The AspectJ^TM Programming Guide

A critical element in the design of any aspect-oriented language is the join point model. The join point model provides the common frame of reference that makes it possible to define the dynamic structure of crosscutting concerns. This chapter describes AspectJ's dynamic join points, in which join points are certain well-defined points in the execution of the program.

AspectJ provides for many kinds of join points, but this chapter discusses only one of them: method call join points. A method call join point encompasses the actions of an object receiving a method call. It includes all the actions that comprise a method call, starting after all arguments are evaluated up to and including return (either normally or by throwing an exception).

Each method call at runtime is a different join point, even if it comes from the same call expression in the program. Many other join points may run while a method call join point is executing -- all the join points that happen while executing the method body, and in those methods called from the body. We say that these join points execute in the dynamic context of the original call join point.

In AspectJ, pointcuts pick out certain join points in the program flow. For example, the pointcut

call(void Point.setX(int))

picks out each join point that is a call to a method that has the signature void Point.setX(int) — that is, Point's void setX method with a single int parameter.

A pointcut can be built out of other pointcuts with and, or, and not (spelled &&, ||, and !). For example:

call(void Point.setX(int)) ||
call(void Point.setY(int))

picks out each join point that is either a call to setX or a call to setY.

Pointcuts can identify join points from many different types — in other words, they can crosscut types. For example,

call(void FigureElement.setXY(int,int)) ||
call(void Point.setX(int))              ||
call(void Point.setY(int))              ||
call(void Line.setP1(Point))            ||
call(void Line.setP2(Point));

picks out each join point that is a call to one of five methods (the first of which is an interface method, by the way).

In our example system, this pointcut captures all the join points when a FigureElement moves. While this is a useful way to specify this crosscutting concern, it is a bit of a mouthful. So AspectJ allows programmers to define their own named pointcuts with the pointcut form. So the following declares a new, named pointcut:

pointcut move():
    call(void FigureElement.setXY(int,int)) ||
    call(void Point.setX(int))              ||
    call(void Point.setY(int))              ||
    call(void Line.setP1(Point))            ||
    call(void Line.setP2(Point));

and whenever this definition is visible, the programmer can simply use move() to capture this complicated pointcut.

The previous pointcuts are all based on explicit enumeration of a set of method signatures. We sometimes call this name-based crosscutting. AspectJ also provides mechanisms that enable specifying a pointcut in terms of properties of methods other than their exact name. We call this property-based crosscutting. The simplest of these involve using wildcards in certain fields of the method signature. For example, the pointcut

call(void Figure.make*(..))

picks out each join point that's a call to a void method defined on Figure whose the name begins with "make" regardless of the method's parameters. In our system, this picks out calls to the factory methods makePoint and makeLine. The pointcut

call(public * Figure.* (..))

picks out each call to Figure's public methods.

But wildcards aren't the only properties AspectJ supports. Another pointcut, cflow, identifies join points based on whether they occur in the dynamic context of other join points. So

cflow(move())

picks out each join point that occurs in the dynamic context of the join points picked out by move(), our named pointcut defined above. So this picks out each join points that occurrs between when a move method is called and when it returns (either normally or by throwing an exception).

So pointcuts pick out join points. But they don't do anything apart from picking out join points. To actually implement crosscutting behavior, we use advice. Advice brings together a pointcut (to pick out join points) and a body of code (to run at each of those join points).

AspectJ has several different kinds of advice. Before advice runs as a join point is reached, before the program proceeds with the join point. For example, before advice on a method call join point runs before the actual method starts running, just after the arguments to the method call are evaluated.

before(): move() {
    System.out.println("about to move");
}

After advice on a particular join point runs after the program proceeds with that join point. For example, after advice on a method call join point runs after the method body has run, just before before control is returned to the caller. Because Java programs can leave a join point 'normally' or by throwing an exception, there are three kinds of after advice: after returning, after throwing, and plain after (which runs after returning or throwing, like Java's finally).

after() returning: move() {
    System.out.println("just successfully moved");
}

Around advice on a join point runs as the join point is reached, and has explicit control over whether the program proceeds with the join point. Around advice is not discussed in this section.

