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Advanced AspectJ
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This chapter covers
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Using reflection support to access join point information Using aspect precedence to coordinate multiple aspects Creating reusable aspects with aspect association Exception softening and privileged aspects
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Accessing join point information
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The core concepts presented earlier equipped you with basic AspectJ constructs so that you can begin to implement crosscutting functionality in your system. For complex applications involving the creation of reusable aspects and the use of multiple aspects, you will need advanced AspectJ concepts and constructs to provide you with additional options for design and implementation. This chapter introduces more advanced features of AspectJ, such as aspect precedence and aspect association. Unlike the earlier chapters, where concepts build on top of one another, this chapter contains a collection of constructs that each stand alone.
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4.1 Accessing join point information via reflection
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Reflective support in AspectJ provides programmatic access to the static and dynamic information associated with the join points. For example, using reflection, you can access the name of the currently advised method as well as the argument objects to that method. The dynamic context that can be captured using reflective support is similar to that captured using this(), target(), and args() pointcuts only the mechanism to obtain the information is different. The most common use of this reflective information is in aspects that implement logging and similar functionaliy. We have already used simple reflective support to write the JoinPointTraceAspect in chapter 2. In this section, we examine the details of reflective support.
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While you can always use reflection to obtain the dynamic context, the preferred way is to use the this(), target(), and args() pointcuts. The reflective way of accessing information has poor performance, lacks static type checking, and is cumbersome to use. However, there are times when you need to use reflection because you need to access dynamic context and little information is available or required about the advised join points. For instance, you cannot easily use an args() pointcut to capture arguments for all logged methods, since each method may take a different number and type of arguments. Further, the logging aspect s advice doesn t need to care about the type of the argument objects because the only interaction of the logging aspect with those objects is to print them.
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AspectJ provides reflective access by making three special objects available in each advice body: thisJoinPoint, thisJoinPointStaticPart, and thisEnclosingJoinPointStaticPart. These objects are much like the special variable this that
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Advanced AspectJ
is available in each instance method in Java to provide access to the execution object. The information contained in these three objects is of two types: dynamic information and static information:
Dynamic information consists of the kind of information that changes with each invocation of the same join points. For example, two different calls to the method Account.debit() will probably have different account objects and debit amounts. Static information is information that does not change between the multiple executions. For example, the name and source location of a method remain the same during different invocations of the method.
Each join point provides one object that contains dynamic information and two objects that contain static information about the join point and its enclosing join point. Let s examine the information in each of these special objects. We will examine the API to access the information from these objects in section 4.1.1:
thisJoinPoint This object of type JoinPoint contains the dynamic infor-
mation of the advised join point. It gives access to the target object, the execution object, and the method arguments. It also provides access to the static information for the join point, using the getStaticPart() method. You use thisJoinPoint when you need dynamic information related to the join point. For example, if you want to log the execution object and method arguments, you would use the thisJoinPoint object. thisJoinPointStaticPart This object of type JoinPoint.StaticPart contains the static information about the advised join point. It gives access to the source location, the kind (method-call, method-execution, field-set, field-get, and so forth), and the signature of the join point. You use thisJoinPointStaticPart when you need the structural context of the join point, such as its name, kind, source location, and so forth. For example, if you need to log the name of the methods that are executed, you would use the thisJoinPointStaticPart object. thisEnclosingJoinPointStaticPart This object of type JoinPoint.StaticPart contains the static information about the enclosing join point, which is also refered to as the enclosing context. The enclosing context of a join point depends on the kind of join point. For example, for a methodcall join point, the enclosing join point is the execution of the caller method, whereas for an exception-handler join point, the enclosing join point is the method that surrounds the catch block. You use the thisEn-
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