Hands-on Programming with AspectJ®

Matt Chapman
Matthew Webster
Mik Kersten


In this tutorial you will solve some basic programming tasks using AspectJ. The tasks progress from writing non-functional, development-only aspects to writing aspects that augment a deployed program with crosscutting features. This follows the same progression most users see in their own adoption of AspectJ.

Since this is a hands-on tutorial, you will be working with a live AspectJ distribution. The example code we will be working with is a simple figure editor, along with JUnit tests for each exercise. We will break up into groups of two to three people per computer to foster discussion within the group as well as with the presenters.

If you have a laptop running a recent version of Windows, MacOS or Linux, feel free to bring it along. We will provide CDs and other installation media for a standalone AspectJ system, including the figure editor code these exercises are based on and unit tests for the exercises. If you don't have a laptop with you, don't worry about it.

These notes consist of four sections of exercises, a quick reference to AspectJ syntax, and a UML diagram of a figure editor program.

If you receive these tutorial notes early, feel free to have a quick look, especially at the UML diagram and quick reference. But you'll be cheating yourself if you try to do the exercises early; you'll learn a lot more by working through it in groups during the tutorial proper.

AspectJ Development in Eclipse

The AspectJ Development Tools (AJDT) plugins for Eclipse provide support for using the AspectJ compiler in the Eclipse environment. Different versions of AJDT are available to support both the latest release version of Eclipse, and the latest milestone build. As part of the tutorial we will be providing distributions of base eclipse, the AJDT plugins and the exercises. Should you wish to install the exercises into an existing configuration on your machine, you can download the exercises plugin via the following URL:


Simply copy this JAR file to your eclipse/plugins folder.


The key components to have installed are:

The best way to verify your installation is by attempting to create one of the 4 tutorial projects. To do this, open File>New>Other... as shown here (or press the New button on the toolbar):

You should then see the following option:

If you do, congratulations, you are ready to go !!

1. Static Invariants

The easiest way to get started with AspectJ is to use it to enforce static invariants, also known as compile time checks. These can be as simple as verifying a particular API is never called or as sophisticated as verifying your layered architecture is adhered to.

The tutorial plugin includes four eclipse projects representing the four major parts of this tutorial. Use the menus described in the previous two screenshots to create each exercise - create the first project now, Exercise1.

1.a. Find old tracing

Sample Exercise: The main point of this exercise is to make sure your configuration works.

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 far longer than is necessary. Type in the aspect below to signal an error at compile time if this mistake is made.

This is your first aspect, in order to create the necessary Eclipse resource into which you can type the answer, use the New Aspect Wizard. You can access this wizard in two ways:

1. From the New Java Class drop-down on the toolbar:

2. From the File>New menu:

For this first aspect, fill it in as shown below:

After clicking Finish you should find yourself in the new aspect, change it to look like this following code


package answers;

import figures.*;

aspect Answer {
    declare error
        : get(java.io.PrintStream System.out) && within(figures..*)
        : "illegal access to System.out";
Note: For every task in this tutorial, you should work in the same Answer aspect that you create at the start of the exercise, comment out each previous answer as you move to the next task - if you don't then you may unexpectedly get some test failures.

After saving, a build will take place and you'll find one incorrect trace in SlothfulPoint:

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.

After successfully finding the error, edit your program to remove the illegal tracing call.

Make sure your program still passes the JUnit test tests.CoreTest (which it should also pass at the beginning of all exercises) before continuing. Every exercise in this tutorial includes tests, each of which should be run individually. To run a particular test, locate it in the test-src folder, then, right click the test and navigate to the Run>JUnit Test menu option as shown here:

If it runs OK, you should get the trusty green bar:

When it passes, you are ready to move on!

1.b. Mandate setters

Task: Signal a warning for assignments outside of setter methods.

Tools: set, withincode, the void set*(..) pattern

One common coding convention is that no private field should be assigned to outside of setter methods. Write an aspect to signal a warning at compile time for these illegal assignment expressions.

This is going to look like

aspect Answer {
    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.CoreTest before continuing. Make sure you get eleven warnings from this:

Wait to fix them until the next exercise.

1.c. Refine setters mandate

Task: Allow assignmnents inside of constructors.

Tools: the new(..) pattern

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 assigned to 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

Rewrite these two occurrences so as not to violate the convention. Make sure your program still passes the JUnit test tests.CoreTest before continuing.

Congratulations, 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. Try to resist moving on to the next section until we discuss solutions as a group.

2. Dynamic invariants

The next step in AspectJ adoption is often to augment a test suite by including additional dynamic tests, sometimes called runtime checks.

Tutorial attendees typically progress at different speeds through these exercises. Throughout this tutorial, if you finish early, see what the people around you are doing and if they need help. Don't help them out of charity, help them out of naked self-interest—we promise you'll learn a lot about using AspectJ by explaining it.

2.a. Check a simple precondition

Sample Exercise: We've provided the answer to this exercise to get you started. Feel free to think a bit, but don't get stuck on this one.

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:

But after compiling in the aspect...


package answers;

import figures.*;

aspect Answer {
    before(int newValue): set(int Point.*) && args(newValue) {
        if (newValue < 0) {
            throw new IllegalArgumentException("too small");

2.b. Check another precondition

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.

2.c. Check yet another precondition

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.

2.d. Assure input

Task: Pass tests.Test2d.

Tools: around advice

Instead of throwing an exception when one of Point's int fields is 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 Answer {
    void around(int val): <Pointcut> {
        <Do something with val>

2.e. Check a postcondition

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.

2.f. Check another postcondition

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.

3. Tracing

Tracing is one of the classic AspectJ applications, and is often the first where AspectJ is used on deployed code.

3.a. Simple tracing

Task: Pass tests.Test3a.

Tools: Log.write(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 (this is in the support package, so remember to import it into your answer aspect). Write a message into the log with the static write(String) method.

3.b. Exposing a value

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

3.c. More specialized logging

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 to see the trace message we expect you to write in the log.

3.d. Logging extended to checking an invariant

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.

3.e. Better error messages for 3.d.

Task: Pass tests.Test3e.

Extend your solution to problem (d) by using the string representation of the Point's containing group as the msg part of the IllegalStateException.

4. Caching

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.

4.a. Make a constant override

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 field FigureElement.MAX_BOUNDS.

Write an aspect to implement this change. You can override Group's getBounds() method entirely with around advice intercepting the method.

4.b. Make a constant cache

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.

4.c. Invalidate, part 1

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.

4.d. Invalidate, part 2

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.

4.e. Invalidate, part 3

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.

Congratulations! Not only have you learned about how to program in AspectJ, you have worked through exercises paralleling a common AspectJ adoption strategy. You should be able to pick up AspectJ and use it to improve your own software's crosscutting modularity.

You can find the current binaries, source, documentation and an active user community for AspectJ at