after(FigureElement fe, int x, int y) returning:
        ...SomePointcut... {
    ...SomeBody...
}

after(FigureElement fe, int x, int y) returning:
        ...SomePointcut... {
    System.out.println(fe + " moved to (" + x + ", " + y + ")");
}

after(FigureElement fe, int x, int y) returning:
        call(void FigureElement.setXY(int, int))
        && target(fe)
        && args(x, y) {
    System.out.println(fe + " moved to (" + x + ", " + y + ")");
}

pointcut setXY(FigureElement fe, int x, int y):
    call(void FigureElement.setXY(int, int))
    && target(fe)
    && args(x, y);

after(FigureElement fe, int x, int y) returning: setXY(fe, x, y) {
    System.out.println(fe + " moved to (" + x + ", " + y + ").");
}

Inter-type declarations in AspectJ are declarations that cut across classes and their hierarchies. They may declare members that cut across multiple classes, or change the inheritance relationship between classes. Unlike advice, which operates primarily dynamically, introduction operates statically, at compile-time.

Consider the problem of expressing a capability shared by some existing classes that are already part of a class hierarchy, i.e. they already extend a class. In Java, one creates an interface that captures this new capability, and then adds to each affected class a method that implements this interface.

AspectJ can express the concern in one place, by using inter-type declarations. The aspect declares the methods and fields that are necessary to implement the new capability, and associates the methods and fields to the existing classes.

Suppose we want to have Screen objects observe changes to Point objects, where Point is an existing class. We can implement this by writing an aspect declaring that the class Point Point has an instance field, observers, that keeps track of the Screen objects that are observing Points.

aspect PointObserving {
    private Vector Point.observers = new Vector();
    ...
}

The observers field is private, so only PointObserving can see it. So observers are added or removed with the static methods addObserver and removeObserver on the aspect.

aspect PointObserving {
    private Vector Point.observers = new Vector();

    public static void addObserver(Point p, Screen s) {
        p.observers.add(s);
    }
    public static void removeObserver(Point p, Screen s) {
        p.observers.remove(s);
    }
    ...
}

Along with this, we can define a pointcut changes that defines what we want to observe, and the after advice defines what we want to do when we observe a change.

aspect PointObserving {
    private Vector Point.observers = new Vector();

    public static void addObserver(Point p, Screen s) {
        p.observers.add(s);
    }
    public static void removeObserver(Point p, Screen s) {
        p.observers.remove(s);
    }

    pointcut changes(Point p): target(p) && call(void Point.set*(int));

    after(Point p): changes(p) {
        Iterator iter = p.observers.iterator();
        while ( iter.hasNext() ) {
            updateObserver(p, (Screen)iter.next());
        }
    }

    static void updateObserver(Point p, Screen s) {
        s.display(p);
    }
}

Note that neither Screen's nor Point's code has to be modified, and that all the changes needed to support this new capability are local to this aspect.

Aspects wrap up pointcuts, advice, and inter-type declarations in a a modular unit of crosscutting implementation. It is defined very much like a class, and can have methods, fields, and initializers in addition to the crosscutting members. Because only aspects may include these crosscutting members, the declaration of these effects is localized.

Like classes, aspects may be instantiated, but AspectJ controls how that instantiation happens -- so you can't use Java's new form to build new aspect instances. By default, each aspect is a singleton, so one aspect instance is created. This means that advice may use non-static fields of the aspect, if it needs to keep state around:

aspect Logging {
    OutputStream logStream = System.err;

    before(): move() {
        logStream.println("about to move");
    }
}

Aspects may also have more complicated rules for instantiation, but these will be described in a later chapter.

This first example shows how to increase the visibility of the internal workings of a program. It is a simple tracing aspect that prints a message at specified method calls. In our figure editor example, one such aspect might simply trace whenever points are drawn.

aspect SimpleTracing {
    pointcut tracedCall():
        call(void FigureElement.draw(GraphicsContext));

    before(): tracedCall() {
        System.out.println("Entering: " + thisJoinPoint);
    }
}

This code makes use of the thisJoinPoint special variable. Within all advice bodies this variable is bound to an object that describes the current join point. The effect of this code is to print a line like the following every time a figure element receives a draw method call:

Entering: call(void FigureElement.draw(GraphicsContext))

To understand the benefit of coding this with AspectJ consider changing the set of method calls that are traced. With AspectJ, this just requires editing the definition of the tracedCalls pointcut and recompiling. The individual methods that are traced do not need to be edited.

