This tutorial consists of the attendees solving AspectJ programming tasks (with unit tests) in pairs or triples and discussing the answers with the group and with the tutorial presenters. The exercises work with a figure editor together with JUnit test cases. They progress, as most users do in their adoption of AspectJ, from non-functional, development-only aspects to aspects which augment a deployed program with crosscutting features.
These notes consist of four sections of exercises, a quick reference to AspectJ syntax, and a UML diagram of a figure editor program. While you should feel free to have a quick look through these notes before the tutorial, please do not try to seriously read or do the exercises; you'll learn a lot more by working through it in groups.
At the beginning of the tutorial we will make available a binary
package that includes the tests, the base code, JUnit, and a
distribution of AspectJ. All it needs is information about where Java
lives (so set your JAVA_HOME environment variable). It assumes that
you unzip it in c:\ (on Windows) or in your home directory (on Linux):
If you put it somewhere else, edit setpaths or setpaths.bat, as
appropriate. Once all this is done, run setpaths.bat
or
source setpaths
to export some other needed environment
variables.
All the files in the program are listed in base.lst, including
test cases and an empty answer aspect,
answers/Answer.java
. Therefore, if you write your
answers there, all you need to do is compile base.lst, either in an
IDE or with
$ ajc -argfile base.lst
Before you move onto another exercise, though, make sure to copy your answer into a different file so we can discuss the answers together:
> copy answers/Answer.java answers/2a.java (Windows) $ cp answers/Answer.java answers/2a.java (Unix)
After building the system, you should invoke Java on the compiled test class. On the command-line, this this would be
$ java tests.Test2a
(For these exercises, when we give examples of execution we will show the command-line use, but of course if you are using JBuilder, Forte/NetBeans, Emacs, or Eclipse, use the appropriate compile and execute tools.)
The default test, tests.Test
, performs some
rudimentary tests on figure elements, and so is a useful test to run
periodically. Looking at the JUnit tests for each exercise may also
be helpful.
Again, we will be looking at some solutions and having discussion, which is much more difficult without incremental solutions. So when you go from one exercise to the next, make sure to save your work in a file and go on to work in a different file, even if you plan to duplicate some code.
You may use whatever editor or environment you choose to work through these exercises. We provide a simple code-browser that can work well as an editor for these short exercises, in addition to providing better visualization of how aspects affect the system:
$ ajbrowser base.lst
With the browser you can edit code (including the
answers/Answer.java
file), and after saving hit the build
button to start an ajc compile. We recommend you start up another
shell, though, to run the JUnit tests (and don't forget to run the
setpaths
script when you open the new shell): You could
set up the run button to run a test through the Options menu, but
we've found this is fairly cumbersome.
The easiest way to get started with AspectJ is to use it to enforce static invariants.
Sample Exercise: The main point of this exercise is to make sure your configuration works. Type in the answer below into your answer file, make sure you get the desired compile-time error, remove the offending line, and make sure you pass the JUnit test.
Task: Signal an error for calls to
System.out.println
.
The way that we are all taught to print "hello world" from Java is
to use System.out.println()
, so that is what we typically
use for one-off debugging traces. It's a common mistake to leave
these in your system longer than is necessary. Type in the aspect
below to force an error at compile time if this mistake is made.
When you use this on the given system, you'll find one incorrect
trace in SlothfulPoint
.
$ ajc -argfile base.lst ./figures/SlothfulPoint.java:38 illegal access to System.out 1 error
Remove the illegal tracing call.
Make sure your program still passes the JUnit test
tests.Test
(which it should also pass at the beginning of
all exercises) before continuing.
$ java tests.Test .... Time: 0.03 OK (4 tests)
Answer:
package answers; import figures.*; aspect Answer1a { declare error : get(java.io.PrintStream System.out) && within(figures..*) : "illegal access to System.out"; }
Note that this answer does not say that the call to the
println()
method is incorrect, rather, that the field get
of the out
field is illegal. This will also catch those
users who bind System.out to a static field to save typing.
Task: Signal a warning for assignments outside of setter methods.
Tools: set
, withincode
,
signature void set*(..)
