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Learning advanced EJB concepts
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5.3.2 What are interceptors
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Essentially the EJB rendition of AOP, interceptors are objects that are automatically triggered when an EJB method is invoked (interceptors are not new concepts and date back to technologies like CORBA). While EJB 3 interceptors provide sufficient functionality to handle most common crosscutting concerns (such as in our logging example), they do not try to provide the level of functionality that a fullscale AOP package such as AspectJ offers. On the flip side, EJB 3 interceptors are also generally a lot easier to use. Recall our discussion in section 5.1 on how the EJB object provides services such as transactions and security. In essence, the EJB object is essentially a sophisticated built-in interceptor that makes available a whole host of functionality. If you wanted to, you could create your own EJB -esque services using interceptors. In the pure AOP world, interception takes place at various points (called point cuts) including at the beginning of a method, at the end of a method, and when an exception is triggered. If you are familiar with AOP, an EJB interceptor is the most general form of interception it is an around invoke advice. EJB 3 interceptors are triggered at the beginning of a method and are around when the method returns; they can inspect the method return value or any exceptions thrown by the method. Interceptors can be applied to both session and message-driven beans. Let s examine business method interceptors further by implementing basic logging on the PlaceBid session bean from chapter 2. Once you understand how this works, applying it to an MDB should be a snap. Figure 5.3 shows a
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Figure 5.3 Business interceptors are typically used to implement common code. The ActionBazaarLogger implements common logging code used by all EJBs in the ActionBazaar system.
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AOP in the EJB world: interceptors
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business method interceptor that implements common logging code in the ActionBazaar application. Listing 5.2 contains the code for our interceptors. The interceptor attached to the addBid method will print a log message to the console each time the method is invoked. In a real-world application, this could be used as debugging information (and perhaps printed out using java.util.logging or Log4J).
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Listing 5.2 EJB business method interceptors
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@Stateless public class PlaceBidBean implements PlaceBid { ... @Interceptors(ActionBazaarLogger.class) public void addBid(Bid bid) { ... } }
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Attaching interceptor
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public class ActionBazaarLogger { @AroundInvoke Specifying interceptor method public Object logMethodEntry( InvocationContext invocationContext) throws Exception { System.out.println("Entering method: + invocationContext.getMethod().getName()); return invocationContext.proceed(); } }
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Let s take a bird s-eye view of this code before analyzing each feature in detail in the coming sections. The interceptor class, ActionBazaarLogger, is attached to the addBid method of the PlaceBid stateless session bean using the @javax. interceptor.Interceptors annotation b. The ActionBazaarLogger object s logMethodEntry method is annotated with @javax.interceptor.AroundInvoke and will be invoked when the addBid method is called C. The logMethodEntry method prints a log message to the system console, including the method name entered using the javax.interceptor.InvocationContext. Finally, the invocation context s proceed method is invoked to signal to the container that the addBid invocation can proceed normally. We will now start a detailed analysis of the code, starting with attaching the interceptor using the @Interceptors annotation.
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5.3.3 Specifying interceptors
The @Interceptors annotation allows you to specify one or more interceptor classes for a method or class. In listing 5.2 we attach a single interceptor to the addBid method:
@Interceptors(ActionBazaarLogger.class) public void addBid (...
You can also apply the @Interceptors annotation to an entire class. When you do, the interceptor is triggered if any of the target class s methods are invoked. For example, if the ActionBazaarLogger is applied at the class level as in the following code, our logMethodEntry method will be invoked when the PlaceBid class s addBid or addTimeDelayedBid method is called by the client (imagine that the addTimeDelayedBid method adds a bid after a specified interval of time):
@Interceptors(ActionBazaarLogger.class) @Stateless public class PlaceBidBean implements PlaceBid { public void addBid (... public void addTimeDelayedBid (... }
As we explained, the @Interceptors annotation is fully capable of attaching more than one interceptor either at a class or method level. All you have to do is provide a comma-separated list as a parameter to the annotation. For example, a generic logger and a bidding statistics tracker could be added to the PlaceBid session bean as follows:
@Interceptors({ActionBazaarLogger.class, BidStatisticsTracker.class}) public class PlaceBidBean { ... }
Besides specifying method- and class-level interceptors, you can create what is called a default interceptor. A default interceptor is essentially a catchall mechanism that attaches to all the methods of every bean in the EJB module. Unfortunately, you cannot specify these kinds of interceptors by using annotations and must use deployment descriptor settings instead. We won t discuss deployment descriptors in any great detail at this point, but we ll show you how setting the ActionBazaarLogger class as a default interceptor for the ActionBazaar application might look:
<assembly-descriptor> <interceptor-binding> <ejb-name>*</ejb-name> <interceptor-class> actionbazaar.buslogic.ActionBazaarLogger
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