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Encoder QR Code 2d barcode in Java A simple example, shown in listing 4.19, might illustrate this concept better.

A simple example, shown in listing 4.19, might illustrate this concept better.
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Listing 4.19 TestAssociationScope.java
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public class TestAssociationScope { public static void main(String[] args) { A a = new A(); a.m(); } } class A { public void m() { B b = new B(); b.m(); } } class B { public void m() { } } aspect TestAspect { before() : !within(TestAspect) { System.out.println(thisJoinPoint); } }
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Advanced AspectJ
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When we compile and run this program, we see logging of each executed join point:
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> ajc *.java > java TestAssociationScope staticinitialization(TestAssociationScope.<clinit>) execution(void TestAssociationScope.main(String[])) call(A()) staticinitialization(A.<clinit>) preinitialization(A()) initialization(A()) execution(A()) call(void A.m()) execution(void A.m()) call(B()) staticinitialization(B.<clinit>) preinitialization(B()) initialization(B()) execution(B()) call(void B.m()) execution(void B.m())
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Now let s modify TestAspect to use the perthis() association.
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aspect TestAspect perthis(execution(void A.*())) { before() : !within(TestAspect) { System.out.println(thisJoinPoint); } }
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When we compile and run this again, we see that only the methods that match the execution(void A.*())) pointcut are advised:
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> ajc *.java > java TestAssociationScope execution(void A.m()) call(B()) call(void B.m())
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4.3.5 Comparing object association with member introduction It is possible to avoid using the perthis()/pertarget() association with a judicious use of static crosscutting using introduced fields. In that case, instead of keeping the state in an aspect, you introduce that state to the object being aspected. This kind of modification often leads to simpler design. For example, consider this aspect, which associates an aspect instance with each Account object. The aspect s state _minimumBalance effectively becomes part of the Account object s state:
public aspect MinimumBalanceAspect perthis(this(Account)) { private float _minimumBalance; ... methods and advice using _minimumBalance }
Aspect association
Now if we want to use member introduction instead of association, we can change the aspect in the following way:
public aspect MinimumBalanceAspect { private float Account._minimumBalance; ... methods and advice using _minimumBalance }
In this snippet, we use the member introduction mechanism to associate a new member _minimumBalance with each Account object. The result is identical in both snippets a new state is associated with each Account object. Certain reusable aspects, such as cache management, that need to work with diverse types of objects may not have any common shared type. For example, Customer and Account probably have no class or interface common to their inheritance hierarchy. Therefore, to introduce a state, you will first need to specify a common type using a declare parent. For example, you can declare the interface Cacheable to be a parent type of Account and Customer. Then you may introduce the required state to Cacheable. This way, you get the same effect as per-object association using a simple introduction mechanism. Developing reusable aspects using introduction instead of per-object association can get tricky. The main reason is that a reusable base aspect, unaware of the application-specific classes, cannot use the declare parents construct to specify a common type. While you can get around this issue by using a complex design, per-object association can offer an elegant alternative solution. When you re using per-object associations, the base aspect includes an abstract pointcut that associates the aspect with the object at the matching join points. Then, all that a derived aspect needs to do is provide a definition for that pointcut so that it captures join points whose associated objects need additional per-object state. 9 (section 9.7.2) provides a concrete example of the simplification of a reusable aspect using per-object association. The choice between use of per-object association and member introduction is a balance between elegance and simplicity. Experience is usually the best guide. 4.3.6 Accessing aspect instances Aspect instances are created automatically by the system according to the association specification. To access their state from outside the aspect, however, you will need to get its instance. For example, in a profiling aspect that collects duration for the execution of profiled methods, typically you would keep the profile data inside the profile aspect. When you need to retrieve this data, say from
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