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Introducing AspectJ
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whose body is identical to the advice body. The synthesized method name for the advice, such as before0$ajc(), is purely for internal purposes. Now let s see the equivalent code for the MessageCommunicator class, after it has been aspected by MannersAspect:
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public class MessageCommunicator { public static void deliver(String message) { MannersAspect.aspectInstance.before0$ajc(); System.out.println(message); } public static void deliver(String person, String message) { MannersAspect.aspectInstance.before0$ajc(); System.out.print(person + ", " + message); } }
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Recall that the deliverMessage() pointcut in MannersAspect is defined to capture both of the overloaded deliver() methods in MessageCommunicator. To show the effect of advice to the join points captured by deliverMessage(), the identical modification must be made to both methods. Accordingly, we see that the MannersAspect.aspectInstance.before0$ajc() call is made from both methods. The resulting code looks simple because MannersAspect itself is simple. For complex aspects, the woven code is accordingly complex.
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Thinking about the language semantics in terms of the transformed code helps in taking the mystery out of AspectJ. It also makes you appreciate the hard work that the AspectJ compiler is performing and the hard work that you no longer need to perform! However, such thinking has inherent within it the danger of bogging down too much in the details of the transformed code. A better approach is to start thinking in terms of language semantics instead of transformation.
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Now that you are familiar with the basic flavor of the AspectJ programming language, let s jump into the details. In the rest of this chapter, we examine the join point model and aspects, followed by a brief description of the AspectJ implementation. In chapter 3, we look at the syntax of AspectJ code as it is used in pointcuts, advice, and introductions.
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The join point model
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2.4 The join point model
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The join point is the most fundamental concept in AspectJ. A join point is any identifiable execution point in a system. A call to a method is a join point, and so is a method s execution. Even an assignment to a variable or a return statement is a join point. In addition, the following are all join points: an object construction, a conditional check, a comparison, an exception handler, and even for, while, and do/while loops. In AspectJ, the join points are the places where we can interject the crosscutting actions; therefore, it is necessary to understand the join point model in order to specify the weaving rules using pointcuts and advice. Not all of the join points in a system are available for your use. The join points that you can select in pointcuts are called exposed join points. In order to prevent implementation-dependent or unstable crosscutting, AspectJ deliberately exposes only a subset of all the possible join points in the system. AspectJ, for example, does not expose for loops because you can easily change a for loop to a while loop that functions in the same manner. If such a change were to be made, all of the advice to the join point for the for loop would no longer be valid since the loop would no longer exist. Some of the join points exposed by AspectJ include method calls and execution, object instantiation, field access, and exception handlers. The exposed join points in the system are the only possible places in the code where we can augment or alter the main program execution. All join points also have a context associated with them. For example, a call to a join point in a method has the caller object, the target object, and the arguments of the method available as the context. Similarly, the exception handler join point would have the current object and the thrown exception as the context. As we will see in chapter 3, certain pointcuts can capture this context and pass it to advice to be used in the advice body to make decisions based on the context. For example, a pointcut capturing a join point in a debit() method in the Account class may collect the amount to be debited as context so that advice to the join points can check it against the minimum balance requirement. In figure 2.1, the UML sequence diagram shows a graphical representation of join points in an ATM transaction example, which illustrates some of the places where you could introduce a new or alternative crosscutting behavior. In the sequence diagram, we see several join points that are encountered when an ATM object invokes a debit() method on an Account object. The first join point is the call to the debit() method itself. During the execution of the debit() method, the join points for the execution of the getBalance() and setBalance() methods are encountered, and so on. Method calls aren t the only join
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