
only works were aspects are not required to compile the pure-java
classes. If the aspects are required, then the Java classes
have to be compiled with the aspects using the AspectJ compiler.)
The AspectJ 1.2 release had the
WeavingURLClassLoader, and the 1.2.1 release introduced
the aj.bat script for Java 1.4.
The AspectJ 5 release introduces much better support for
load-time weaving, including declaring concrete aspects
in XML files and integrating with Java 5 and BEA JRocket
JVM's. See .
Some have asked about only weaving particular classes
specified at run-time.
Aspects should work across an entire namespace, and problems
will likely result from weaving
some classes but not others. Also, it's confusing to
specify crosscutting both in the aspect and in the
list of runtime classes; the crosscutting specification
should be in the aspect itself,
where it can be processed by tools.
And just to state the obvious:
do not use bytecode weaving, at load-time or otherwise,
to modify .class files protected by license,
without permission from the licensor.
AspectJ Project DevelopmentI'm interested in the code implementing AspectJ.
Most people do not need to see the code for AspectJ;
they can download the binary distribution for documentation
and tools for writing AspectJ programs.
For people who want to know how the AspectJ technology works,
the source code is the best resource, until we write some
proper white papers
(see ).
To get and compile the Java source code for the AspectJ
distribution, see
.
Bear in mind when looking at the code that there are many
ways to implement the AspectJ language, and the code inspected
might be an initial version of a new architecture (e.g., bytecode
weaving).
How can I get involved with developing the AspectJ project?
For those who want to contribute to the project,
here's a general list of ways to do so, in no particular order:
Participate effectively in the mailing lists.
The quality of the mailing lists makes a huge difference
in the ability of new and experienced AspectJ users
to write good code. For guidance on effective
participation, see
and
.
Also, the time that experienced users take in answering emails
can directly translate to time developers can use (instead)
for fixing bugs or adding features.
Write bugs. Good bugs, especially with test cases,
are always appreciated. We especially like proposals for
new XLint messages, since they are
sometimes easy to implement and help users learn
AspectJ, and for other implementable features
grounded in a compelling use-case.
Write test cases for compiler bugs without test cases.
Compiler bugs without test cases are much less likely to be fixed;
until they are rendered in code, they might be user mistakes,
and they might duplicate another bug or actually cover many bugs.
Find them by searching open compiler bugs and picking out
any which do not have test case attachments or a comment that
a test case has been written.
Here is a query for open compiler bugs:
http://bugs.eclipse.org/bugs/buglist.cgi?product=AspectJ&component=Compiler&bug_status=UNCONFIRMED&bug_status=NEW&bug_status=ASSIGNED&bug_status=REOPENED
For how to write test cases, see
.
Write patches to fix bugs.
If you particularly need a bug to be fixed, or if you're interested in
learning about the system, then get the source code and try to fix the
bug. Most likely you'll want to email aspectj-dev@eclipse.org to
declare your intentions and the approach you propose (based on having
looked at the code).
Mailing the list gives those experienced with the code a chance to
guide you away from pitfalls. To submit the patch, attach it to
the bug. (When creating patches, do so on a per-module basis; that
means if fixing the bug involves changes to three modules, submit
three patches.)
Write patches for other reasons.
Often documentation needs to be fixed, or there may be a small new
feature you'd like to see. You can just do it and then submit it
as a patch to a bug you create. As with bugs, in some cases you
might want to declare your intentions on the mailing list to avoid
wasting time on something that's been fixed but not committed or
on an approach that will be fruitless.
How do I get and compile the source code for AspectJ?
AspectJ 1.0 source code is available in an archive available
with the 1.0 downloads. It contains instructions for building
from sources.
AspectJ 1.1+ source code is available through CVS using the
CVS Root dev.eclipse.org:/cvsroot/technology.
For more information on accessing the CVS tree at eclipse.org,
see the documentation from http://eclipse.org. Find
specific instructions in the AspectJ tree at
org.aspectj/modules/build/readme-build-and-test-aspectj.html.
If you would like to use Ant to checkout the sources, build the
distribution, and test everything, see
org.aspectj/modules/build/release/build.xml.
