The AspectJ Language
Introduction
The previous chapter, , was a brief
overview of the AspectJ language. You should read this chapter to
understand AspectJ's syntax and semantics. It covers the same
material as the previous chapter, but more completely and in much
more detail.
We will start out by looking at an example aspect that we'll build
out of a pointcut, an introduction, and two pieces of advice. This
example aspect will gives us something concrete to talk about.
The Anatomy of an Aspect
This lesson explains the parts of AspectJ's aspects. By reading this
lesson you will have an overview of what's in an aspect and you will
be exposed to the new terminology introduced by AspectJ.
An Example Aspect
Here's an example of an aspect definition in AspectJ:
The FaultHandler consists of one inter-type
field on Server (line 03), two methods (lines
05-07 and 09-11), one pointcut definition (line 13), and two pieces
of advice (lines 15-17 and 19-22).
This covers the basics of what aspects can contain. In general,
aspects consist of an association of other program entities,
ordinary variables and methods, pointcut definitions, inter-type declarations,
and advice, where advice may be before, after or around advice. The
remainder of this lesson focuses on those crosscut-related
constructs.
Pointcuts
AspectJ's pointcut definitions give names to pointcuts. Pointcuts
themselves pick out join points, i.e. interesting points in the
execution of a program. These join points can be method or
constructor invocations and executions, the handling of exceptions,
field assignments and accesses, etc. Take, for example, the
pointcut definition in line 13:
This pointcut, named services, picks out those
points in the execution of the program when
Server objects have their public methods called.
It also allows anyone using the services
pointcut to access the Server object whose
method is being called.
The idea behind this pointcut in the
FaultHandler aspect is that
fault-handling-related behavior must be triggered on the calls to
public methods. For example, the server may be unable to proceed
with the request because of some fault. The calls of those methods
are, therefore, interesting events for this aspect, in the sense
that certain fault-related things will happen when these events
occur.
Part of the context in which the events occur is exposed by the
formal parameters of the pointcut. In this case, that consists of
objects of type Server. That formal parameter
is then being used on the right hand side of the declaration in
order to identify which events the pointcut refers to. In this
case, a pointcut picking out join points where a Server is the
target of some operation (target(s)) is being composed
(, meaning and) with a pointcut
picking out call join points (call(...)). The calls are identified
by signatures that can include wild cards. In this case, there are
wild cards in the return type position (first *), in the name
position (second *) and in the argument list position (..); the
only concrete information is given by the qualifier
public.
Pointcuts pick out arbitrarily large numbers of join points of a
program. But they pick out only a small number of
kinds of join points. Those kinds of join
points correspond to some of the most important concepts in
Java. Here is an incomplete list: method call, method execution,
exception handling, instantiation, constructor execution, and
field access. Each kind of join point can be picked out by its
own specialized pointcut that you will learn about in other parts
of this guide.
Advice
A piece of advice brings together a pointcut and a body of code to
define aspect implementation that runs at join points picked out by
the pointcut. For example, the advice in lines 15-17 specifies that
the following piece of code
is executed when instances of the Server class
have their public methods called, as specified by the pointcut
services. More specifically, it runs when those
calls are made, just before the corresponding methods are executed.
The advice in lines 19-22 defines another piece of implementation
that is executed on the same pointcut:
But this second method executes after those operations throw
exception of type FaultException.
There are two other variations of after advice: upon successful
return and upon return, either successful or with an exception.
There is also a third kind of advice called around. You will see
those in other parts of this guide.
Join Points and Pointcuts
Consider the following Java class:
In order to get an intuitive understanding of AspectJ's join points
and pointcuts, let's go back to some of the basic principles of
Java. Consider the following a method declaration in class Point:
This piece of program says that that when method named
setX with an int argument
called on an object of type Point, then the method
body { this.x = x; } is executed. Similarly, the
constructor of the class states that when an object of type
Point is instantiated through a constructor with
two int arguments, then the constructor body
{ this.x = x; this.y = y; } is executed.
One pattern that emerges from these descriptions is
When something happens, then something gets executed.
In object-oriented programs, there are several kinds of "things that
happen" that are determined by the language. We call these the join
points of Java. Join points consist of things like method calls,
method executions, object instantiations, constructor executions,
field references and handler executions. (See the for a complete listing.)
