# The AspectJ Language

## Introduction

The previous chapter, , was a brief overview of the AspectJ language. You should read this chapter to understand AspectJ's syntax and semantics. It covers the same material as the previous chapter, but more completely and in much more detail.

We will start out by looking at an example aspect that we'll build out of a pointcut, an introduction, and two pieces of advice. This example aspect will gives us something concrete to talk about.

## The Anatomy of an Aspect

This lesson explains the parts of AspectJ's aspects. By reading this lesson you will have an overview of what's in an aspect and you will be exposed to the new terminology introduced by AspectJ.

### An Example Aspect

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

     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.

### Pointcuts

AspectJ's pointcut definitions give names to pointcuts. Pointcuts themselves pick out join points, i.e. interesting points in the execution of a program. These join points can be method or constructor invocations and executions, the handling of exceptions, field assignments and accesses, etc. Take, for example, the pointcut definition in line 13:

    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.

### Advice

A piece of advice brings together a pointcut and a body of code to define aspect implementation that runs at join points picked out by the pointcut. For example, the advice in lines 15-17 specifies that the following piece of code

    {
      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.

## Join Points and Pointcuts

Consider the following Java class:

    class Point {
        private int x, y;

        Point(int x, int y) { this.x = x; this.y = y; }

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

        int getX() { return x; }
        int getY() { return y; }
    }

In order to get an intuitive understanding of AspectJ's join points and pointcuts, let's go back to some of the basic principles of Java. Consider the following a method declaration in class Point:

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

This piece of program says that that when method named `setX` with an `int` argument called on an object of type `Point`, then the method body `{ this.x = x; }` is executed. Similarly, the constructor of the class states that when an object of type `Point` is instantiated through a constructor with two `int` arguments, then the constructor body `{ this.x = x; this.y = y; }` is executed.

One pattern that emerges from these descriptions is

> When something happens, then something gets executed.

In object-oriented programs, there are several kinds of "things that happen" that are determined by the language. We call these the join points of Java. Join points consist of things like method calls, method executions, object instantiations, constructor executions, field references and handler executions. (See the for a complete listing.)

Pointcuts pick out these join points. For example, the pointcut

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

picks out each call to `setX(int)` or `setY(int)` when called on an instance of `Point`. Here's another example:

    pointcut ioHandler(): within(MyClass) && handler(IOException);

This pointcut picks out each the join point when exceptions of type `IOException` are handled inside the code defined by class `MyClass`.

Pointcut definitions consist of a left-hand side and a right-hand side, separated by a colon. The left-hand side consists of the pointcut name and the pointcut parameters (i.e. the data available when the events happen). The right-hand side consists of the pointcut itself.

### Some Example Pointcuts

Here are examples of pointcuts picking out

when a particular method body executes

  `execution(void Point.setX(int))`

when a method is called

  `call(void Point.setX(int))`

when an exception handler executes

  `handler(ArrayOutOfBoundsException)`

when the object currently executing (i.e. `this`) is of type `SomeType`

  `this(SomeType)`

when the target object is of type `SomeType`

  `target(SomeType)`

when the executing code belongs to class `MyClass`

  `within(MyClass)`

when the join point is in the control flow of a call to a `Test`'s no-argument `main` method

  `cflow(call(void Test.main()))`

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

- It is possible to use wildcards. So

  1. `execution(* *(..))`

  2. `call(* set(..))`

  means (1) the execution of any method regardless of return or parameter types, and (2) the call to any method named `set` regardless of return or parameter types -- in case of overloading there may be more than one such `set` method; this pointcut picks out calls to all of them.

- You can select elements based on types. For example,

  1. `execution(int *())`

  2. `call(* setY(long))`

  3. `call(* Point.setY(int))`

  4. `call(*.new(int, int))`

  means (1) the execution of any method with no parameters that returns an `int`, (2) the call to any `setY` method that takes a `long` as an argument, regardless of return type or declaring type, (3) the call to any of `Point`'s `setY` methods that take an `int` as an argument, regardless of return type, and (4) the call to any classes' constructor, so long as it takes exactly two `int`s as arguments.

- You can compose pointcuts. For example,

  1. `target(Point) && call(int *())`

  2. `call(* *(..)) && (within(Line) || within(Point))`

  3. `within(*) && execution(*.new(int))`

  4. `!this(Point) && call(int *(..))`

  means (1) any call to an `int` method with no arguments on an instance of `Point`, regardless of its name, (2) any call to any method where the call is made from the code in `Point`'s or `Line`'s type declaration, (3) the execution of any constructor taking exactly one `int` argument, regardless of where the call is made from, and (4) any method call to an `int` method when the executing object is any type except `Point`.

- You can select methods and constructors based on their modifiers and on negations of modifiers. For example, you can say:

  1. `call(public * *(..))`

  2. `execution(!static * *(..))`

  3. `execution(public !static * *(..))`

  which means (1) any call to a public method, (2) any execution of a non-static method, and (3) any execution of a public, non-static method.

