# Hands-on Programming with AspectJ®

Erik Hilsdale
Mik Kersten
http://www.eclipse.com/aspectj

## Overview

In this tutorial you will solve some canonical 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.

### Command-line usage

While the AspectJ system is well integrated with a number of IDEs, it can also be used as a command-line compiler. The standalone package we provide (containing the tests, the base code, JUnit, and a distribution of AspectJ) needs 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.

Each time you open a new shell window run `setpaths.bat` or `source setpaths` to export some other needed environment variables.

In general, 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

The default test, `tests.CoreTest`, performs some rudimentary tests on figure elements, and so is a useful test to run periodically. You should also look at the JUnit tests for each exercise as you do it.

Again, we will be looking at solutions and having discussion, which is much more difficult without incremental solutions. So when you go from one exercise to the next, save your work in a file before going on to the next exercise even if you plan to duplicate some code.

When we give examples of execution in these exercises we will show the command-line use, but of course you should use the appropriate compile and execute tools if you are using the AspectJ browser, Emacs, or Eclipse.

## 1. Static Invariants

The easiest way to get started with AspectJ is to use it to enforce static invariants.

### 1.a. Find old tracing

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

Answer:

    package answers;

    import figures.*;

    aspect Answer1a {
        declare error
            : get(java.io.PrintStream System.out) && within(figures..*)
            : "illegal access to System.out";
    }

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

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 you have successfully used this aspect, 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.

    $ java tests.CoreTest
    ....
    Time: 0.03

    OK (4 tests)

### 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 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.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

    2 errors

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.

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,

    $ 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");
            }
        }
    }

### 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 A {
        void around(int val): <Pointcut> {
            <Do something with val>
            proceed(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 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.

### 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

> http://www.eclipse.org/aspectj