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The advice to accountOperationExecution() prints the static context of the captured join point, the aspect instance, and the Account object captured by the pointcut. Note that when used from advice, the object this refers to the instance of an aspect and not the execution object at a join point. Next let s write a simple test program (listing 4.16) that creates two Account objects and calls methods on them.
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Listing 4.16 TestAssociation.java: testing associations
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public class TestAssociation { public static void main(String[] args) throws Exception { SavingsAccount account1 = new SavingsAccount(12245); SavingsAccount account2 = new SavingsAccount(67890); account1.credit(100); account1.debit(100); account2.credit(100); account2.debit(100); } }
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When we compile the classes and run the TestAssociation program, we see output similar to the following:
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> ajc *.java > java TestAssociation Creating aspect instance JoinPoint: execution(void Account.credit(float)) aspect: AssociationDemoAspect@187aeca object: SavingsAccount@e48e1b JoinPoint: execution(void Account.debit(float)) aspect: AssociationDemoAspect@187aeca object: SavingsAccount@e48e1b JoinPoint: execution(void Account.credit(float)) aspect: AssociationDemoAspect@187aeca object: SavingsAccount@12dacd1 JoinPoint: execution(void Account.debit(float)) aspect: AssociationDemoAspect@187aeca object: SavingsAccount@12dacd1
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Aspect instance creation
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The output shows that only one instance of the aspect is created, and that instance is available to all advice in the aspect.
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Aspect association
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4.3.2 Per-object association Oftentimes, reusable base aspects need to keep some per-object state consisting of the data that is associated with each object, without having sufficient information about the type of objects that will participate in the static crosscutting mechanism of member introduction. Consider a cache-management aspect that needs to track the last access time for each object in the cache so that it can remove from the cache objects that are not accessed for a long duration. Since such cache management is a reusable concept, we want to create a reusable base aspect. By associating a separate aspect instance with each object under cache management and by keeping the field definition for the last accessed time inside the base aspect, we can track the required information for each cache-managed object. The per-object association feature lets us associate a new aspect instance with an execution or target object by using a pointcut. In the following snippet, a new aspect instance is associated with each new execution object using perthis(), which matches the abstract access() pointcut:
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public abstract aspect CacheManagementAspect perthis(access()) { ... aspect's state instance members such as the last accessed time abstract pointcut access(); ... advice to access() pointcut to update the last accessed time ... advice using the aspect s state }
As an example, we can enable cache management in a banking application by simply creating a subaspect that provides a definition for the abstract access() pointcut:
public aspect BankingCacheManagementAspect extends CacheManagementAspect { pointcut access() : execution(* banking..Account+.*(..)) || execution(* banking..Customer+.*(..)); }
Now whenever a join point that is captured by the access() pointcut executes (such as the debit() method), and the execution object is not previously associated with a BankingCacheManagementAspect instance, a new instance of the aspect is created and associated with the execution object. The same scenario will take place with Customer objects as well. Effectively, the aspect s state now forms a part of each execution object s state. The advice in the base and derived aspects may then use the state of the aspect as if it were the cached object s state.
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With per-object associations, an aspect instance is associated with each object matching the association specification. You can specify two kinds of perobject associations:
perthis() Associates a separate aspect instance with the execution object (this) for the join point matching the pointcut specified inside perthis() pertarget() Associates a separate aspect instance with the target object for the join point matching the pointcut specified inside pertarget()
With object associations, the aspect instance is created when executing a join point of a matching object for the first time. Once an association is created between an object and an instance of the declaring aspect, the association is good for the lifetime of the object. Specifically, executing another matching join point on the same object does not create a new aspect with the object. Figure 4.3 illustrates object association using a UML sequence diagram. To illustrate, let s modify the aspect AssociationDemoAspect. Listing 4.17 shows the use of the perthis() association with the accountOperationExecution pointcut.
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