barcode 128 generator vb.net Distinguish Between Stateful and Stateless Session Beans in Java

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Distinguish Between Stateful and Stateless Session Beans
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Both local and remote interfaces provide the following services to a client:
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n Delegate business method invocations on a session bean instance. n Return the session object s home interface. n Test to determine whether a session object is identical to another session object. n Remove a session object.
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Any method on a session bean class that is to be made visible to a client must be added to the bean s remote interface. This makes it possible to hide session bean methods from clients as well as make different methods of a session bean available using different interfaces. Figure 7-2 illustrates how the client will see the EJB session bean interfaces. When the application is deployed, the container or application server will use the interfaces defined by the enterprise bean provider and create EJBHome, EJBObject, stub, and tie classes:
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n The EJBHome class is used to create instances of the session bean class and
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the EJBObject class.
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n The EJBObject class provides access to the desired methods of the session bean. n The stub classes act as proxies to the remote EJBObjects. n The tie classes provide the call and dispatch mechanisms that bind the proxy
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to the EJBObject.
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FIGURE 7-2
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EJB session bean interface exposed to a client
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7: Enterprise JavaBeans and the EJB Container Model
Creating a Remote Interface (Prior to EJB 3.0) All remote interfaces must extend the class javax.ejb.EJBObject. The following is an example:
public interface ValidateInputRemote extends EJBObject { .. }
Methods in Remote Interfaces (Prior to EJB 3.0) The enterprise bean provider provides the session bean s remote interface, which extends javax.ejb.EJBObject, and the EJB container implements this interface. An enterprise bean s remote interface provides the client s view of a session object and defines the business methods that are callable by the client. All business methods declared in the remote interface must have the same parameters and same return value types as the bean class. It is not necessary for all bean class methods to be exposed to the remote client. All business methods declared in the remote interface must also throw at least the same exceptions as those in the bean class. They must also throw the java.rmi .RemoteException exception, because EJBs are dependent on the RMI package, specifically the java.rmi.Remote package, for distributed processing. Normally, the container or application server being used will generate the necessary remote interface code. The container should also update this code when changes are made to the bean class. The method names and the signatures in the remote interface must be identical to the method names and signatures of the business methods defined by the enterprise bean. This is different from the home interface, where method signatures must match, as method names can be different. In addition to business methods that may be defined in the remote interface, the methods listed here and shown in Figure 7-3 must be contained inside the remote interface:
n getEJBHome()
This method returns a reference to the session bean s home
interface.
n getHandle()
This method returns a handle for the EJBObject. This handle can be used at a later time to reobtain a reference to the EJBObject. A session object handle can be serialized to a persistent data store to enable the retrieval of a session object even beyond the lifetime of a client process. This is assuming that the EJB container does not crash or time out the session object, thereby destroying it.
Distinguish Between Stateful and Stateless Session Beans
n getPrimaryKey()
This method is not to be used for session beans. It returns the session bean object s primary key, but since individual session objects are to be used only by the specific client that creates them, they are intended to appear anonymous. If getPrimaryKey() is called looking for the identity of a session object, the method will throw an exception. This is different from entity objects, which expose their identity as primary keys. This method is used to test whether the EJBObject passed is identical to the invoked EJBObject. This method is used to remove a session bean object.
n isIdentical() n remove()
Note that these methods are included automatically by virtue of inheritance. A typical remote interface definition for a session bean looks something like this:
import javax.ejb.*; import java.rmi.*; import java.util.*; public interface ValidateInputRemote extends EJBObject { public void isInt (double amount) throws RemoteException; public void isNum (double amount) throws RemoteException; }
Creating a Home Interface (Prior to EJB 3.0) Session beans are instantiated when a client makes a call to one of the create() methods defined in the home interface. The home interface contains a create() method for every corresponding ejbCreate() method in the bean class.
FIGURE 7-3
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