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The wormhole pattern
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8.2.1 The current solution If you don t use AspectJ, there are two ways to pass the caller s context in a multithreaded environment: you can pass additional parameters containing context or you can use thread-specific storage to set and access the context information. In either case, multiple modules are involved in the logic that is passing the context. The first way of passing a parameter causes API pollution every method in the execution stack must have extra parameters to pass on the context collected. The second way requires the caller to create a ThreadLocal variable to store the context information and set its context. While the second approach avoids API pollution, it entails changes in both caller and callee implementation and requires an understanding of how the context is stored. 8.2.2 An overview of the wormhole pattern The basic idea behind the wormhole pattern, shown in figure 8.1, is to specify two pointcuts: one for the caller and the other for the callee, with the former collecting the context to be transferred through the wormhole. Then you specify the wormhole at the places of execution of the callee s join points in the control flow of a caller s join points.
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Figure 8.1 The wormhole pattern. Each horizontal bar shows a level in the call. The wormhole makes the object in the caller plane available to the methods in the called plane without passing the object through the call stack.
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In figure 8.1, each level in the call stack is depicted as a plane in the space. To transfer context from one plane to another, you would normally have to pass it on to the next plane until it reached the desired location. The wormhole pattern provides a path that cuts directly through the planes, which avoids having the context trickle through the levels from caller to callee. 8.2.3 The pattern template Let s create a template for the pattern that will allow you to plug the pattern into your system by simply replacing the entities in the template with concrete ones in your system:
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public aspect WormholeAspect { pointcut callerSpace(<caller context>) : <caller pointcut>; pointcut calleeSpace(<callee context>) : <callee pointcut>; pointcut wormhole(<caller context>, <callee context>) : cflow(callerSpace(<caller context>)) && calleeSpace(<callee context>); // advices to wormhole around(<caller context>, <callee context>) : wormhole(<caller context>, <callee context>) { ... advice body } }
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In this template we define a pointcut in the caller s space that collects the associated context. Similarly, we define a pointcut in the callee s space. The collected context in both cases could be an execution and target object as well as any parameters to the methods involved. We then create a wormhole through these two spaces with a pointcut that defines the join points captured by the calleeSpace() pointcut in the control flow of the join points captured by the callerSpace() pointcut. Since we have the context available for both of these join points, we can write advice to the wormhole() pointcut using this information. Let s look at a simple example using this pattern; for more complex and complete examples, refer to chapters 11 and 12. The aspect in listing 8.10 creates a wormhole between a transaction initiator such as an ATM, teller, or Internet bank and the actual account operations.
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The wormhole pattern
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Listing 8.10 AccountTransactionAspect.java
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public aspect AccountTransactionAspect { pointcut transactionSystemUsage(TransactionSystem ts) : execution(* TransactionSystem.*(..)) && this(ts); Using transaction
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system operations
Using account operations
pointcut accountTransactions(Account account, float amount) : this(account) && args(amount) && (execution(public * Account.credit(float)) || execution(public * Account.debit(float))); pointcut wormhole(TransactionSystem ts, Account account, float amount) : cflow(transactionSystemUsage(ts)) && accountTransactions(account, amount); before(TransactionSystem ts, Account account, float amount) returning : wormhole(ts, account, amount) { ... log the operation along with information about ... transaction system, perform authorization, etc. } }
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