When debugging, programmers often invest considerable effort in figuring out a good set of trace points to use when looking for a particular kind of problem. When debugging is complete or appears to be complete it is frustrating to have to lose that investment by deleting trace statements from the code. The alternative of just commenting them out makes the code look bad, and can cause trace statements for one kind of debugging to get confused with trace statements for another kind of debugging.

With AspectJ it is easy to both preserve the work of designing a good set of trace points and disable the tracing when it isn t being used. This is done by writing an aspect specifically for that tracing mode, and removing that aspect from the compilation when it is not needed.

This ability to concisely implement and reuse debugging configurations that have proven useful in the past is a direct result of AspectJ modularizing a crosscutting design element the set of methods that are appropriate to trace when looking for a given kind of information.

Our second example shows you how to do some very specific profiling. Although many sophisticated profiling tools are available, and these can gather a variety of information and display the results in useful ways, you may sometimes want to profile or log some very specific behavior. In these cases, it is often possible to write a simple aspect similar to the ones above to do the job.

For example, the following aspect counts the number of calls to the rotate method on a Line and the number of calls to the set* methods of a Point that happen within the control flow of those calls to rotate:

aspect SetsInRotateCounting {
    int rotateCount = 0;
    int setCount = 0;

    before(): call(void Line.rotate(double)) {
        rotateCount++;
    }

    before(): call(void Point.set*(int))
              && cflow(call(void Line.rotate(double))) {
        setCount++;
    }
}

In effect, this aspect allows the programmer to ask very specific questions like

How many times is the rotate method defined on Line objects called?

How many times are methods defined on Point objects whose name begins with "set" called in fulfilling those rotate calls?

Many programmers use the "Design by Contract" style popularized by Bertand Meyer in Object-Oriented Software Construction, 2/e. In this style of programming, explicit pre-conditions test that callers of a method call it properly and explicit post-conditions test that methods properly do the work they are supposed to.

AspectJ makes it possible to implement pre- and post-condition testing in modular form. For example, this code

aspect PointBoundsChecking {

    pointcut setX(int x):
        (call(void FigureElement.setXY(int, int)) && args(x, *))
        || (call(void Point.setX(int)) && args(x));

    pointcut setY(int y):
        (call(void FigureElement.setXY(int, int)) && args(*, y))
        || (call(void Point.setY(int)) && args(y));

    before(int x): setX(x) {
        if ( x < MIN_X || x > MAX_X )
            throw new IllegalArgumentException("x is out of bounds.");
    }

    before(int y): setY(y) {
        if ( y < MIN_Y || y > MAX_Y )
            throw new IllegalArgumentException("y is out of bounds.");
    }
}

implements the bounds checking aspect of pre-condition testing for operations that move points. Notice that the setX pointcut refers to all the operations that can set a Point's x coordinate; this includes the setX method, as well as half of the setXY method. In this sense the setX pointcut can be seen as involving very fine-grained crosscutting — it names the the setX method and half of the setXY method.

Even though pre- and post-condition testing aspects can often be used only during testing, in some cases developers may wish to include them in the production build as well. Again, because AspectJ makes it possible to modularize these crosscutting concerns cleanly, it gives developers good control over this decision.

The property-based crosscutting mechanisms can be very useful in defining more sophisticated contract enforcement. One very powerful use of these mechanisms is to identify method calls that, in a correct program, should not exist. For example, the following aspect enforces the constraint that only the well-known factory methods can add an element to the registry of figure elements. Enforcing this constraint ensures that no figure element is added to the registry more than once.

aspect RegistrationProtection {

    pointcut register(): call(void Registry.register(FigureElement));

    pointcut canRegister(): withincode(static * FigureElement.make*(..));

    before(): register() && !canRegister() {
        throw new IllegalAccessException("Illegal call " + thisJoinPoint);
    }
}

This aspect uses the withincode primitive pointcut to denote all join points that occur within the body of the factory methods on FigureElement (the methods with names that begin with "make"). This is a property-based pointcut because it identifies join points based not on their signature, but rather on the property that they occur specifically within the code of another method. The before advice declaration effectively says signal an error for any calls to register that are not within the factory methods.