Pointcuts pick out these join points. For example, the pointcut
picks out each call to setX(int) or
setY(int) when called on an instance of
Point. Here's another example:
This pointcut picks out each the join point when exceptions of type
IOException are handled inside the code defined by
class MyClass.
Pointcut definitions consist of a left-hand side and a right-hand side,
separated by a colon. The left-hand side consists of the pointcut name
and the pointcut parameters (i.e. the data available when the events
happen). The right-hand side consists of the pointcut itself.
Some Example Pointcuts
Here are examples of pointcuts picking out
when a particular method body executes
execution(void Point.setX(int))
when a method is called
call(void Point.setX(int))
when an exception handler executes
handler(ArrayOutOfBoundsException)
when the object currently executing
(i.e. this) is of type
SomeType
this(SomeType)
when the target object is of type SomeType
target(SomeType)
when the executing code belongs to
class MyClass
within(MyClass)
when the join point is in the control flow of a call to a
Test's no-argument main
method
cflow(call(void Test.main()))
Pointcuts compose through the operations or
("||"), and
("") and not
("!").
It is possible to use wildcards. So
execution(* *(..))
call(* set(..))
means (1) the execution of any method regardless of return or
parameter types, and (2) the call to any method named
set regardless of return or parameter types
-- in case of overloading there may be more than one such
set method; this pointcut picks out calls to
all of them.
You can select elements based on types. For example,
execution(int *())
call(* setY(long))
call(* Point.setY(int))
call(*.new(int, int))
means (1) the execution of any method with no parameters that
returns an int, (2) the call to any
setY method that takes a
long as an argument, regardless of return
type or declaring type, (3) the call to any of
Point's setY methods that
take an int as an argument, regardless of
return type, and (4) the call to any classes' constructor, so
long as it takes exactly two ints as
arguments.
You can compose pointcuts. For example,
target(Point) call(int *())
call(* *(..)) (within(Line) || within(Point))
within(*) execution(*.new(int))
!this(Point) call(int *(..))
means (1) any call to an int method with no
arguments on an instance of Point,
regardless of its name, (2) any call to any method where the
call is made from the code in Point's or
Line's type declaration, (3) the execution of
any constructor taking exactly one int
argument, regardless of where the call is made from, and
(4) any method call to an int method when
the executing object is any type except Point.
You can select methods and constructors based on their
modifiers and on negations of modifiers. For example, you can
say:
call(public * *(..))
execution(!static * *(..))
execution(public !static * *(..))
which means (1) any call to a public method, (2) any
execution of a non-static method, and (3) any execution of a
public, non-static method.
Pointcuts can also deal with interfaces. For example, given the
interface
the pointcut call(* MyInterface.*(..)) picks
out any call to a method in MyInterface's
signature -- that is, any method defined by
MyInterface or inherited by one of its a
supertypes.
call vs. execution
When methods and constructors run, there are two interesting times
associated with them. That is when they are called, and when they
actually execute.
AspectJ exposes these times as call and execution join points,
respectively, and allows them to be picked out specifically by
call and execution pointcuts.
So what's the difference between these join points? Well, there are a
number of differences:
Firstly, the lexical pointcut declarations
within and withincode match
differently. At a call join point, the enclosing code is that of
the call site. This means that call(void m())
withincode(void m()) will only capture
directly recursive calls, for example. At an execution join point,
however, the program is already executing the method, so the
enclosing code is the method itself: execution(void m())
withincode(void m()) is the same as
execution(void m()).
Secondly, the call join point does not capture super calls to
non-static methods. This is because such super calls are different in
Java, since they don't behave via dynamic dispatch like other calls to
non-static methods.
The rule of thumb is that if you want to pick a join point that
runs when an actual piece of code runs (as is often the case for
tracing), use execution, but if you want to pick
one that runs when a particular signature is
called (as is often the case for production aspects), use
call.
