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3.2.3 Specifying bean business interfaces
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In section 3.1, we introduced you to EJB interfaces. Now let s explore a bit more how they work with stateless session beans. Client applications can invoke a stateless session bean in three different ways. In addition to local invocation within the same JVM and remote invocation through RMI, stateless beans can be invoked remotely as web services. Three types of business interfaces correspond to the different access types; each is identified through a distinct annotation. Let s take a detailed look at these annotations. Local interface A local interface is designed for clients of stateless session beans collocated in the same container (JVM) instance. You designate an interface as a local business interface by using the @Local annotation. The following could be a local interface for the BidManagerBean class in listing 3.1:
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Building business logic with session beans
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@Local public interface BidManagerLocal { void addBid(Bid bid); void cancelBid(Bid bid); List<Bid> getBids(Item item); }
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Local interfaces don t require any special measures in terms of either defining or implementing them. Remote interface Clients residing outside the EJB container s JVM instance must use some kind of remote interface. If the client is also written in Java, the most logical and resourceefficient choice for remote EJB access is Java Remote Method Invocation (RMI). In case you are unfamiliar with RMI, we provide a brief introduction to RMI in appendix A. For now, all you need to know is that it is a highly efficient, TCP/IPbased remote communication API that automates most of the work needed for calling a method on a Java object across a network. EJB 3 enables a stateless bean to be made accessible via RMI through the @Remote annotation. The BidManager business interface in our example uses the annotation to make the bean remotely accessible:
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@Remote public interface BidManager extends Remote { ... }
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A remote business interface may extend java.rmi.Remote as we ve done here, although this is optional. Typically the container will perform byte-code enhancements during deployment to extend java.rmi.Remote if your bean interface does not extend it. Remote business interface methods are not required to throw java.rmi.RemoteException unless the business interface extends the java.rmi. Remote interface. Remote business interfaces do have one special requirement: all parameters and return types of interface methods must be Serializable. This is because only Serializable objects can be sent across the network using RMI. Web service endpoint interface The third type of interface is specific to stateless session beans that you haven t seen yet: the web service endpoint interface (also known as SEI). The ability to expose a stateless session bean as a SOAP-based web service is one of the most powerful features of EJB 3. All you need to do to make a bean SOAP accessible is mark a
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Stateless session beans
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business interface with the @javax.jws.WebService annotation. The following defines a simple web service endpoint interface for the BidManagerBean:
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@WebService public interface BidManagerWS { void addBid(Bid bid); List<Bid> getBids(Item item); }
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Note we have omitted the cancelBid bean method from the interface; we don t want this functionality to be accessible via a web service, although it is accessible locally as well as remotely through RMI. The @WebService annotation doesn t place any special restrictions on either the interface or the implementing bean. We discuss EJB web services in greater detail in chapter 15. Working with multiple business interfaces Although it is tempting, you cannot mark the same interface with more than one access type annotation. For example, you cannot mark the BidManager interface in listing 3.1 with both the @Local and @Remote annotations instead of creating separate BidManagerLocal (local) and BidManager (remote) interfaces, although both interfaces expose the exact same bean methods. However, a business interface can extend another interface, and you can remove code duplication by creating a business interface that has common methods and business interfaces that extend the common parent interface. For example, you can create a set of interfaces utilizing OO inheritance as follows:
public interface BidManager{ void addBid(Bid bid); List<Bid> getBids(Item item); } @Local public interface BidManagerLocal extends BidManager { void cancelBid(Bid bid); } @Remote public interface BidManagerRemote extends BidManagerLocal { } @WebService public interface BidManagerWS extends BidManager { }
If you want, you can apply the @Local, @Remote, or @WebService annotation in the bean class without having to implement the business interface as follows:
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