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Postconditions often access more data than preconditions. This data includes: The result of the invoked method The values of the parameters object attributes before and/or after the execution of the invoked method The implementation of postconditions is illustrated by the example which purposely contains a bug that is shown in Listing 9-6. Listing 9-6. An Example of an Incorrect Increment Function package aop.contracts.postconditions; public class PostConditionExample2 { public int increment(int p) { return p++; }
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CHAPTER 9 QUALITY OF SERVICE AND AOP
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public static void main(String[] args) { PostConditionExample2 t = new PostConditionExample2(); System.out.println("incr mente 1 : "+t.increment(1)); } } The body of the increment method should be return ++p;. For this method, the postcondition is implemented by the interceptor that is shown in Listing 9-7. Listing 9-7. A Postcondition Interceptor for the Increment Function package aop.contracts.postconditions; import java.lang.reflect.Method; import import import import import org.jboss.aop.Interceptor; org.jboss.aop.Invocation; org.jboss.aop.InvocationResponse; org.jboss.aop.InvocationType; org.jboss.aop.MethodInvocation;
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public class PostConditionInterceptor implements Interceptor { public String getName() { return "PostConditionInterceptor"; } public InvocationResponse invoke(Invocation invocation) boolean incrementInvocation = false; int incrementParameterValue = 0; throws Throwable {
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if (invocation.getType() == InvocationType.METHOD) { MethodInvocation methodInvocation = (MethodInvocation)invocation; Method method = methodInvocation.method; if ("increment".equals(method.getName())) { incrementInvocation = true; incrementParameterValue = ((Integer) methodInvocation.arguments[0]) .intValue(); } } InvocationResponse rsp = invocation.invokeNext(); if (incrementInvocation) { int result = ((Integer) rsp.getResponse()).intValue(); if (result != (incrementParameterValue + 1)) { ... throw new Error(errorMsg.toString()); }
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CHAPTER 9 QUALITY OF SERVICE AND AOP
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} return rsp; } } The interceptor is activated by adding the pointcut in Listing 9-8 to the jboss-aop.xml file. Listing 9-8. The Deployment of the Postcondition Interceptor <interceptor-pointcut methodFilter="ALL" fieldFilter="NONE" constructorFilter="NONE" class="aop.contracts.postconditions.PostConditionExample"> <interceptors> <interceptor class="aop.contracts.postconditions.PostConditionInterceptor"/> </interceptors> </interceptor-pointcut> Now, if the program is executed, the trace messages in Listing 9-9 are obtained. Listing 9-9. The Trace of the Increment Function when the Postcondition is Applied java.lang.Error: Unfulfilled post-condition for increment. The result (1) is not equal to an incrementation of the passed parameter (1). ... Exception in thread "main" This example shows that a clean Design by Contract approach, implemented with AOP, will help you detect bugs concerning program values.
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Implementing Invariants
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There are two types of invariants class and internal that we have spoken about. A class invariant usually needs to be checked after the execution of a method that can potentially modify the state of the object. Not all methods should be checked. Indeed, an intermediate method, which is part of a higher-level method defined for the same class, may leave the object in an inconsistent state. It is only when the higher-level method ends that the object should be left in a consistent state. Selecting the methods that the invariant is checked for can be achieved either by advising only certain methods or by using cflow. The latter solution disables the checking of the invariant when the advised method is in the control flow of a higher-level method (usually belonging to the same class). This solution is more generic but slightly less efficient than the former one because the aspect system adds some run-time tests to determine whether the current method execution belongs to a given control flow. These run-time tests are called residues. The advice code in Listing 9-10 shows the implementation of an invariant that ensures that a state field always equals 1.
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CHAPTER 9 QUALITY OF SERVICE AND AOP
Listing 9-10. Advice Code for a state Invariant ... public InvocationResponse invoke(Invocation invocation) throws Throwable { InvocationResponse rsp = invocation.invokeNext(); if (invocation.getType() == InvocationType.METHOD) { MethodInvocation methodInvocation = (MethodInvocation)invocation; Method method = methodInvocation.method; Object target = methodInvocation.targetObject; int state = target.getClass().getDeclaredField("state").getInt(target); if (state != 1) { throw new Error("Broken class invariant."); } } return rsp; } ... As you can see, testing an invariant is similar to testing a postcondition, except that the target object s state has to be accessed through the java.lang.reflect API.
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