To check out the source code in Eclipse go to (File > new > Other > CVS > Checkout Projects from CVS). You'll need about 125 MB of space for the source and build.
Host: dev.eclipse.org,
Repository Path: /cvsroot/technology,
user name: anonymous,
password: (your email address),
connection type: pserver,
default port.
Then select the individual modules you want to check out (you probably want all of them bar aspectj-attic and java5) and click Next and choose to check out the modules you selected as Java projects.
Once thats done each module you checked out should show up as a project in the package explorer.
If you have problems after this point you can view the build instructions that come with AspectJ by going in the package explorer to: build > readme-build-and-test-aspectj.html.
To get the modules to build you have to set some classpath variables (Window > Preferences > Java > Build Path > Classpath Variables):
Name: JAVA_HOME, Value: (wherever your Java JDK is installed)
Name: JRE14_LIB, Value: (wherever your Java 4 Runtime is installed)\jre\lib\rt.jar
Name: JRE15_LIB, Value: (wherever your Java 5 Runtime is installed)\jre\lib\rt.jar
Name: ASPECTJRT_LIB, Value: (wherever your workspace is)\lib\aspectj\lib\aspectjrt.jar. To find out where your workspace is go to File > Switch Workspace.
The org.aspectj.lib project is an AspectJ project so
you also have to have AJDT installed. For the latest AJDT release and
download instructions visit the
AJDT Downloads page.
When you've added the variables click OK to do a full rebuild, then run the tests by going in the Package Explorer to:
run-all-junit-tests > testsrc > (default package) > RunTheseBeforeYouCommitTests.java
and running this as a JUnit test (right click and select Run As > JUnit Test).
Don't worry about any errors that appear in the console output,
just check that there are no failures in the JUnit view (Window > Show View > Other > Java > JUnit).
If that finishes with no Failures and a full green bar you have the AspectJ compiler source and it's building and testing properly.
Further details:
You can check out the entire modules directory and build using the
Ant build script modules/build/build.xml.
All required libraries are included in modules/lib/,
(including Ant 1.5.1 in modules/lib/ant).
If you are using Eclipse, you can check out any modules/
subdirectory as an eclipse Java project.
Depending on what you are trying to build, you need not check out
all modules; as of this writing, here are the modules to get
for building the specific parts of AspectJ:
For any builds: build, lib
For the documentation: docs
For the compiler: bridge, util, testing-util,
weaver, asm, org.eclipse.jdt.core, org.aspectj.ajdt.core,
and runtime.
For the AspectJ distribution, the ajbrowser modules,
plus aspectj5rt and org.aspectj.lib.
For the test harness (or to run the release build
scripts and tests): testing, testing-client, and testing-drivers.
To run the test suite: the test harness modules, plus
tests.
Note that module interdependencies are recorded only in the eclipse
modules/{module}/.classpath
files and may
change, so the list above may not be correct when you read it.
How do I build AspectJ and integrate it into AJDT?
To build AspectJ, first get the source tree as
described in . Once you have
a development environment set up, copy the
build/sample-local.properties file
to build/local.properties and within this file point the
java14.home and java15.home
to the corresponding places on your machine.
To build AspectJ on the command line:
Open a command prompt
Navigate to the build directory within your AspectJ workspace
(to find out where your workspace is go to File >
Switch Workspace within Eclipse).
Run ant clean to remove the files from
previously built AspectJ versions.
Run ant to build AspectJ. The built files are created in
your_eclipse_installation_directory/aspectj_development_workspace/aj-build.
To import a locally built AspectJ into AJDT first follow the
instructions on
How do I setup an AJDT development environment in Eclipse?
for setting up an AJDT development environment and running the
correctness tests. Then:
Create a file aspectjlib.properties within
the org.aspectj.ajde project and add the following two lines
aspectj.lib.dir=C:/eclipse/aspectj-workspace/aj-build/dist/tools/lib
aspectj.doc.dir=C:/eclipse/aspectj-workspace/aj-build/dist/ide/eclipse/org.aspectj.ajde.doc/doc
making sure to change the path to correspond to your set up.
Run the build.xml file in org.aspectj.ajde
with the plugin jars target:
Right click on the build.xml file in the
org.aspectj.ajde plugin
Select Run As > Ant build...