This advice throws a runtime exception at certain join points, but AspectJ can do better. Using the declare error form, we can have the compiler signal the error.

aspect RegistrationProtection {

    pointcut register(): call(void Registry.register(FigureElement));
    pointcut canRegister(): withincode(static * FigureElement.make*(..));

    declare error: register() && !canRegister(): "Illegal call"
}

When using this aspect, it is impossible for the compiler to compile programs with these illegal calls. This early detection is not always possible. In this case, since we depend only on static information (the withincode pointcut picks out join points totally based on their code, and the call pointcut here picks out join points statically). Other enforcement, such as the precondition enforcement, above, does require dynamic information such as the runtime value of parameters.

Configuration management for aspects can be handled using a variety of make-file like techniques. To work with optional aspects, the programmer can simply define their make files to either include the aspect in the call to the AspectJ compiler or not, as desired.

Developers who want to be certain that no aspects are included in the production build can do so by configuring their make files so that they use a traditional Java compiler for production builds. To make it easy to write such make files, the AspectJ compiler has a command-line interface that is consistent with ordinary Java compilers.

The first example production aspect shows how one might implement some simple functionality where it is problematic to try and do it explicitly. It supports the code that refreshes the display. The role of the aspect is to maintain a dirty bit indicating whether or not an object has moved since the last time the display was refreshed.

Implementing this functionality as an aspect is straightforward. The testAndClear method is called by the display code to find out whether a figure element has moved recently. This method returns the current state of the dirty flag and resets it to false. The pointcut move captures all the method calls that can move a figure element. The after advice on move sets the dirty flag whenever an object moves.

aspect MoveTracking {
    private static boolean dirty = false;

    public static boolean testAndClear() {
        boolean result = dirty;
        dirty = false;
        return result;
    }

    pointcut move():
        call(void FigureElement.setXY(int, int)) ||
        call(void Line.setP1(Point))             ||
        call(void Line.setP2(Point))             ||
        call(void Point.setX(int))               ||
        call(void Point.setY(int));

    after() returning: move() {
        dirty = true;
    }
}

Even this simple example serves to illustrate some of the important benefits of using AspectJ in production code. Consider implementing this functionality with ordinary Java: there would likely be a helper class that contained the dirty flag, the testAndClear method, as well as a setFlag method. Each of the methods that could move a figure element would include a call to the setFlag method. Those calls, or rather the concept that those calls should happen at each move operation, are the crosscutting concern in this case.

The AspectJ implementation has several advantages over the standard implementation:

The structure of the crosscutting concern is captured explicitly. The moves pointcut clearly states all the methods involved, so the programmer reading the code sees not just individual calls to setFlag, but instead sees the real structure of the code. The IDE support included with AspectJ automatically reminds the programmer that this aspect advises each of the methods involved. The IDE support also provides commands to jump to the advice from the method and vice-versa.

Evolution is easier. If, for example, the aspect needs to be revised to record not just that some figure element moved, but rather to record exactly which figure elements moved, the change would be entirely local to the aspect. The pointcut would be updated to expose the object being moved, and the advice would be updated to record that object. The paper An Overview of AspectJ (available linked off of the AspectJ web site -- http://eclipse.org/aspectj), presented at ECOOP 2001, presents a detailed discussion of various ways this aspect could be expected to evolve.

The functionality is easy to plug in and out. Just as with development aspects, production aspects may need to be removed from the system, either because the functionality is no longer needed at all, or because it is not needed in certain configurations of a system. Because the functionality is modularized in a single aspect this is easy to do.

The implementation is more stable. If, for example, the programmer adds a subclass of Line that overrides the existing methods, this advice in this aspect will still apply. In the ordinary Java implementation the programmer would have to remember to add the call to setFlag in the new overriding method. This benefit is often even more compelling for property-based aspects (see the section Providing Consistent Behavior).

