barcode 128 generator vb.net 7: Enterprise JavaBeans and the EJB Container Model in Java

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7: Enterprise JavaBeans and the EJB Container Model
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// Call the getMsg() method on the EJB object. // The remote reference will delegate the call // to the bean code, receive the response and // then pass it to this client. System.out.println(mySLSessionRemote.getMsg()); // When finished with the remote reference, // remove it and the EJB container will then // destroy the EJB object. mySLSessionRemote.remove(); } private static Context getInitialContext() throws Exception { // This implementation is specific to the Weblogic // server and will differ for other server vendors. String providerUrl = "t3://localhost:7001"; String icFactory = "weblogic.jndi.WLInitialContextFactory"; String user = null; String password = null; Properties properties = null; try { properties = new Properties(); properties.put(Context.INITIAL_CONTEXT_FACTORY, icFactory); properties.put(Context.PROVIDER_URL, providerUrl); if (user != null) { properties.put(Context.SECURITY_PRINCIPAL, user); properties.put(Context.SECURITY_CREDENTIALS, password == null "" : password); } return new InitialContext(properties); } catch(Exception e) { System.out.println( "Unable to connect to JNDI server at " + providerUrl); throw e; } } }
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Here is the output information provided by the client:
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This is a message from a stateless session bean!
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Here is the output information provided by the client and the application server console:
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...when the jar is deployed to the server... Sat Jan 20 12:58:55 EST 2007:SLSessionBean:4536570 ejbCreate()
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Distinguish Between Stateful and Stateless Session Beans
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Sat Jan ...when Sat Jan ...when Sat Jan Sat Jan
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20 12:58:55 EST 2007:SLSessionBean:2076276 the client application is executed... 20 12:59:15 EST 2007:SLSessionBean:2076276 the server is shut down... 20 13:01:33 EST 2007:SLSessionBean:2076276 20 13:01:33 EST 2007:SLSessionBean:4536570
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ejbCreate() getMsg() ejbRemove() ejbRemove()
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The next section will cover the client side of the calls in a little more detail.
Calling Stateless Session Beans from a Client
When a client calls an EJB, the ultimate goal is to gain the benefits derived from executing the business methods on a bean class. Before the EJB can get access to these methods, it must first find the EJBHome interface necessary to make an instance of the EJB class. The first step in the process of finding the EJBHome starts with creating an InitialContext class.
InitialContext The InitialContext class acts as the client s interface to the JNDI interface. The InitialContext may contain information that will allow a client to bind to many naming services such as JNDI, CORBA Common Object Service ( COS ), and Domain Naming Service (DNS). Using the InitialContext class allows a client to have to maintain only a single interface to any naming service in the client s environment that supports JNDI. If any problems are encountered with the creation of the InitialContext object or with calling one of its methods, the javax.naming .NamingException will be thrown. More on Type Narrowing In a stand-alone Java application, if a Java object such as Object is returned from a Hashtable, the return type of the method get() will be the supertype Object instead of the derived type String. Here s an example:
Hashtable hash = ..; ..; hash.get("keyToAStringElement");
It is up to the developer to cast, or narrow, the return value of the method get() to the proper object type. For example:
String aString = (String) hash.get("keyToAStringElement");
In the EJB application, once an object reference is obtained by a client, the method javax.rmi.PortableRemoteObject.narrow() must be used to perform type-narrowing for its client-side representation of its home and remote interfaces.
7: Enterprise JavaBeans and the EJB Container Model
The javax.rmi.PortableRemoteObject class is part of the RMI-IIOP standard extension. Type-narrowing ensures that the client programs are interoperable with different EJB containers. Once the InitialContext has been used as the starting point for looking up a specific JNDI registered object, the javax.rmi.PortableRemoteObject .narrow() method should be called to perform type-narrowing of the client-side representations of the home interface.
Finding Objects and Interfaces:The JNDI Clients that have access to the JNDI API may use this API to look up enterprise beans, resources such as databases, and data in environment variables. From the earlier example, the client application locates an EJB with the following:
// Get a reference to the home object Object obj = ctx.lookup("SLSessionRemoteHome");
The name that a Java Platform EE client uses to refer to an EJB does not necessarily have to be identical to the JNDI name of the EJB deployed in the EJB container or application server. The level of indirection provided by the ability to map Java Platform EE client names to JNDI registered EJBs gives great flexibility to distributed applications by allowing the client to use names that reference EJBs that make logical sense to the client. The client even has the ability to reference a single EJB with different names, when it makes sense to do so. This flexibility comes in handy when either the client code or the server code changes dynamically. The name that a stand-alone Java client uses to refer to an EJB using a JNDI lookup method must be identical to the EJBs JNDI name in the EJB container or application server.
Creating an Instance Using EJBHome Finally, after the home reference has been found, narrowed, and called, the home reference s create() method can be called, returning the remote reference upon which the desired business methods can be invoked. The syntax for the calling of the create() method may look somewhat like this:
SLSessionRemote mySLSessionRemote = home.create();
Remember that create() returns a reference to a remote interface, not the bean object itself. This means that certain programming practices that may be taken for granted in Java may not be used with EJBs. For example, objects are passed to EJBs via the arguments of the method calls and results are passed back to the client
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