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another thread that will print the latest profile information, you have to get the aspect instance first. For all types of aspect associations, you can get the aspect instance using the static method aspectOf() that is available for each aspect. The method returns an instance of the aspect. For a profiler case, we could retrieve the data as follows:
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Map profileData = ProfilerAspect.aspectOf().getProfileData();
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If the getProfileData() method were static (which would require data returned to be marked static), we could directly access the data using ProfilerAspect.getProfileData() irrespective of the association specification. In certain cases, such as when using third-party aspects or aspects with a class as a base, it may not be possible to mark certain members static due to other design considerations. In any case, marking a certain state static for easy access may not be a good practice and may prevent reusability through the use of different aspect associations. Each aspect contains two static methods aspectOf() to obtain the associated aspect instance and hasAspect() to check if an instance is associated. For aspects with default and control-flow association, both these methods take no arguments, whereas for aspects with per-object association, these methods take one argument of type Object to specify the object for which the associated aspect instance is sought. In all cases, the aspectOf() method returns the instance of an aspect if one is associated; otherwise, it throws a NoAspectBoundException. The method hasAspect() returns true if an aspect instance is associated; otherwise, it returns false. Note that since an aspect instance with a control-flow-based association lives only during the control flow (or below, for percflowbelow()), you can get the aspect instance only in the control flow associated with the aspect.
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4.4 Exception softening
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Java specifies two categories of exceptions that can be thrown by a method: checked and unchecked exceptions. When an exception is checked, callers must deal with it either by catching the exception or by declaring that they can throw it. Unchecked exceptions, which can be either RuntimeException or Error, do not need to be dealt with explicitly. Exception softening allows checked exceptions thrown by specified pointcuts to be treated as unchecked ones. Softening eliminates the need to either catch the exception or declare it in the caller s method specification.
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Exception softening
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The softening feature helps to modularize the crosscutting concerns of exception handling. For example, you can soften a RemoteException thrown in a Remote Method Invocation (RMI)-based system to avoid handling the exception at each level. This may be a useful strategy in some situations. For instance, if you know that you are using local objects of RMI-capable classes that won t throw any RemoteException, you can soften those exceptions. To soften exceptions, you use the declare soft construct that takes the following form:
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declare soft : <ExceptionTypePattern> : <pointcut>;
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If a method is throwing more than one checked exception, you will have to individually soften each one. In listing 4.20, the aspect declares the softening of an exception thrown by the TestSoftening.perform() method. The method now behaves as if it is throwing an org.aspectj.lang.SoftException, which extends RuntimeException.
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Listing 4.20 TestSoftening.java: code for testing the effect of softening an exception
import java.rmi.RemoteException; public class TestSoftening { public static void main(String[] args) { TestSoftening test = new TestSoftening(); test.perform(); } public void perform() throws RemoteException { throw new RemoteException(); } }
Compiling the TestSoftening class will result in a compiler error, since main() neither catches the exception nor declares that it is throwing that exception:
> ajc TestSoftening.java F:\aspectj-book\ch04\section4.4\TestSoftening.java:6 Unhandled exception type RemoteException test.perform(); ^^^^^^^^^^^^^^
Listing 4.21 shows SofteningTestAspect, which softens the RemoteException thrown by the join point that corresponds to the call to the TestSoftening.perform() method.
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Listing 4.21 Softening aspect
import java.rmi.RemoteException; public aspect SofteningTestAspect { declare soft : RemoteException : call(void TestSoftening.perform()); }
By softening the exception, we can compile the code without errors. When we run the program, we see a call stack due to a thrown SoftException:
> ajc TestSoftening.java SofteningTestAspect.java > java TestSoftening Exception in thread "main" org.aspectj.lang.SoftException at TestSoftening.main(TestSoftening.java:6)
An aspect declaring an exception for a join point wraps the join point execution in a try/catch block. The catch block catches the original exception, and the throw block throws a SoftException that wraps the original exception. This means that in listing 4.21, if we were to specify execution instead of call in the pointcut, the compiler would still give us a compiler error for the unhandled exception. To illustrate this, let s look at the code in listings 4.20 and 4.21 again. First let s see that compiling TestSoftening together with SofteningTestAspect results in a woven TestSoftening class that looks like the following:
import java.rmi.RemoteException; public class TestSoftening { public static void main(String[] args) { TestSoftening test = new TestSoftening(); try { test.perform(); } catch (RemoteException ex) { throw new SoftException(ex); } } public void perform() throws RemoteException { throw new RemoteException(); } }
The portion marked in bold shows the effective code that was inserted due to SofteningTestAspect . As you see, the RemoteException is now caught by the main() method, which throws a SoftException wrapping the caught exception. Since the SoftException is an unchecked exception, main() no longer needs to declare that it can throw it.
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