In the resultant dialog navigate to the Targets tab
Ensure plugin jars is the only selected target
Click Run
Refresh the org.aspectj.ajde, org.aspectj.runtime
and org.aspectj.weaver plugins.
Where do I find developer documentation on building and testing AspectJ source code?
Find the developer documentation in HTML files in the CVS tree,
inside the build and testing modules
(i.e., in org.aspectj/modules/build/...).
Most pertinant:
../build/readme-build-and-test-aspectj.html
describes how to build the AspectJ distribution in Eclipse
and in Ant.
../build/readme-docs-module.html
describes the AspectJ documentation sources and
how to build the documentation using Ant.
../build/readme-tests-module.html
describes the all the tests
in the tests module.
../build/readme-writing-compiler-tests.html
describes how to write compiler tests that can be run by
the AspectJ test harness.
../build/readme-testing-drivers-module.html
describes the test harness used to run the compiler tests
in the tests module.
../build/readme-testing-drivers-module.html
describes the test harness used to run the compiler tests
in the testing module.
How should I submit test cases for bugs?
You can attach files to a bug after it has been created.
The code of course should replicate the actual behavior
described in the bug when run on the target version.
If you have a single source file, you can attach it directly,
describing in the comments the expected result
(e.g., error on line 14, or successful compile/run).
The most helpful form for describing the test scenario
and the expected results are the test definitions
described next.
For more complex bugs requiring many files,
create a zip file of a directory containing all the files
and an XML test definition file.
The XML test definition file contains specifications
for how to compile, recompile, or run the test sources.
Complete documentation is available in the CVS tree
at tests/readme-writing-compiler-tests.html
but here is a sample file with some example definitions,
preceded by comments showing the directory layout
of the files referred to in the test definitions.
]]>I'd like to run my test case. How do I get the test harness?
The test harness is not distributed.
To build it, get the source tree as
described in and then
build the build-testing-drivers target:
cd build
../lib/ant/bin/ant -f build.xml build-testing-drivers
This produces
../aj-build/jars/testing-drivers-all.jar
which you can run as described in
tests/readme-tests-module.html.
BCEL is used by AspectJ but it's not actively developed. Will you change?
The AspectJ bytecode weaver has used BCEL for bytecode manipulation
since its first release. We have upgraded it extensively, to improve
performance, support Java 5, etc. The BCEL developers have not
incorporated our patches, so we continue to maintain our own version.
Ours has been optimized for the AspectJ weaver and battle-hardened
over years of development and use. At some point in the future,
the AspectJ weaver might be restructured to make it easy to see
whether another bytecode package offers the same stability,
functionality, and performance, but for now we prefer using something
that we know works well.
In the AspectJ 5 release, the weaver has been restructured to
use reflection where possible. Otherwise, it
continues to use BCEL, but does not hold BCEL structures in
memory after our evaluation completes.
Getting Help
How do I find out more about AspectJ?
Visit the AspectJ project web site:
http://eclipse.org/aspectj.
How do I submit a bug report?You can submit a bug from
http://bugs.eclipse.org/bugs/enter_bug.cgi?product=AspectJ
.
If it seems to be a bug in the compiler,
please attach a small test case (source code)
to reproduce the problem.
For more information on writing compiler test cases, see
.
If you are unable to submit a test case, consider submitting traces,
ajcore files, and/or .class dump files, as described in the
AspectJ Problem Diagnosis Guide.
How do I communicate with other AspectJ users?
You can reach other AspectJ users by using the
aspectj-users mailing list. You can subscribe to the list or view the
list archives from the AspectJ home page
http://eclipse.org/aspectj
.
How can I search the email archives or the web site?
It is very effective to do a google search of the form,
http://www.google.com/search?q=site:eclipse.org+cflowbelow
,
and you can use the eclipse.org search at
http://www.eclipse.org/search/search.cgi
.
You can also check the old archives available for download from
the AspectJ home page
http://eclipse.org/aspectj
.
How should I write email queries?
Here's the general form of a good email:
Describe the big picture of what you are trying to do...
Describe what you think it takes, in AspectJ terms
(concepts, syntax, and semantics) from the
Programming Guide...
Show the AspectJ code you are using, what output it
produces when run, and what output you expect...
