Types of Enterprise Beans

Session Beans

  • associated with one EJB Client
  • created and destroyed by the particular EJB Client
  • nonpersistant, do not survive a server shutdown/crash.
  • obviously used to model some service

Two types of session beans exist: stateful and stateless

The class implementing either stateless or stateful session bean implements the SessionBean interface: 

public interface SessionBean extends EnterpriseBean {
   void setSessionContext(SessionContext sessionContext)
      throws EJBException, RemoteException;
    // called by container to provide the bean a "session" context 
    // to allow it to access the runtime information

   void ejbRemove() throws EJBException, RemoteException;
    // the container notifies the bean that the bean instance 
    // is to be removed

   void ejbActivate() throws EJBException, RemoteException;
    // notifies the stateful bean that it has been activated 
    // (so it may eventually reacquire system resources that 
    // were released when the bean was previously passivated)

   void ejbPassivate() throws EJBException, RemoteException;
    // notifies the stateful bean that it is about to be passivated 
    // by the container
}

Entity Beans

  • always have states
  • may be shared by multiple EJB Clients
  • their states can be persisted and stored across multiple invocations
  • survive server shutdown/crash
  • encapsulate in their object reference a unique ID that points to their state

The class implementing entity bean implements the EntityBean interface: 

public interface EntityBean extends EnterpriseBean {
   public void setEntityContext(EntityContext ctx); 
    // called by container to pass a reference to the EntityContext interface 
    // to the bean instance. The entity context interface provides methods
    // to access properties of the runtime context

   public void unsetEntityContext();
    // called by the container before it terminates the life of the current 
    // instance of the entity bean. The bean can free resources that have 
    // been allocated during the setEntityContext() call (if any)
 
   public void ejbRemove();
    // removes the database entry or entries associated with this particular 
    // entity bean.  The container calls this method when a client invokes 
    // a remove() method

   public void ejbActivate();
    // notifies an entity bean that it has been activated. The container 
    // invokes this method on the instance selected from the pool
    // of available instances and assigned to a specific entity object 
    // identity. When activated, the entity bean instance has 
    // the opportunity to acquire additional resources it might need 

   public void ejbPassivate();
    // notifies an entity bean that it is about to be deactivated
    // -that is, the instance is about to be disassociated from an entity 
    // object identity and returned to the pool of available instances. 
    // The instance has the chance to release any resources allocated 
    // during the ejbActive() method which it might not want to hold

   public void ejbLoad();
    // refreshes the data the entity object represents from the database 

   public void ejbStore();
    // stores the data the entity object represents in the database 
}
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