The crosscutting structure of context passing can be a significant source of complexity in Java programs. Consider implementing functionality that would allow a client of the figure editor (a program client rather than a human) to set the color of any figure elements that are created. Typically this requires passing a color, or a color factory, from the client, down through the calls that lead to the figure element factory. All programmers are familiar with the inconvenience of adding a first argument to a number of methods just to pass this kind of context information.

Using AspectJ, this kind of context passing can be implemented in a modular way. The following code adds after advice that runs only when the factory methods of Figure are called in the control flow of a method on a ColorControllingClient.

aspect ColorControl {
    pointcut CCClientCflow(ColorControllingClient client):
        cflow(call(* * (..)) && target(client));

    pointcut make(): call(FigureElement Figure.make*(..));

    after (ColorControllingClient c) returning (FigureElement fe):
            make() && CCClientCflow(c) {
        fe.setColor(c.colorFor(fe));
    }
}

This aspect affects only a small number of methods, but note that the non-AOP implementation of this functionality might require editing many more methods, specifically, all the methods in the control flow from the client to the factory. This is a benefit common to many property-based aspects while the aspect is short and affects only a modest number of benefits, the complexity the aspect saves is potentially much larger.

This example shows how a property-based aspect can be used to provide consistent handling of functionality across a large set of operations. This aspect ensures that all public methods of the com.bigboxco package log any Errors they throw to their caller (in Java, an Error is like an Exception, but it indicates that something really bad and usually unrecoverable has happened). The publicMethodCall pointcut captures the public method calls of the package, and the after advice runs whenever one of those calls throws an Error. The advice logs that Error and then the throw resumes.

aspect PublicErrorLogging {
    Log log = new Log();

    pointcut publicMethodCall():
        call(public * com.bigboxco.*.*(..));

    after() throwing (Error e): publicMethodCall() {
        log.write(e);
    }
}

In some cases this aspect can log an exception twice. This happens if code inside the com.bigboxco package itself calls a public method of the package. In that case this code will log the error at both the outermost call into the com.bigboxco package and the re-entrant call. The cflow primitive pointcut can be used in a nice way to exclude these re-entrant calls:

after() throwing (Error e):
        publicMethodCall() && !cflow(publicMethodCall()) {
    log.write(e);
}

The following aspect is taken from work on the AspectJ compiler. The aspect advises about 35 methods in the JavaParser class. The individual methods handle each of the different kinds of elements that must be parsed. They have names like parseMethodDec, parseThrows, and parseExpr.

aspect ContextFilling {
    pointcut parse(JavaParser jp):
        call(* JavaParser.parse*(..))
        && target(jp)
        && !call(Stmt parseVarDec(boolean)); // var decs
                                              // are tricky

    around(JavaParser jp) returns ASTObject: parse(jp) {
        Token beginToken = jp.peekToken();
        ASTObject ret = proceed(jp);
        if (ret != null) jp.addContext(ret, beginToken);
        return ret;
     }
}

This example exhibits a property found in many aspects with large property-based pointcuts. In addition to a general property based pattern call(* JavaParser.parse*(..)) it includes an exception to the pattern !call(Stmt parseVarDec(boolean)). The exclusion of parseVarDec happens because the parsing of variable declarations in Java is too complex to fit with the clean pattern of the other parse* methods. Even with the explicit exclusion this aspect is a clear expression of a clean crosscutting modularity. Namely that all parse* methods that return ASTObjects, except for parseVarDec share a common behavior for establishing the parse context of their result.

The process of writing an aspect with a large property-based pointcut, and of developing the appropriate exceptions can clarify the structure of the system. This is especially true, as in this case, when refactoring existing code to use aspects. When we first looked at the code for this aspect, we were able to use the IDE support provided in AJDE for JBuilder to see what methods the aspect was advising compared to our manual coding. We quickly discovered that there were a dozen places where the aspect advice was in effect but we had not manually inserted the required functionality. Two of these were bugs in our prior non-AOP implementation of the parser. The other ten were needless performance optimizations. So, here, refactoring the code to express the crosscutting structure of the aspect explicitly made the code more concise and eliminated latent bugs.

