Aspectj Cookbook

Recipe 7.2. Capturing a Constructor When It Is Executing

Problem

You want to capture a constructor that matches a specific signature when it is executing.

Solution

Use the execution(Signature) pointcut with the additional new keyword as part of the signature. The syntax of the execution(Signature) pointcut when using in relation to constructors is:

pointcut <pointcut name>(<any values to be picked up>) : execution(<optional modifier> <class>.new(<parameter types>));

Discussion

The execution(Signature) pointcut has three key characteristics when it is used to capture the execution of a constructor:

1. The execution(Signature) pointcut with the new keyword triggers join points when a class constructor is executing.

2. The exact point at which the join point is triggered cannot be prior to the class's constructor being invoked. This prevents overriding of the returned object.

3. The implementation, but not the type of object being constructed, of the constructor method can be overridden using around() advice.

Example 7-2 shows the execution(Signature) capturing the execution of the MyClass constructor that takes an int and a String as parameters.

Example 7-2. Using the execution(Signature) pointcut to capture join points within a specific constructor

public aspect ExecutionNewRecipe { /* Specifies calling advice when any constructor executes that meets the following signature rules: Class Name: MyClass Method Name: new (This is a keyword indicating the constructor call) Method Parameters: int, String */ pointcut myClassConstructorWithIntAndStringPointcut( ) : execution(MyClass.new (int, String)); // Advice declaration before( ) : myClassConstructorWithIntAndStringPointcut( ) { System.out.println( "---------------- Aspect Advice Logic -----------------"); System.out.println( "In the advice picked by " + "myClassConstructorWithIntAndOthersPointcut( )"); System.out.println( "The current type of object under construction is: "); System.out.println(thisJoinPoint.getThis( ).getClass( )); System.out.println( "Signature: " + thisJoinPoint.getSignature( )); System.out.println( "Source Line: " + thisJoinPoint.getSourceLocation( )); System.out.println( "------------------------------------------------------"); } }

Figure 7-2 shows how the execution(Signature) pointcut is applied to constructors.

See Also

Recipe 4.1 shows some of the wildcard variations that can be used in a Signature; describes how the call(Signature) pointcut is applied to constructors and is similar to this recipe's pointcut, with the exception that the call(Signature) pointcut has the power to override the object that is being constructed; Recipes Recipe 4.1 and Recipe 4.4 respectively show how to define the call(Signature) and execution(Signature) pointcuts to capture join points from regular methods; Chapter 13 describes the different types of advice available in AspectJ.