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AOP in the EJB world: interceptors
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</interceptor-class> </interceptor-binding> </assembly-descriptor>
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An interesting question that might have already crossed your mind is what would happen if you specified default, class-, and method-level interceptors for a specific target method (yes, this is perfectly legal). In which order do you think the interceptors would be triggered Somewhat counterintuitive to how Java scoping typically works, the interceptors are called from the larger scope to the smaller scope. That is, the default interceptor is triggered first, then the class-level interceptor, and finally the methodlevel interceptor. Figure 5.4 shows this behavior. If more than one interceptor is applied at any given level, they are executed in the order in which they are specified. In our ActionBazaarLogger and BidStatisticsTracker example, the ActionBazaarLogger is executed first since it appears first in the comma-separated list in the @Interceptors annotation:
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@Interceptors({ActionBazaarLogger.class, BidStatisticsTracker.class})
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Unfortunately, the only way to alter this execution order is to use the interceptor-order element in the deployment descriptor; there are no annotations for changing interceptor order. However, you can disable interceptors at the default or class levels if you need to. Applying the @javax.interceptor.ExcludeDefaultInterceptors annotation on either a class or a method disables all default interceptors on the class or method. Similarly the @javax.interceptor.ExcludeClassInterceptors annotation disables class-level interceptors for a method. For example, both default and class-level interceptors may be disabled for the addBid method using the following code:
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@Interceptors(ActionBazaarLogger.class) @ExcludeDefaultInterceptors
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Figure 5.4 The order in which business method interceptors are invoked. Default interceptors apply to all methods of all EJBs in an ejb-jar package. Class-level interceptors apply to all methods of a specific class. Method-level interceptors apply to one specific method in a class. Default application-level interceptors are invoked first, then class-level interceptors, then method-level interceptors.
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@ExcludeClassInterceptors public void addBid (...
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Next we ll take a detailed look at the interceptor classes themselves.
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5.3.4 Implementing business interceptors Like the EJB lifecycle callback methods that we discussed in chapters 3 and 4, business interceptors can be implemented either in the bean class itself or in separate classes. However, we recommend that you create interceptor methods external to the bean class, because that approach allows you to separate crosscutting concerns from business logic and you can share the methods among multiple beans. After all, isn t that the whole point of AOP As you can see in listing 5.2, following the general EJB 3 philosophy, an interceptor class is simply a POJO that may have a few annotations.
Around invoke methods It s important to realize that an interceptor must always have only one method that is designated as the around invoke (AroundInvoke) method. Around invoke methods must not be business methods, which means that they should not be public methods in the bean s business interface(s). An around invoke method is automatically triggered by the container when a client invokes a method that has designated it to be its interceptor. In listing 5.2, the triggered method is marked with the @AroundInvoke annotation:
@AroundInvoke public Object logMethodEntry( InvocationContext invocationContext) throws Exception { System.out.println("Entering method: " + invocationContext.getMethod().getName()); return invocationContext.proceed(); }
In effect, this means that the logMethodEntry method will be executed whenever the ActionBazaarLogger interceptor is triggered. As you might gather from this code, any method designated AroundInvoke must follow this pattern:
Object <METHOD>(InvocationContext) throws Exception
The InvocationContext interface passed in as the single parameter to the method provides a number of features that makes the AOP mechanism extremely flexible. The logMethodEntry method uses just two of the methods included in the interface. The getMethod().getName() call returns the name of the method being intercepted addBid in our case.
AOP in the EJB world: interceptors
The call to the proceed method is extremely critical to the functioning of the interceptor. In our case, we always return the object returned by InvocationContext.proceed in the logMethodEntry method. This tells the container that it should proceed to the next interceptor in the execution chain or call the intercepted business method. On the other hand, not calling the proceed method will bring processing to a halt and avoid the business method (and any other interceptor down the execution chain) from being called. This feature can be extremely useful for procedures like security validation. For example, the following interceptor method prevents the intercepted business method from being executed if security validation fails:
@AroundInvoke public Object validateSecurity(InvocationContext invocationContext) throws Exception { if (!validate(...)) { throw new SecurityException("Security cannot be validated. " + "The method invocation is being blocked."); } return invocationContext.proceed(); }
The InvocationContext interface The InvocationContext interface has a number of other useful methods. Here is the definition of the interface:
public interface InvocationContext { public Object getTarget(); public Method getMethod(); public Object[] getParameters(); public void setParameters(Object[]); public java.util.Map<String,Object> getContextData(); public Object proceed() throws Exception; }
The getTarget method retrieves the bean instance that the intercepted method belongs to. This method is particularly valuable for checking the current state of the bean through its instance variables or accessor methods. The getMethod method returns the method of the bean class for which the interceptor was invoked. For AroundInvoke methods, this is the business method on the bean class; for lifecycle callback interceptor methods, getMethod returns null. The getParameters method returns the parameters passed to the intercepted method as an array of objects. The setParameters method, on the other hand, allows us to change these values at runtime before they are passed to the method.
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