java qr code reader AssociationDemoAspect.java: with perthis() association in Java

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Listing 4.17 AssociationDemoAspect.java: with perthis() association
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public aspect AssociationDemoAspect perthis(accountOperationExecution(Account)) { public AssociationDemoAspect() { System.out.println("Creating aspect instance"); } pointcut accountOperationExecution(Account account) : (execution(* Account.credit(..)) || execution(* Account.debit(..))) && this(account); before(Account account) : accountOperationExecution(account) { System.out.println("JoinPoint: " + thisJoinPointStaticPart + "\n\taspect: " + this + "\n\tobject: " + account); } }
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Figure 4.3 This sequence diagram shows aspect creation and association points for object-based association. For this illustration, we specify the perthis
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(execution(* Account. *(..))) association. An aspect is
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created for each object when the join point matching the pointcut is first executed for that object. The aspect association then remains valid during the lifetime of the object. Notice that no new aspect instance is created when the debit() method is invoked on the account1 object.
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Now when we compile this using the modified aspect and run the test program, we get following output:
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> ajc *.java > java TestAssociation Creating aspect instance JoinPoint: execution(void Account.credit(float)) aspect: AssociationDemoAspect@e48e1b object: SavingsAccount@12dacd1 JoinPoint: execution(void Account.debit(float)) aspect: AssociationDemoAspect@e48e1b object: SavingsAccount@12dacd1 Creating aspect instance JoinPoint: execution(void Account.credit(float)) aspect: AssociationDemoAspect@1ad086a object: SavingsAccount@10385c1 JoinPoint: execution(void Account.debit(float)) aspect: AssociationDemoAspect@1ad086a object: SavingsAccount@10385c1
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Aspect instance creation
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The output shows:
Two instances of AssociationDemoAspect are created. Each aspect is created right before the execution of the first join point with each Account object. In each advice body, the same aspect instance is available for each join point on an object.
To associate an aspect instance with the target object for a matching join point instead of the execution object, you use pertarget() instead of perthis(). 4.3.3 Per-control-flow association As with per-object association, you sometimes need per-control-flow association to store per-control-flow states in implementations. You can think of control flow as a conceptual object that encapsulates the thread of execution encompassing a given join point. The per-control-flow state then is data associated with this conceptual control-flow object. With per-control-flow association, an aspect instance is associated with each control flow matching the association specification. Consider the following snippet of a reusable base aspect providing transaction management. This aspect needs to store states needed by the transaction management, such as a JDBC connection object used by all operations:
public abstract aspect TransactionManagementAspect percflow(transacted()) { ... aspect state: ... instance members such as the connection object used
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abstract pointcut transacted(); ... advice using the aspect state }
We can then introduce a transaction management capability in a banking application by extending this aspect and providing a definition for the abstract transacted() pointcut:
public aspect BankingTransactionManagementAspect extends TransactionManagementAspect { pointcut transacted() : execution(* banking..Account+.*(..)) || execution(* banking..Customer+.*(..)); }
In this aspect, we introduced transaction management into a banking system by simply specifying the operations that need transaction management support in the definition of the abstract pointcut transacted(). This will capture the execution of appropriate methods in banking-related classes. The bulk of transaction management logic resides in the reusable base TransactionManagementAspect aspect. There are a few ways to achieve the goal of creating reusable aspects that need to keep some per-control-flow state without using a control-flow-based association. For example, you could use a thread-specific storage such as ThreadLocal to manage the control flow s state. In many cases, however, using an aspect association creates a simpler implementation. You can specify two kinds of per-control-flow object associations:
percflow() Associates a separate aspect instance with the control flow at the join point matching the pointcut specified inside percflow() percflowbelow() Associates a separate aspect instance with the control flow below the join point matching the pointcut specified inside percflowbelow()
Much like the perthis and pertarget cases, once an association is made between a control flow and an aspect instance, it continues to exist for the lifetime of that control flow. Figure 4.4 illustrates the effect of control-flow-based association. In figure 4.4, we consider an aspect that associates the aspect instance with the control flow of join points that match the execution of any method in the Account class. We see that six aspect instances are created one each for the toplevel credit() and debit() executions, and two each for getBalance() and setBalance() , which are called from the credit() and debit() methods. Each aspect instance continues to exist until its join point s execution completes.
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