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80 Chapter 4 A client s session duration could be as long as a browser window is open, perhaps connecting to an e-commerce site with deployed session beans. It could also be as long as your Java applet is running, as long as a standalone application is open, or as long as another bean is using your bean. The length of the client s session generally determines how long a session bean is in use that is where the term session bean originated. The EJB container is empowered to destroy session beans if clients time out. If your client code is using your beans for 10 minutes, your session beans might live for minutes or hours, but probably not weeks, months, or years. Typically session beans do not survive application server crashes, nor do they survive machine crashes. They are in-memory objects that live and die with their surrounding environments. In contrast, entity beans can live for months or even years because entity beans are persistent objects. Entity beans are part of a durable, permanent storage, such as a database. Entity beans can be constructed in memory from database data, and they can survive for long periods of time. Session beans are nonpersistent. This means that session beans are not saved to permanent storage, whereas entity beans are. Note that session beans can perform database operations, but the session bean itself is not a persistent object. Session Bean Subtypes All enterprise beans hold conversations with clients at some level. A conversation is an interaction between a client and a bean, and it is composed of a number of method calls between the client and the bean. A conversation spans a business process for the client, such as configuring a frame-relay switch, purchasing goods over the Internet, or entering information about a new customer. The two subtypes of session beans are stateful session beans and stateless session beans. Each is used to model different types of these conversations. Stateful Session Beans Some business processes are naturally drawn-out conversations over several requests. An example is an e-commerce Web store. As a user peruses an online e-commerce Web site, the user can add products to the online shopping cart. Each time the user adds a product, we perform another request. The consequence of such a business process is that the components must track the user s state (such as a shopping cart state) from request to request.
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Introduction to Session Beans CHAPTER 4 A session bean represents work being performed for client code that is calling it. Session beans are business process objects that implement business logic, business rules, algorithms, and workflow. For example, a session bean can perform price quoting, order entry, video compression, banking transactions, stock trades, database operations, complex calculations, and more. They are reusable components that contain logic for business processes. Let s examine the characteristics of session beans in detail and then code up a stateful session bean. Session Bean Lifetime A key difference between session beans and other bean types is the scope of their lives. A session bean instance is a relatively short-lived object. It has roughly the lifetime equivalent of a session or of the client code that is calling the session bean. Session bean instances are not shared between multiple clients. For example, if the client code contacted a session bean to perform order entry logic, the EJB container is responsible for creating an instance of that session bean component. When the client later disconnects, the application server may destroy the session bean instance.
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PA R T Two PA R T Two The Triad of Beans In Part Two of this book, we ll focus on the development details for implementing an EJB application. We ll discuss the three types of enterprise beans: session beans (Chapter 4), entity beans (Chapter 6), and message- driven beans (Chapter 9). We ll also explore their subtypes: stateless session beans, stateful session beans, session beans as Web Services (Chapter 5), bean-managed persistent entity beans (Chapter 7), and container-managed persistent entity beans (Chapter . Not only will we cover each of these conceptually, but we ll also write an example for each bean type. We ll end Part Two with a discussion of container-provided services (Chapter 10), such as security, the environment, and calling beans from other beans. Part Two is essential for those of you who are ready to delve into EJB programming fundamentals. It is essential groundwork to prepare yourself for the more advanced topics, such as transactions and EJB design strategies, which are coming in Part Three.
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Writing Your First Bean 75 // Bean implementation public class HelloBean implements SessionBean, HelloBusinessMethods { public String hello() { return Hello, World! ; } <...define other required callbacks...> } The only problem with this approach is that the local interface throws remote exceptions. If you can live with that, then this design strategy works. Summary In this chapter, you learned how to write the component interfaces, home interface, enterprise bean class, deployment descriptor, and Ejb-jar file. You also saw how to call beans using JNDI and RMI-IIOP. Congratulations are in order: It took a while, but you ve successfully completed your first Enterprise JavaBeans deployment!
