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Introduction to EJBs: Part 3, Page 2

  • July 31, 2002
  • By Sams Publishing
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How Do I Deploy an EJB?

After an EJB is packaged, it can be deployed in an appropriate J2EE server. There is no limit to the number of times an EJB can be deployed as a part of different applications.

Remember that J2EE defines a separate role for the application deployer. It may be that for particular installations, databases, or other resource names need to be changed to match the local environment. When configuring the application, the deployer can alter this EJB or enterprise application metadata.

Plugging into the Container

When an EJB is deployed into a particular EJB container, the EJB must be plugged into that container. To do this, an EJBObject must be generated based on the EJB's remote interface. This EJBObject will be specific to that EJB container and will contain code that allows it to interface with that container to access security and transaction information. The container will examine the metadata supplied with the EJB to determine what type of security and transaction code is required in the EJBObject.

The container will also generate the home interface implementation so that calls to create, find, and destroy EJB instances are delegated to container-defined methods.

The container will examine the EJB and enterprise application metadata and hook up resource references. It will also provide an environment for the application components.

Finally, the container will register the home interface of the EJB with JNDI. This allows other application components to create and find EJBs of this type.

Performing the Deployment

As mentioned previously, when deploying an EJB or enterprise application, the application developer taking on the J2EE role of deployer can choose to alter certain of the metadata relating to the configuration of the application. Although this can be done manually, it is usually done through a GUI tool to make things easier and to keep things consistent.

After the EJB has been deployed, any subsequent changes to its functionality will mean that the EJB must be re-deployed. If the enterprise application or EJB is no longer needed, it should be undeployed from the container.

How Do I Use an EJB?

Given that EJBs are middle-tier business components, they are of little use without a client to drive them. As mentioned earlier, those clients can be Web components, standalone Java clients, or other EJBs.

Regardless of the type of client, using an EJB requires the same set of steps—namely, discovery, retrieval, use, and disposal. These steps are covered in the next three sections.

Discovery

To create or find an EJB, the client must call the appropriate method on the EJB's home interface. Consequently, the first step for the client is to get hold of a remote reference to the home interface. On Day 3, you looked at naming services and how these can be used to register information in a distributed environment. In a J2EE environment, such a naming service is accessible through JNDI and can be used to store references to EJB home interfaces.

The EJB container will have registered the home interface using the JNDI name specified during deployment (as part of the deployment descriptor). This is the name that the client should use to look up the home interface. Recall from the EJB deployment descriptor shown in Listing 4.5 that the EJB name specified was Agency. When deploying the EJB, the deployer has a chance to set the JNDI name by which clients will find this EJB. In this case, you would expect the deployer to simply set a JNDI name of ejb/Agency so that the client could find the home interface by looking up java:comp/env/ejb/Agency. The following code shows the initial lookup required:

try
 {
  InitialContext ic = new InitialContext();
  Object lookup = ic.lookup("java:comp/env/ejb/Agency");
AgencyHome home =
     (AgencyHome)PortableRemoteObject.narrow(lookup, AgencyHome.class);
  ...
 }
 catch (NamingException ex) { /* Handle it */ }
 catch (ClassCastException ex) { /* Handle it */ }

As you can see, because the reference returned from JNDI is just an object, you must narrow it to the home interface type you expect—in this case, AgencyHome. If there are any problems with the JNDI access or if the wrong object type is returned, a NamingException or ClassCastException will be thrown.

There is no magic here. The object returned by the JNDI is simply an RMI remote object stub. This stub represents the home interface remote object created by the container when the EJB was deployed. This can be seen in Figure 4.2.

Now that you have a reference to the home interface, you can create the EJB you want to use.