The big picture helps others redirect you to other approaches.
Using AspectJ terms helps others correct mistakes in thinking
about the problem (the most common being to confuse join points
and pointcuts).
The code is key to clarifying your question and getting a good
response. On the mail list, someone can reply by fixing your
code. In bugs, the developers can reproduce the problem immediately
and start analyzing the fix.
The code should not be incomplete; it should run (or fail) as-is,
without additional libraries or source files.
For the mail lists, we try to follow the conventions for open-source
discussions that help avoid "the tragedy of the commons."
For example conventions, see
http://jakarta.apache.org/site/mail.html
and
http://www.tuxedo.org/%7Eesr/faqs/smart-questions.html
.
How do I write bugs for IDE support?
Bugs appearing in the IDE's may apply to the affected IDE
or to the compiler. Compiler stack traces in IDE message windows
are prefixed "Internal Compiler Error" and should be written up
as compiler bugs. If you are unsure, try redoing the compile
from the command line.
Bug report for the IDE extensions go to their respective projects,
listed in
(including bug reports for the AJDE Eclipse support,
which you can submit at
http://bugs.eclipse.org/bugs/enter_bug.cgi?product=AJDT
).
One of the benefits of open-source is that you can
find and fix the bug for yourself; when you submit
the fix back to us, we can validate the fix for you
and incorporate it into the next release.
You can submit a patch by attaching it to the bug.
How do I write bugs for the AspectJ compiler?
The best compiler bug report is a reproducible test case,
standalone code that demonstrates the problem.
Sometimes with aspects, a test case requires several
files, if not some way to capture the behavior.
Here's how we recommend submitting test cases:
Write the test case so that when the compiler bug
is fixed, the test completes normally without output
(e.g., expected compiler errors are issued,
or classes produced run correctly). This usually
means writing one or more source files.
In the bug report, briefly summarize the bug.
If it is not obvious, be sure to specify
the expected output/behavior (e.g., compiler error on line 32)
and, if the compile should complete, the main class to run.
Submit the bugs via the web form
http://bugs.eclipse.org/bugs/enter_bug.cgi?product=AspectJ
.
Attach the test case to the bug.
The test case may be a single file
or it may be multiple files in a single zip archive,
of the form discussed in
.
Can you recommend reading or teaching material for AspectJ?
The documentation available in the distribution is the
best source for language and usage questions. You can also find
selected AspectJ papers and presentations on the
PARC AspectJ page.
For links to Aspect-oriented programming materials in general, see
http://aosd.net.
Where can our group get consulting and support?
The best thing to to is join and email the
aspectj-dev@eclipse.org mailing list.
What has changed since the last FAQ version?
Entries changed recently:
About the AspectJ ProjectWhat does the fact that AspectJ is an Open Source
Project mean to me?
Open source protects your interest in a correct, long-lived,
up-to-date, and widely-accepted implementation of AspectJ.
With the source code, you control your own destiny
in perpetuity. You can continue to use the implementation
and update it as necessary to fix bugs and add things you need.
Because the code is available to all, anyone can find
and fix bugs. There is no need to hope for it to be fixed
in the next product release. Those who encounter the bugs
are motivated to fix them, and there are more eyeballs on
the code than in closed-source, so the quality tends to be high.
This can be particularly true for the AspectJ community,
which tends to be highly skilled.
The same is true of new features or behavior, so the
implementation should be up-to-date. This is important as
the field of AOP develops, to capture the latest solutions.
For a programming language which forms the basis of
an entire solution stack, open source facilitates the kind
of adoption -- tool integrations and significant projects --
that develop and prove the technology for wider adoption. This
limits delays caused by waiting for the completion of standards
process or promulgation by industry leaders, and also provides
the proofs necessary for such adoption.
What are your plans to make AspectJ a general feature
of Java supported by Sun and the other key players in the Java
Industry?
Although we are committed to making AspectJ available to a wide
range of users, it is too early to decide on a strategy. Some
options include continuing AspectJ as a stand-alone product,
integrating it into IDEs, or possibly incorporating it into
standard Java with Sun's blessing.
We currently focus on developing for the 1.1 implementation
which improves AspectJ in key areas: rapid
incremental compilation, bytecode weaving, and IDE integration.