- Pointcuts can also deal with interfaces. For example, given the interface

      interface MyInterface { ... }

  the pointcut `call(* MyInterface.*(..))` picks out any call to a method in `MyInterface`'s signature -- that is, any method defined by `MyInterface` or inherited by one of its a supertypes.

### call vs. execution

When methods and constructors run, there are two interesting times associated with them. That is when they are called, and when they actually execute.

AspectJ exposes these times as call and execution join points, respectively, and allows them to be picked out specifically by `call` and `execution` pointcuts.

So what's the difference between these join points? Well, there are a number of differences:

Firstly, the lexical pointcut declarations `within` and `withincode` match differently. At a call join point, the enclosing code is that of the call site. This means that `call(void m()) && withincode(void m())` will only capture directly recursive calls, for example. At an execution join point, however, the program is already executing the method, so the enclosing code is the method itself: `execution(void m()) && withincode(void m())` is the same as `execution(void m())`.

Secondly, the call join point does not capture super calls to non-static methods. This is because such super calls are different in Java, since they don't behave via dynamic dispatch like other calls to non-static methods.

The rule of thumb is that if you want to pick a join point that runs when an actual piece of code runs (as is often the case for tracing), use `execution`, but if you want to pick one that runs when a particular signature is called (as is often the case for production aspects), use `call`.

### Pointcut composition

Pointcuts are put together with the operators and (spelled `&&`), or (spelled `||`), and not (spelled `!`). This allows the creation of very powerful pointcuts from the simple building blocks of primitive pointcuts. This composition can be somewhat confusing when used with primitive pointcuts like `cflow` and `cflowbelow`. Here's an example:

`cflow(P)` picks out each join point in the control flow of the join points picked out by P. So, pictorially:

      P ---------------------
        \
         \  cflow of P
          \

What does `cflow(P) && cflow(Q)` pick out? Well, it picks out each join point that is in both the control flow of P and in the control flow of Q. So...

              P ---------------------
                \
                 \  cflow of P
                  \
                   \
                    \
      Q -------------\-------
        \             \
         \  cflow of Q \ cflow(P) && cflow(Q)
          \             \

Note that P and Q might not have any join points in common... but their control flows might have join points in common.

But what does `cflow(P && Q)` mean? Well, it means the control flow of those join points that are both picked out by P and picked out by Q.

       P && Q -------------------
              \
               \ cflow of (P && Q)
                \

and if there are no join points that are both picked by P and picked out by Q, then there's no chance that there are any join points in the control flow of `(P && Q)`.

Here's some code that expresses this.

    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 it's own advice. (See for more details.)

### Pointcut Parameters

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 `Point`s, 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)));

because `p1` is only bound when calling `setX`, and `p2` is only bound when calling `setY`, but the pointcut picks out all of these join points and tries to bind both `p1` and `p2`.

### Example: `HandleLiveness`

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 {}

## Advice

Advice defines pieces of aspect implementation that execute at well-defined points in the execution of the program. Those points can be given either by named pointcuts (like the ones you've seen above) or by anonymous pointcuts. Here is an example of an advice on a named pointcut:

      pointcut setter(Point p1, int newval): target(p1) && args(newval)
                                             (call(void setX(int) ||
                                              call(void setY(int)));

      before(Point p1, int newval): setter(p1, newval) {
          System.out.println("About to set something in " + p1 +
                             " to the new value " + newval);
      }

And here is exactly the same example, but using an anonymous pointcut:

      before(Point p1, int newval): target(p1) && args(newval)
                                    (call(void setX(int)) ||
                                     call(void setY(int))) {
          System.out.println("About to set something in " + p1 +
                             " to the new value " + newval);
      }

Here are examples of the different advice:

This before advice runs just before the join points picked out by the (anonymous) pointcut:

      before(Point p, int x): target(p) && args(x) && call(void setX(int)) {
          if (!p.assertX(x)) return;
      }

This after advice runs just after each join point picked out by the (anonymous) pointcut, regardless of whether it returns normally or throws an exception:

      after(Point p, int x): target(p) && args(x) && call(void setX(int)) {
          if (!p.assertX(x)) throw new PostConditionViolation();
      }

This after returning advice runs just after each join point picked out by the (anonymous) pointcut, but only if it returns normally. The return value can be accessed, and is named `x` here. After the advice runs, the return value is returned:

      after(Point p) returning(int x): target(p) && call(int getX()) {
          System.out.println("Returning int value " + x + " for p = " + p);
      }

This after throwing advice runs just after each join point picked out by the (anonymous) pointcut, but only when it throws an exception of type `Exception`. Here the exception value can be accessed with the name `e`. The advice re-raises the exception after it's done:

      after() throwing(Exception e): target(Point) && call(void setX(int)) {
          System.out.println(e);
      }

This around advice traps the execution of the join point; it runs instead of the join point. The original action associated with the join point can be invoked through the special `proceed` call:

    void around(Point p, int x): target(p)
                              && args(x)
                              && call(void setX(int)) {
        if (p.assertX(x)) proceed(p, x);
        p.releaseResources();
    }

## Inter-type declarations

Aspects can declare members (fields, methods, and constructors) that are owned by other types. These are called inter-type members. Aspects can also declare that other types implement new interfaces or extend a new class. Here are examples of some such inter-type declarations:

This declares that each `Server` has a `boolean` field named `disabled`, initialized to `false`:

      private boolean Server.disabled = false;

It is declared `private`, which means that it is private to the aspect: only code in the aspect can see the field. And even if `Server` has another private field named `disabled` (declared in `Server` or in another aspect) there won't be a name collision, since no reference to `disabled` will be ambiguous.