Pointcut composition
Pointcuts are put together with the operators and (spelled
&&), or (spelled ||),
and not (spelled !). This allows the creation
of very powerful pointcuts from the simple building blocks of
primitive pointcuts. This composition can be somewhat confusing
when used with primitive pointcuts like cflow
and cflowbelow. Here's an example:
cflow(P) picks out
each join point in the control flow of the join points picked out
by P. So, pictorially:
P ---------------------
\
\ cflow of P
\
What does cflow(P) &&
cflow(Q) pick out? Well, it
picks out each join point that is in both the control flow of
P and in the control flow of
Q. So...
P ---------------------
\
\ cflow of P
\
\
\
Q -------------\-------
\ \
\ cflow of Q \ cflow(P) && cflow(Q)
\ \
Note that P and
Q might not have any join points in
common... but their control flows might have join points in common.
But what does cflow(P
&& Q) mean? Well, it
means the control flow of those join points that are both picked
out by P and picked out by
Q.
P && Q -------------------
\
\ cflow of (P && Q)
\
and if there are no join points that are both
picked by P and picked out by
Q, then there's no chance that there are
any join points in the control flow of
(P &&
Q).
Here's some code that expresses this.
The !within(A)
pointcut above is required to avoid the printPC
pointcut applying to the System.out.println
call in the advice body. If this was not present a recursive call
would result as the pointcut would apply to it's own advice.
(See for more details.)
Pointcut Parameters
Consider again the first pointcut definition in this chapter:
As we've seen, this pointcut picks out each call to
setX(int) or setY(int)
methods where the target is an instance of
Point. The pointcut is given the name
setters and no parameters on the left-hand
side. An empty parameter list means that none of the context from
the join points is published from this pointcut. But consider
another version of version of this pointcut definition:
This version picks out exactly the same join points. But in this
version, the pointcut has one parameter of type
Point. This means that any advice that uses this
pointcut has access to a Point from each join
point picked out by the pointcut. Inside the pointcut definition
this Point is named p is
available, and according to the right-hand side of the definition,
that Point p comes from the
target of each matched join point.
Here's another example that illustrates the flexible mechanism for
defining pointcut parameters:
This pointcut also has a parameter of type
Point. Similar to the
setters pointcut, this means that anyone using
this pointcut has access to a Point from each
join point. But in this case, looking at the right-hand side we
find that the object named in the parameters is not the target
Point object that receives the call; it's the
argument (also of type Point) passed to the
equals method when some other
Point is the target. If we wanted access to both
Points, then the pointcut definition that would
expose target Point p1 and argument
Point p2 would be
Let's look at another variation of the setters pointcut:
In this case, a Point object and an
int value are exposed by the named
pointcut. Looking at the the right-hand side of the definition, we
find that the Point object is the target object,
and the int value is the called method's
argument.
The use of pointcut parameters is relatively flexible. The most
important rule is that all the pointcut parameters must be bound at
every join point picked out by the pointcut. So, for example, the
following pointcut definition will result in a compilation error:
because p1 is only bound when calling
setX, and p2 is only bound
when calling setY, but the pointcut picks out
all of these join points and tries to bind both
p1 and p2.
Example: <literal>HandleLiveness</literal>
The example below consists of two object classes (plus an exception
class) and one aspect. Handle objects delegate their public,
non-static operations to their Partner
objects. The aspect HandleLiveness ensures that,
before the delegations, the partner exists and is alive, or else it
throws an exception.
Advice
Advice defines pieces of aspect implementation that execute at
well-defined points in the execution of the program. Those points can
be given either by named pointcuts (like the ones you've seen above)
or by anonymous pointcuts. Here is an example of an advice on a named
pointcut:
And here is exactly the same example, but using an anonymous
pointcut:
Here are examples of the different advice:
This before advice runs just before the join points picked out by the
(anonymous) pointcut:
This after advice runs just after each join point picked out by the
(anonymous) pointcut, regardless of whether it returns normally or throws
an exception:
This after returning advice runs just after each join point picked
out by the (anonymous) pointcut, but only if it returns normally.
The return value can be accessed, and is named x
here. After the advice runs, the return value is returned:
This after throwing advice runs just after each join point picked out by
the (anonymous) pointcut, but only when it throws an exception of type
Exception. Here the exception value can be accessed
with the name e. The advice re-raises the exception
after it's done:
This around advice traps the execution of the join point; it runs
instead of the join point. The original action
associated with the join point can be invoked through the special
proceed call:
Inter-type declarations
Aspects can declare members (fields, methods, and constructors) that
are owned by other types. These are called inter-type members.
