Recipe4.3.Capturing the Target of a Method Call

Recipe 4.3. Capturing the Target of a Method Call

Problem

You want to capture the object being called as a method is invoked.

Solution

Create a pointcut that specifies a single parameter of the same type as the target of the method call that you want to capture. Use the call(Signature) and target(Type | Identifier) pointcuts to capture the invocation of a method and then to bind the single identifier to the object that the method is being called upon.

Discussion

Example 4-3 shows the call(Signature) pointcut being used to declare an interest in all methods that match the signature MyClass.foo(int,String). The captureCallTarget(MyClass) pointcut requires a MyClass object as specified by the myObject identifier. The myObject identifier is then bound to the object that is being called by the MyClass.foo(int,String) method by the target(Type | Identifier) pointcut.

Example 4-3. Capturing the object upon which the MyClass.foo(..) method is invoked

public aspect CaptureCallTargetRecipe  {    /*    Specifies calling advice whenever a method    matching the following rules gets called:        Class Name: MyClass    Method Name: foo    Method Return Type: void    Method Parameters: an int followed by a String    */    pointcut captureCallTarget(MyClass myObject) :        call(void MyClass.foo(int, String)) &&        target(myObject);    // Advice declaration    before(MyClass myObject) : captureCallTarget(myObject)    {       System.out.println(          "------------------- Aspect Advice Logic --------------------");       System.out.println(          "In the advice attached to the call point cut");       System.out.println("Captured target object for the method call: "           + myObject);       System.out.println(          "------------------------------------------------------------");    } }

The before( ) advice can access the single identifier declared on the captureCallTar-get(MyClass) pointcut by including the myObject identifier in its signature and then binding that identifier to the captureCallTarget(MyClass) pointcut.

See Also

The call(Signature) pointcut is described in Recipe 4.1; Recipe 4.1 also shows some of the wildcard variations that can be used in a Signature; Recipe 11.2 discusses the target(Type | Identifier) pointcut; combining pointcut logic using a logical AND (&&) is shown in Recipe 12.2; the before( ) form of advice is shown in Recipe 13.3; the calling context that is available to advice is covered in Chapter 13.