Retrieval and Use

You can now call the create() method you saw defined on the AgencyHome interface in Listing 4.4 as follows:

 try 
 {
  ...
  Agency agency = home.create();
  System.out.println("Welcome to: " + agency.getAgencyName());
  ...
 }
 catch (RemoteException ex) { /* Handle it */ }
 catch (CreateException ex) { /* Handle it */ }

The create() method returns a remote reference to the newly-created EJB. If there are any problems with the EJB creation or the remote connection, a CreateException or RemoteException will be thrown. CreateException is defined in the javax.ejb package, and RemoteException is defined in the java.rmi package, so remember to import these packages at the top of your client class.

Now that you have a reference to an EJB, you can call its methods. The previous code sample shows the getAgencyName() method being called on the returned Agency reference. Again, whenever you call a remote method that is defined in an EJB remote interface, you must be prepared to handle RemoteExceptions.


Note

You will see later that some types of EJB are found rather than created. In this case, all steps are the same except that the create() method is replaced by the appropriate finder method and find-related exceptions must be handled. You still end up with a remote reference to an EJB. All of this is covered later when Entity EJBs are discussed on Day 6.


Disposing of the EJB

You have now created and used an EJB. What happens now? Well, if you no longer need the EJB, you can get rid of it in exactly the same way that you would get rid of a local Java object or a remote Java object defined using RMI—by setting its reference to null as follows:

 // No longer need the agency EJB instance
 agency = null;

When the local RMI runtime detects that the remote object no longer has any local references, it will trigger remote garbage collection for that object, which means that its remote reference will time out. This will result in the object being de-referenced at the server-side. In the case of the simple Agency bean (a stateless Session bean), this will cause the bean to be destroyed.

Although it is possible to use the remove() method to get rid of the EJB, you would not normally use this for such a simple bean. Use of this method is discussed in more detail on Days 5 and 6.

Running the Client

You are now in a position to write a simple application client for the Agency EJB. After you have written it, you will want to compile and run it.

Before compiling your client, you should ensure that you have j2ee.jar on your classpath. This JAR file lives in the lib directory under J2EE_HOME. If you are using an enterprise IDE, you may find that all the relevant classes are already in your classpath.

To compile and run the client, you will need the following:

  • The J2EE classes. These must be accessible through the classpath.

  • Access to the EJB's home and remote interface class files via the classpath.

  • RMI stubs for the home and remote interfaces. These can either be installed on the local classpath or downloaded dynamically from the EJB server.

  • If the client does not have the JNDI name of the EJB compiled in, you may want to provide this on the command line or through a system property.

When you deploy the EJB, you should be able to ask the container for a client JAR file. This client JAR file will contain all of the classes and interfaces needed to compile the client (as defined in the previous bulleted list). You should add this client JAR file to your classpath when compiling your client.

In theory, this should be it. However, you will find that any form of security definition on the server will require you to authenticate yourself before you can run the application. In this case, you must explicitly use the client container to provide the required security mechanism.


Note

The client container is called runclient under the J2EE RI.


Monday's installment: What is an EJB and why use them?

Tuesday's installment: What's in an EJB and how do I create one?

Thursday's installment: Deploying and Using an EJB in the J2EE Reference Implementation


About the Author

Andy Longshaw is a consultant, writer and educator specializing in J2EE, XML, Web-based technologies and components, particularly the design and architecture decisions required to use these technologies successfully. Andy has been explaining technology for most of the last decade as a trainer and in conference sessions. A wild rumor suggests that some people have managed to stay awake in these sessions. Despite being well educated and otherwise fairly normal, Andy still subjects himself, and his family, to "trial by unpredictability" by watching Manchester City FC far more often than is healthy.

Andy and the other authors work for Content Master Ltd., a technical authoring company in the United Kingdom specializing in the production of training and educational materials. For more information on Content Master, please see their web site at www.contentmaster.com

Source of this material

This is the first of four installments that make up Chapter 4: Introduction to EJBs from the book Sams Teach Yourself J2EE in 21 Days (ISBN:0-672-32384-2) written by Martin Bond, Dan Haywood, Debbie Law, Andy Longshaw, and Peter Roxburgh, published by Sams Publishing.

To access the full Table of Contents for the book





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