Here's an example of an aspect definition in AspectJ:

 1 aspect FaultHandler {
 2
 3   private boolean Server.disabled = false;
 4
 5   private void reportFault() {
 6     System.out.println("Failure! Please fix it.");
 7   }
 8
 9   public static void fixServer(Server s) {
10     s.disabled = false;
11   }
12
13   pointcut services(Server s): target(s) && call(public * *(..));
14
15   before(Server s): services(s) {
16     if (s.disabled) throw new DisabledException();
17   }
18
19   after(Server s) throwing (FaultException e): services(s) {
20     s.disabled = true;
21     reportFault();
22   }
23 }

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.

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:

pointcut services(Server s): target(s) && call(public * *(..))

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.

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

{
  if (s.disabled) throw new DisabledException();
}

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:

{
  s.disabled = true;
  reportFault();
}

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.

Here are examples of pointcuts picking out

Pointcuts compose through the operations or ("||"), and ("&&") and not ("!").

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.

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.

public class Test {
    public static void main(String[] args) {
        foo();
    }
    static void foo() {
        goo();
    }
    static void goo() {
        System.out.println("hi");
    }
}

aspect A  {
    pointcut fooPC(): execution(void Test.foo());
    pointcut gooPC(): execution(void Test.goo());
    pointcut printPC(): call(void java.io.PrintStream.println(String));

    before(): cflow(fooPC()) && cflow(gooPC()) && printPC() && !within(A) {
        System.out.println("should occur");
    }

    before(): cflow(fooPC() && gooPC()) && printPC() && !within(A) {
        System.out.println("should not occur");
    }
}

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 its own advice. (See the section called “Infinite loops” for more details.)

Consider again the first pointcut definition in this chapter:

  pointcut setter(): target(Point) &&
                     (call(void setX(int)) ||
                      call(void setY(int)));

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:

  pointcut setter(Point p): target(p) &&
                            (call(void setX(int)) ||
                             call(void setY(int)));

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:

  pointcut testEquality(Point p): target(Point) &&
                                  args(p) &&
                                  call(boolean equals(Object));

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

  pointcut testEquality(Point p1, Point p2): target(p1) &&
                                             args(p2) &&
                                             call(boolean equals(Object));

Let's look at another variation of the setters pointcut:

pointcut setter(Point p, int newval): target(p) &&
                                      args(newval) &&
                                      (call(void setX(int)) ||
                                       call(void setY(int)));

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:

  pointcut badPointcut(Point p1, Point p2):
      (target(p1) && call(void setX(int))) ||
      (target(p2) && call(void setY(int)));

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.

  class Handle {
    Partner partner = new Partner();

    public void foo() { partner.foo(); }
    public void bar(int x) { partner.bar(x); }

    public static void main(String[] args) {
      Handle h1 = new Handle();
      h1.foo();
      h1.bar(2);
    }
  }

  class Partner {
    boolean isAlive() { return true; }
    void foo() { System.out.println("foo"); }
    void bar(int x) { System.out.println("bar " + x); }
  }

  aspect HandleLiveness {
    before(Handle handle): target(handle) && call(public * *(..)) {
      if ( handle.partner == null  || !handle.partner.isAlive() ) {
        throw new DeadPartnerException();
      }
    }
  }

  class DeadPartnerException extends RuntimeException {}

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

  private int Foo.x;

  int Foo.x;

then everything in the aspect's package (which may or may not be Foo's package) can access x.

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.

  class Point  {
      int x, y;

      public void setX(int x) { this.x = x; }
      public void setY(int y) { this.y = y; }

      public static void main(String[] args) {
          Point p = new Point();
          p.setX(3); p.setY(333);
      }
  }

  aspect PointAssertions {

      private boolean Point.assertX(int x) {
          return (x <= 100 && x >= 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;
          }
      }
  }

(The code for this example is in InstallDir/examples/tjp.)

A join point is some point in the execution of a program together with a view into the execution context when that point occurs. Join points are picked out by pointcuts. When a program reaches a join point, advice on that join point may run in addition to (or instead of) the join point itself.