Aspects can also declare that other types implement new interfaces or
extend a new class. Here are examples of some such inter-type
declarations:
This declares that each Server has a
boolean field named disabled,
initialized to false:
It is declared private, which means that it is
private to the aspect: only code in the aspect
can see the field. And even if Server has
another private field named disabled (declared in
Server or in another aspect) there won't be a name
collision, since no reference to disabled will be
ambiguous.
This declares that each Point has an
int method named getX with no
arguments that returns whatever this.x is:
Inside the body, this is the
Point object currently executing. Because the
method is publically declared any code can call it, but if there is
some other Point.getX() declared there will be a
compile-time conflict.
This publically declares a two-argument constructor for
Point:
This publicly declares that each Point has an
int field named x, initialized
to zero:
Because this is publically declared, it is an error if
Point already has a field named
x (defined by Point or by
another aspect).
This declares that the Point class implements the
Comparable interface:
Of course, this will be an error unless Point
defines the methods required by Comparable.
This declares that the Point class extends the
GeometricObject class.
An aspect can have several inter-type declarations. For example, the
following declarations
publicly declare that Point has both a String field
name and a void method
setName(String) (which refers to the
name field declared by the aspect).
An inter-type member can only have one target type, but often you may
wish to declare the same member on more than one type. This can be
done by using an inter-type member in combination with a private
interface:
This declares a marker interface HasName, and also declares that any
type that is either Point,
Line, or Square implements that
interface. It also privately declares that all HasName
object have a String field called
name, and publically declares that all
HasName objects have a String
method getName() (which refers to the privately
declared name field).
As you can see from the above example, an aspect can declare that
interfaces have fields and methods, even non-constant fields and
methods with bodies.
Inter-type Scope
AspectJ allows private and package-protected (default) inter-type declarations in
addition to public inter-type declarations. Private means private in
relation to the aspect, not necessarily the target type. So, if an
aspect makes a private inter-type declaration of a field
Then code in the aspect can refer to Foo's
x field, but nobody else can. Similarly, if an
aspect makes a package-protected introduction,
then everything in the aspect's package (which may or may not be
Foo's package) can access x.
Example: <literal>PointAssertions</literal>
The example below consists of one class and one aspect. The aspect
privately declares the assertion methods of
Point, assertX and
assertY. It also guards calls to
setX and setY with calls to
these assertion methods. The assertion methods are declared
privately because other parts of the program (including the code in
Point) have no business accessing the assert
methods. Only the code inside of the aspect can call those
methods.
= 0);
}
private boolean Point.assertY(int y) {
return (y <= 100 && y >= 0);
}
before(Point p, int x): target(p) && args(x) && call(void setX(int)) {
if (!p.assertX(x)) {
System.out.println("Illegal value for x"); return;
}
}
before(Point p, int y): target(p) && args(y) && call(void setY(int)) {
if (!p.assertY(y)) {
System.out.println("Illegal value for y"); return;
}
}
}
]]>
thisJoinPoint
AspectJ provides a special reference variable,
thisJoinPoint, that contains reflective
information about the current join point for the advice to use. The
thisJoinPoint variable can only be used in the
context of advice, just like this can only be used
in the context of non-static methods and variable initializers. In
advice, thisJoinPoint is an object of type org.aspectj.lang.JoinPoint.
One way to use it is simply to print it out. Like all Java objects,
thisJoinPoint has a toString()
method that makes quick-and-dirty tracing easy:
The type of thisJoinPoint includes a rich
reflective class hierarchy of signatures, and can be used to access
both static and dynamic information about join points such as the
arguments of the join point:
In addition, it holds an object consisting of all the static
information about the join point such as corresponding line number
and static signature:
If you only need the static information about the join point, you may
access the static part of the join point directly with the special
variable thisJoinPointStaticPart. Using
thisJoinPointStaticPart will avoid the run-time
creation of the join point object that may be necessary when using
thisJoinPoint directly.
It is always the case that
One more reflective variable is available:
thisEnclosingJoinPointStaticPart. This, like
thisJoinPointStaticPart, only holds the static
part of a join point, but it is not the current but the enclosing
join point. So, for example, it is possible to print out the calling
source location (if available) with