This declares that each `Point` has an `int` method named `getX` with no arguments that returns whatever `this.x` is:

      public int Point.getX() { return this.x; }

Inside the body, `this` is the `Point` object currently executing. Because the method is publically declared any code can call it, but if there is some other `Point.getX()` declared there will be a compile-time conflict.

This publically declares a two-argument constructor for `Point`:

      public Point.new(int x, int y) { this.x = x; this.y = y; }

This publicly declares that each `Point` has an `int` field named `x`, initialized to zero:

      public int Point.x = 0;

Because this is publically declared, it is an error if `Point` already has a field named `x` (defined by `Point` or by another aspect).

This declares that the `Point` class implements the `Comparable` interface:

      declare parents: Point implements Comparable;

Of course, this will be an error unless `Point` defines the methods required by `Comparable`.

This declares that the `Point` class extends the `GeometricObject` class.

      declare parents: Point extends GeometricObject;

An aspect can have several inter-type declarations. For example, the following declarations

      public String Point.name;
      public void Point.setName(String name) { this.name = name; }

publicly declare that Point has both a String field `name` and a `void` method `setName(String)` (which refers to the `name` field declared by the aspect).

An inter-type member can only have one target type, but often you may wish to declare the same member on more than one type. This can be done by using an inter-type member in combination with a private interface:

      aspect A {
        private interface HasName {}
        declare parents: (Point || Line || Square) implements HasName;

        private String HasName.name;
        public  String HasName.getName()  { return name; }
      }

This declares a marker interface `HasName`, and also declares that any type that is either `Point`, `Line`, or `Square` implements that interface. It also privately declares that all `HasName` object have a `String` field called `name`, and publically declares that all `HasName` objects have a `String` method `getName()` (which refers to the privately declared `name` field).

As you can see from the above example, an aspect can declare that interfaces have fields and methods, even non-constant fields and methods with bodies.

### Inter-type Scope

AspectJ allows private and package-protected (default) inter-type declarations in addition to public inter-type declarations. Private means private in relation to the aspect, not necessarily the target type. So, if an aspect makes a private inter-type declaration of a field

      private int Foo.x;

Then code in the aspect can refer to `Foo`'s `x` field, but nobody else can. Similarly, if an aspect makes a package-protected introduction,

      int Foo.x;

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

### Example: `PointAssertions`

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;
              }
          }
      }

## thisJoinPoint

AspectJ provides a special reference variable, `thisJoinPoint`, that contains reflective information about the current join point for the advice to use. The `thisJoinPoint` variable can only be used in the context of advice, just like `this` can only be used in the context of non-static methods and variable initializers. In advice, `thisJoinPoint` is an object of type `org.aspectj.lang.JoinPoint`.

One way to use it is simply to print it out. Like all Java objects, `thisJoinPoint` has a `toString()` method that makes quick-and-dirty tracing easy:

      class TraceNonStaticMethods {
          before(Point p): target(p) && call(* *(..)) {
              System.out.println("Entering " + thisJoinPoint + " in " + p);
          }
      }

The type of `thisJoinPoint` includes a rich reflective class hierarchy of signatures, and can be used to access both static and dynamic information about join points such as the arguments of the join point:

      thisJoinPoint.getArgs()

In addition, it holds an object consisting of all the static information about the join point such as corresponding line number and static signature:

      thisJoinPoint.getStaticPart()

If you only need the static information about the join point, you may access the static part of the join point directly with the special variable `thisJoinPointStaticPart`. Using `thisJoinPointStaticPart` will avoid the run-time creation of the join point object that may be necessary when using `thisJoinPoint` directly.

It is always the case that

       thisJoinPointStaticPart == thisJoinPoint.getStaticPart()

       thisJoinPoint.getKind() == thisJoinPointStaticPart.getKind()
       thisJoinPoint.getSignature() == thisJoinPointStaticPart.getSignature()
       thisJoinPoint.getSourceLocation() == thisJoinPointStaticPart.getSourceLocation()

One more reflective variable is available: `thisEnclosingJoinPointStaticPart`. This, like `thisJoinPointStaticPart`, only holds the static part of a join point, but it is not the current but the enclosing join point. So, for example, it is possible to print out the calling source location (if available) with

       before() : execution (* *(..)) {
         System.err.println(thisEnclosingJoinPointStaticPart.getSourceLocation())
       }