When using a pointcut that picks out join points of a single kind by name, typicaly the the advice will know exactly what kind of join point it is associated with. The pointcut may even publish context about the join point. Here, for example, since the only join points picked out by the pointcut are calls of a certain method, we can get the target value and one of the argument values of the method calls directly.

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;
    }
}

But sometimes the shape of the join point is not so clear. For instance, suppose a complex application is being debugged, and we want to trace when any method of some class is executed. The pointcut

pointcut execsInProblemClass(): within(ProblemClass)
                             && execution(* *(..));

will pick out each execution join point of every method defined within ProblemClass. Since advice executes at each join point picked out by the pointcut, we can reasonably ask which join point was reached.

Information about the join point that was matched is available to advice through the special variable thisJoinPoint, of type org.aspectj.lang.JoinPoint. Through this object we can access information such as

public class Demo {
    static Demo d;

    public static void main(String[] args){
        new Demo().go();
    }

    void go(){
        d = new Demo();
        d.foo(1,d);
        System.out.println(d.bar(new Integer(3)));
    }

    void foo(int i, Object o){
        System.out.println("Demo.foo(" + i + ", " + o + ")\n");
    }

    String bar (Integer j){
        System.out.println("Demo.bar(" + j + ")\n");
        return "Demo.bar(" + j  + ")";
    }
}

aspect GetInfo {

   static final void println(String s){ System.out.println(s); }

   pointcut goCut(): cflow(this(Demo) && execution(void go()));

   pointcut demoExecs(): within(Demo) && execution(* *(..));

   Object around(): demoExecs() && !execution(* go()) && goCut() {
      println("Intercepted message: " +
          thisJoinPointStaticPart.getSignature().getName());
      println("in class: " +
          thisJoinPointStaticPart.getSignature().getDeclaringType().getName());
      printParameters(thisJoinPoint);
      println("Running original method: \n" );
      Object result = proceed();
      println("  result: " + result );
      return result;
   }

   static private void printParameters(JoinPoint jp) {
      println("Arguments: " );
      Object[] args = jp.getArgs();
      String[] names = ((CodeSignature)jp.getSignature()).getParameterNames();
      Class[] types = ((CodeSignature)jp.getSignature()).getParameterTypes();
      for (int i = 0; i < args.length; i++) {
         println("  "  + i + ". " + names[i] +
             " : " +            types[i].getName() +
             " = " +            args[i]);
      }
   }
}

cflow(this(Demo)) && execution(void go())

(The code for this example is in InstallDir/examples/introduction.)

Like advice, inter-type declarations are members of an aspect. They declare members that act as if they were defined on another class. Unlike advice, inter-type declarations affect not only the behavior of the application, but also the structural relationship between an application's classes.

This is crucial: Publically affecting the class structure of an application makes these modifications available to other components of the application.

Aspects can declare inter-type

This example provides three illustrations of the use of inter-type declarations to encapsulate roles or views of a class. The class our aspect will be dealing with, Point, is a simple class with rectangular and polar coordinates. Our inter-type declarations will make the class Point, in turn, cloneable, hashable, and comparable. These facilities are provided by AspectJ without having to modify the code for the class Point.

public aspect CloneablePoint {

   declare parents: Point implements Cloneable;

   public Object Point.clone() throws CloneNotSupportedException {
      // we choose to bring all fields up to date before cloning.
      makeRectangular();
      makePolar();
      return super.clone();
   }

   public static void main(String[] args){
      Point p1 = new Point();
      Point p2 = null;

      p1.setPolar(Math.PI, 1.0);
      try {
         p2 = (Point)p1.clone();
      } catch (CloneNotSupportedException e) {}
      System.out.println("p1 =" + p1 );
      System.out.println("p2 =" + p2 );

      p1.rotate(Math.PI / -2);
      System.out.println("p1 =" + p1 );
      System.out.println("p2 =" + p2 );
   }
}

public aspect ComparablePoint {

   declare parents: Point implements Comparable;

   public int Point.compareTo(Object o) {
      return (int) (this.getRho() - ((Point)o).getRho());
   }

   public static void main(String[] args){
      Point p1 = new Point();
      Point p2 = new Point();