One common coding convention is that no private field should be set outside of setter methods. Write an aspect to warn at compile time when such an illegal assignment expression exists.
This is going to look like
aspect A { declare warning: <pointcut here> : "bad field set"; }
where the pointcut picks out join points of private field sets outside of setter methods. "Outside", here, means that the code for the assignment is outside the text of the setter.
Make sure your program still passes the JUnit test
tests.Test
before continuing. Make sure you get 11
warnings from this. Wait to fix them until the next exercise.
Task: Allow assignmnents inside of constructors.
Tools: signature new(..)
Look at some of the warnings from the previous exercise. Notice that a lot of them are from within constructors. Actually, the common coding convention is that no private field should be set outside of setter methods or constructors. Modify your answer to signal an actual error at compile time (rather than just a warning) when such an illegal assignment expression exists.
You'll want to add another withincode
primitive
pointcut to deal with the constructors.
After you specify your pointcut correctly, you'll still find that the convention is violated twice in the figures package. You should see the following two errors:
.\figures\Point.java:37 bad field set .\figures\Point.java:38 bad field set 2 errors
Rewrite these two occurrences so as not to violate
the convention. Make sure your program still passes the JUnit test
tests.Test
before continuing.
You've taken your first steps. At this point, check the people to your left and right. If they're stuck somewhere, see if you can help them.
The next step in AspectJ adoption is often to augment a test suite by including additional dynamic tests.
Tutorial attendees typically progress at different speeds through these questions. Throughout this tutorial, if you finish early, see what the people around you are doing and if they need help. Feel free to help them out of naked self-interest; we promise you'll learn a lot about AspectJ by explaining it.
Sample Exercise: We've provided the answer to this exercise below to get you started.
Task: Pass tests.Test2a
.
Tools: args
, before
Write an aspect to throw an IllegalArgumentException
whenever an attempt is made to set one of Point
's
int
fields to a value that is less than zero.
This should make the test case of tests.Test2a
pass,
which wouldn't without your aspect. So before compiling in the
aspect,
$ java tests.Test2a .F..F.... Time: 0.04 There were 2 failures: 1) testTooSmall(tests.Test2a)junit.framework.AssertionFailedError: should have thrown IllegalArgumentException 2) testMove(tests.Test2a)junit.framework.AssertionFailedError: should have thrown IllegalArgumentException FAILURES!!! Tests run: 7, Failures: 2, Errors: 0
But after compiling in the aspect...
$ ajc -argfile base.lst answers/Answer.java $ java tests.Test2a ....... Time: 0.04 OK (7 tests)
Answer:
package answers; import figures.*; aspect Answer2a { before(int newValue): set(int Point.*) && args(newValue) { if (newValue < 0) { throw new IllegalArgumentException("too small"); } } }
Task: Pass tests.Test2b
.
Tools: call
.
Group
is a FigureElement
class that
encapsulates groups of other figure elements. As such, only actual
figure element objects should be added to Group
objects.
Write an aspect to throw an IllegalArgumentException
whenever Group.add()
is called with a null
value.
Look at tests/Test2b.java
to see exactly what we're
testing for.
Task: Pass tests.Test2c
.
Tools: target
Another constraint on a well-formed group is that it should not
contain itself as a member (though it may contain other groups). Write
an aspect to throw an IllegalArgumentException
whenever
an attempt is made to call Group.add()
on a
null
value, or on the group itself.
You will want to use a target
pointcut to expose the
Group
object that is the target of the add
call.
Task: Pass tests.Test2d
.
Tools: around advice
Instead of throwing an exception when one of Point
's
int
fields are set to a negative value, write an aspect
to trim the value to zero. You'll want to use around
advice that exposes the new value of the field assignment with an
args
pointcut, and proceed
with the trimmed
value.
This is going to look something like
aspect A { void around(int val): <Pointcut> { <Do something with val> proceed(val); } }
Task: Pass tests.Test2e
Tools: around advice
A postcondition of a Point
's move
operation is that the Point
's coordinates should change.
If a call to move didn't actually move a point by the desired
offset, then the point is in an illegal state and so an
IllegalStateException
should be thrown.