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74 Chapter 3 There are two good reasons not to implement your bean s component interface: Reason 1. Component interfaces extend interfaces defined by Sun, such as javax.ejb.EJBObject or javax.ejb.EJBLocalObject. These superinterfaces define additional methods intended for client use, and you d therefore have to provide no-op implementations of those methods in your bean. Those methods have no place in your bean class. Reason 2. Let s assume your enterprise bean wants to call a method on a different enterprise bean, and you want to pass a reference to your bean as a parameter to the other bean s method (similar to passing the this parameter in Java). How can you do this in EJB? Remember that all clients call methods on EJB objects, not beans. Thus, if your bean calls another bean, you must pass a reference to your bean s EJB object, rather than a reference to your bean. The other bean should operate on your EJB object, and not your bean, because the other bean is a client, just like any other client, and all clients must go through EJB objects. The danger here is if your enterprise bean class implements your EJB object s remote interface. You could accidentally pass a reference to the bean itself, rather than pass a reference to the bean s EJB object. Because your bean implements the same interface as the EJB object, the compiler would let you pass the bean itself as a this parameter, which is an error. A Solution There is an alternative way to preserve compile-time checks of your method signatures. The approach is to contain your bean s business method signatures within a common superinterface that your remote interface extends and your bean implements. You can think of this superinterface as a business interface that defines your business methods and is independent of EJB. The following example illustrates this concept: // Business interface public interface HelloBusinessMethods { public String hello() throws java.rmi.RemoteException; } // EJB remote interface public interface HelloRemote extends javax.ejb.EJBObject, HelloBusinessMethods { } // EJB local interface public interface HelloLocal extends javax.ejb.EJBLocalObject, HelloBusinessMethods { }
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Writing Your First Bean 73 The actual parameters you need should be part of your EJB container s documentation. See the book s accompanying source code for examples of this. For your EJB client code to work, you must take care to distribute the correct class files on the right machines. If remote client code uses home interfaces and remote interfaces, then you must deploy those class files and the necessary client stub classes in your client environment. And because clients never directly access your bean implementation, you should not deploy your bean classes in your client environment. The Server-Side Output When we run the client, our container shows the following debug log. (Debug logs are great for seeing what your enterprise beans are doing.) setSessionContext() ejbCreate() hello() ejbRemove() As you can see, the container associated our bean with a session context, called create(), delegated a business method to the bean, and then called remove(). Note that some containers may give slightly different output than others it s all implementation-specific and part of EJB product differentiation. Keep this in mind when debugging your beans. The Client-Side Output After running the client, you should see the following output: Hello, World! Implementing Component Interfaces We wrap up this chapter with a quick design strategy. As you probably noticed, our enterprise bean class does not implement its own component interface (either remote interface or local interface). But why not? Doesn t the component interface seem like a natural fit for the interface to your bean? After all, the component interface defines every business method of the bean. Implementing your component interface would be a nifty way to perform compile-time checking to make sure your bean s method signature matches your component interface s signature.
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72 Chapter 3 obj, HelloHome.class); /* * Use the factory to create the Hello EJB Object */ Hello hello = home.create(); /* * Call the hello() method on the EJB object. The * EJB object will delegate the call to the bean, * receive the result, and return it to us. * * We then print the result to the screen. */ System.out.println(hello.hello()); /* * Done with EJB Object, so remove it. * The container will destroy the EJB object. */ hello.remove(); } } Source 3.8 (continued) The client code is self-explanatory. Running the System To try the deployment, you first must bring up the application server. This step varies depending on your vendor. Again, since we want to keep this book vendor- neutral, please see the book s accompanying source code for an example. Next, run the client application. When running the client, you need to supply the client with JNDI environment information. As we explain in Appendix A, JNDI requires a minimum of two properties to retrieve an initial context: The name of the initial context factory. Examples are com.sun.jndi .ldap.LdapCtxFactory for an LDAP JNDI context, and com.sun.jndi .cosnaming.CNCtxFactory for a CORBA Naming Service context. The provider URL, indicating the location of the JNDI tree to use. Examples are ldap://louvre:389/o5Airius.com and corbaloc::raccoon: 3700/NameService.