      System.out.println("p1 =?= p2 :" + p1.compareTo(p2));

      p1.setRectangular(2,5);
      p2.setRectangular(2,5);
      System.out.println("p1 =?= p2 :" + p1.compareTo(p2));

      p2.setRectangular(3,6);
      System.out.println("p1 =?= p2 :" + p1.compareTo(p2));

      p1.setPolar(Math.PI, 4);
      p2.setPolar(Math.PI, 4);
      System.out.println("p1 =?= p2 :" + p1.compareTo(p2));

      p1.rotate(Math.PI / 4.0);
      System.out.println("p1 =?= p2 :" + p1.compareTo(p2));

      p1.offset(1,1);
      System.out.println("p1 =?= p2 :" + p1.compareTo(p2));
   }
}

public aspect HashablePoint {

   public int Point.hashCode() {
      return (int) (getX() + getY() % Integer.MAX_VALUE);
   }

   public boolean Point.equals(Object o) {
      if (o == this) { return true; }
      if (!(o instanceof Point)) { return false; }
      Point other = (Point)o;
      return (getX() == other.getX()) && (getY() == other.getY());
   }

   public static void main(String[] args) {
      Hashtable h = new Hashtable();
      Point p1 = new Point();

      p1.setRectangular(10, 10);
      Point p2 = new Point();

      p2.setRectangular(10, 10);

      System.out.println("p1 = " + p1);
      System.out.println("p2 = " + p2);
      System.out.println("p1.hashCode() = " + p1.hashCode());
      System.out.println("p2.hashCode() = " + p2.hashCode());

      h.put(p1, "P1");
      System.out.println("Got: " + h.get(p2));
   }
}

(The code for this example is in InstallDir/examples/tracing.)

Writing a class that provides tracing functionality is easy: a couple of functions, a boolean flag for turning tracing on and off, a choice for an output stream, maybe some code for formatting the output -- these are all elements that Trace classes have been known to have. Trace classes may be highly sophisticated, too, if the task of tracing the execution of a program demands it.

But developing the support for tracing is just one part of the effort of inserting tracing into a program, and, most likely, not the biggest part. The other part of the effort is calling the tracing functions at appropriate times. In large systems, this interaction with the tracing support can be overwhelming. Plus, tracing is one of those things that slows the system down, so these calls should often be pulled out of the system before the product is shipped. For these reasons, it is not unusual for developers to write ad-hoc scripting programs that rewrite the source code by inserting/deleting trace calls before and after the method bodies.

AspectJ can be used for some of these tracing concerns in a less ad-hoc way. Tracing can be seen as a concern that crosscuts the entire system and as such is amenable to encapsulation in an aspect. In addition, it is fairly independent of what the system is doing. Therefore tracing is one of those kind of system aspects that can potentially be plugged in and unplugged without any side-effects in the basic functionality of the system.

public abstract class TwoDShape {
    protected double x, y;
    protected TwoDShape(double x, double y) {
        this.x = x; this.y = y;
    }
    public double getX() { return x; }
    public double getY() { return y; }
    public double distance(TwoDShape s) {
        double dx = Math.abs(s.getX() - x);
        double dy = Math.abs(s.getY() - y);
        return Math.sqrt(dx*dx + dy*dy);
    }
    public abstract double perimeter();
    public abstract double area();
    public String toString() {
        return (" @ (" + String.valueOf(x) + ", " + String.valueOf(y) + ") ");
    }
}

public class Circle extends TwoDShape {
    protected double r;
    public Circle(double x, double y, double r) {
        super(x, y); this.r = r;
    }
    public Circle(double x, double y) { this(  x,   y, 1.0); }
    public Circle(double r)           { this(0.0, 0.0,   r); }
    public Circle()                   { this(0.0, 0.0, 1.0); }
    public double perimeter() {
        return 2 * Math.PI * r;
    }
    public double area() {
        return Math.PI * r*r;
    }
    public String toString() {
        return ("Circle radius = " + String.valueOf(r) + super.toString());
    }
}