Through all of this our goal is to make AspectJ integrate as
seamlessly as possible with the Java programming language. The
AspectJ language design is becoming more integrated, the compiler
is becoming faster and more integrated, the IDE extensions are
becoming more integrated. All of this is designed to help users
really use AspectJ and give us feedback on it.
As the system is improved and we work more closely
with users, we will be in good position to explore the best path
for AspectJ in the long term.
When will AspectJ work from class files?
When will it work at class-loading time?
Bytecode weaving is in AspectJ 1.1. We believe it
works as described in an email to the users list by Jim Hugugin:
The AspectJ language was designed to support weaving at many different times:
compile, load, or even run-time in the JVM. Weaving into bytecodes at both
compile and load-time will definitely be provided in a future release. This
will allow weaving at compile-time into libraries for which source code is
not available. It will also support aspect-aware class loaders that can
perform weaving at load time on arbitrary classes. One advantage of a
language like AspectJ, rather than an explicit meta-tool like jiapi, is
that it separates the specification of a crosscutting concern from any
particular implementation strategy for weaving.
...AspectJ provides a language that can cleanly
capture crosscutting concerns while preserving the static type checking,
modularity, and composability of Java.
If you have an application for using aspects and bytecode,
please let the AspectJ team know of your requirements.
We expect to have a demonstration classloader available in
the 1.1 release or soon thereafter.
What are the differences between the current and
previously released versions of AspectJ?
The AspectJ team aims to keep the implementation bug-free and
up-to-date with the Java language,
to limit AspectJ language changes to those that
are carefully considered, compelling, and backwards-compatible,
and to deliver those language changes only in significant releases (1.0, 1.1).
VersionDescriptionAspectJ 1.5Upgrade to support Java 5 language and much better
load-time weaving.
See README-150.html
for more details.
AspectJ 1.1A few language changes and clarifications;
bytecode weaving and incremental compilation.
See README-11.html
for more detail.
AspectJ 1.0Many language changes, fixes, cleanup and
clarifications, some significant.
AspectJ 0.8More cleanup of the syntax and semantics.AspectJ 0.7Clean up of the semantics, 0.7 beta 4 is the first
open source release.
AspectJ 0.6Advice and crosscuts get explicit type signatures
which describe the values that are available to advice at a
crosscut.
AspectJ 0.5Improved tool support: better Emacs environment
support and ajdoc to parallel
javadoc. around advice is added, and the
aspect keyword is removed and replaced
by the Java keyword class.
AspectJ 0.4Clear separation of crosscuts and crosscut actions
makes it possible to define extensible library
aspects.
AspectJ 0.3First all Java implementation, also includes many
small language improvements.
AspectJ 0.2General-purpose support for crosscutting. Users could
program any kind of aspects, not just coordination. This
release dropped COOL.
AspectJ 0.1A single domain-specific aspect language, called COOL,
for programming coordination in multi-threaded
programs.
More details for 1.0 and earlier releases are available in
changes.html.
What is the AspectJ development schedule?
Below is a table describing the goals for the major releases.
For information about specific features, search the bug database
for RFE's ("requests for enhancement") by
selecting severity of "enhancement".
Like many open-source projects, we don't make or promise
schedules, but we do follow a pattern of issuing preview releases
which can give observers an idea of when
a particular release might be available.
The AspectJ Development ScheduleVersionDescription1.0Final syntax and semantic changes. Standalone structure
browser. Complete documentation.
1.1Faster incremental compilation, bytecode weaving,
and a small number of language changes.1.2Faster weaving, -inpath option, better error messages,
better handling of binary input and resources
during incremental compilation, faster runtime
1.5 (AspectJ 5)Support for Java 1.5, generic aspects,
annotations, etc. Integrates AspectWerkz-style
load-time weaving.
Will AspectJ support Java 5?
Yes. Java 5 is supported in AspectJ 5.
AspectJ is a registered trademark of Palo Alto Research Center, Incorporated (PARC),
used with permission.
Java and all Java-based marks are trademarks or registered trademarks of
Sun Microsystems, Inc. in the United States and other countries. All other
trademarks are the property of their respective owners.