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Writing Your First Bean 71 The complete client source code is shown in Source 3.8. package examples; import javax.naming.Context; import javax.naming.InitialContext; import java.util.Properties; /** * This class is an example of client code that invokes * methods on a simple stateless session bean. */ public class HelloClient { public static void main(String[] args) throws Exception { /* * Setup properties for JNDI initialization. * * These properties will be read-in from * the command-line. */ Properties props = System.getProperties(); /* * Obtain the JNDI initial context. * * The initial context is a starting point for * connecting to a JNDI tree. We choose our JNDI * driver, the network location of the server, etc. * by passing in the environment properties. */ Context ctx = new InitialContext(props); /* * Get a reference to the home object - the * factory for Hello EJB Objects */ Object obj = ctx.lookup( HelloHome ); /* * Home objects are RMI-IIOP objects, and so * they must be cast into RMI-IIOP objects * using a special RMI-IIOP cast. * * See Appendix A for more details on this. */ HelloHome home = (HelloHome) javax.rmi.PortableRemoteObject.narrow( Source 3.8 HelloClient.java. (continued)
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70 Chapter 3 For clients to locate a home object, you must provide a JNDI nickname for your bean s home object. Clients will use this nickname to identify the home object they want. For example, our Hello World example might have a nickname HelloHome. You specify this nickname using the proprietary vendor-specific files that are bundled with your bean. When you deploy your bean into the container, the container automatically binds the nickname HelloHome to the home object. Then any client on any machine across a multitier deployment can use that nickname to find home objects, without regard to physical machine locations. Clients use the JNDI API to do this. JNDI goes over the network to some naming service, or JNDI tree, to look for the home object, perhaps contacting one or more naming services in the process. Eventually the home object is found, and a reference to it is returned to the client (see Figure 3.3). EJB Container/Server 4: Create EJB Object 3: Create a New EJB Object Home Interface Remote Interface Client Home Object EJB Object 1: Retrieve Home Object Reference 6: Invoke Business Method 5: Return EJB Object Reference Enterprise Bean 7: Delegate Request to Bean 2: Return Home Object Reference JNDI Naming Service Such as LDAP Figure 3.3 Acquiring a reference to a home object.
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Writing Your First Bean 69 resources across a network. Some examples of directory service products are Directory Server (iPlanet), Active Directory (Microsoft), and Lotus Notes Domino Server (IBM). Corporations traditionally have used naming and directory services to store user names, passwords, machine locations, printer locations, and so on. EJB servers exploit naming services to store location information for resources that your application code uses in an enterprise deployment. These resources could be EJB home objects, enterprise bean environment properties, database drivers, message service drivers, and other resources. By using naming services, you can write application code that does not depend on specific machine names or locations. This is all part of the EJB location transparency, and it keeps your code portable. If you decide later that resources should be located elsewhere, your code does not need to be rebuilt because the naming service can simply be updated to reflect the new resource locations. This greatly enhances maintenance of a multitier deployment that may evolve over time. This becomes absolutely necessary when purchasing prewritten software (such as enterprise beans), because your purchased components source code will likely not be made available to you to change. While naming and directory servers have typically run standalone, they can also run in the same process as the application server. Many containers are written in Java, and so their naming and directory services are just bunches of Java classes that run inside of the container. Unless you re using CORBA, the de facto API used to access naming and directory services is JNDI, which we explain in Appendix A. JNDI adds value to your enterprise deployments by providing a standard interface for locating users, machines, networks, objects, and services. For example, you can use the JNDI to locate a printer on your corporate intranet. You can also use it to locate a Java object or to connect with a database. In EJB, JNDI is used to look up home objects. JNDI is also useful for locating resources across an enterprise deployment, including environment properties, database resources, and more; we ll show you how to leverage JNDI for these purposes in Chapter 10. How to Use JNDI to Locate Home Objects To achieve location transparency, EJB containers mask the specific locations of home objects from your enterprise beans client code. Clients do not hard-code the machine names that home objects reside on; rather, they use JNDI to look up home objects. Home objects are physically located somewhere on the net- work perhaps in the address space of an EJB container residing on machine #1, or perhaps on a container residing on machine #2. As a developer who writes client code to use beans, you don t care.
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