Note that because we're dealing with how the coordinates change during move, we need some way of getting access to the coordinates both before and after the move, in one piece of advice.
Task: Pass tests.Test2f
Tools: the Rectangle(Rectangle)
constructor, the Rectangle.translate(int, int)
method.
FigureElement
objects have a getBounds()
method that returns a java.awt.Rectangle
representing the
bounds of the object. An important postcondition of the general
move
operation on a figure element is that the figure
element's bounds rectangle should move by the same amount as the
figure itself. Write an aspect to check for this postcondition --
throw an IllegalStateException
if it is violated.
Tracing is one of the classic AspectJ applications, and is often the first where AspectJ is used on deployed code.
Task: Pass tests.Test3a
.
Tools: Log.log(String)
,
thisJoinPoint.toString()
, execution
,
within
Write an aspect to log the execution of all public methods
in the figures package. To do this, use the utility class
Log
(with an import from support.Log
)
and call Log.log(String)
Task: Pass tests.Test3b
.
Tools: target
AspectJ can expose the target object at a join point for tracing. In this exercise, you will print not only the join point information, but also the target object, with the form
thisJoinPointInfo at targetObject
Task: Pass tests.Test3c
.
Tools: args
.
Write an aspect to log whenever a Point
is added to a
group. The args
pointcut allows you to select join points
based on the type of a parameter to a method call.
Look at the test case for details about the tested log message.
Task: Pass tests.Test3d
.
Tools: inter-type field declaration
Make sure that a Point is never added to more than one Group. To do so, associate a boolean flag with each Point using an inter-type declaration, such as
boolean Point.hasBeenAdded = false;
Check and set this flag with the same kind of advice from your
answer to problem (c). Throw an IllegalStateException
if
the point has already been added.
Task: Pass tests.Test3e
.
Tools:
Extend your solution to problem (d) by using the string
representation of the Point's containing group as the msg
part of the IllegalStateException
.
Computation of the bounding box of Group
objects
needs to deal with all aggregate parts of the group, and this
computation can be expensive. In this section, we will explore
various ways of reducing this expense.
Optional: In all of these exercises, you should only deal with points that are added directly to Groups, rather than those that are added "indirectly" through Lines and Boxes. You should handle those points contained in Lines and Boxes only if time permits.
Task: Pass tests.Test4a
.
Tools: around
,
FigureElement.MAX_BOUNDS
Group
's getBounds()
method could be
understood to be a conservative approximation of the bounding box of a
group. If that is true, then it would be a legal (and much faster)
implementation of getBounds()
to simply always return a
rectangle consisting of the entire canvas. The entire canvas is returned
by the static method FigureElement.MAX_BOUNDS
.
Write an aspect to implement this change. You can override
Group
's getBounds()
method entirely with
around advice intercepting the method.
Task: Pass tests.Test4b
.
Tools: inter-type field.
Instead of making the (very) conservative approximation of
getBounds()
from part (a), write an aspect instead that
remembers the return value from the first time
getBounds()
has been called on a Group
, and
returns that first Rectangle
for every subsequent
call.
Hint: You can use an inter-type declaration to keep some
state for every Group
object.
Task: Pass tests.Test4c
.
Tools: before
While caching in this way does save computation, it will lead to
incorrect bounding boxes if a Group
is ever moved.
Change your aspect so that it invalidates the cache whenever the
move()
method of Group
is called.
Task: Pass tests.Test4d
.
Of course, part (c) didn't really solve the problem. What if a
Point
that is part of a Group
moves?
Whenever either of a Point's fields are set it should invalidate the
caches of all enclosing groups. Use your solution to problem 3c to
modify your invalidation criteria in this way, but note that this is
slightly different than the problem in 3c: Here you care about fields,
where there you cared about method calls.
Task: Pass tests.Test4e
.
Tools: You're on you're own
Did you really do part (d) correctly? Run the JUnit test
tests.Test4e
to see. If you pass, congratulations, now
go help other people. Otherwise, you have fallen prey to our cruel
trap: Remember that whenever a point moves it should invalidate the
caches of all enclosing groups.