7.2. The Life Cycle of a Stateless Session Bean
Just as the entity bean has a well-defined life cycle, so does the stateless session bean. The stateless session bean's life cycle has two states: the Does Not Exist state and the Method-Ready Pool. The Method-Ready Pool is similar to the instance pool used for entity beans. This is one of the significant life-cycle differences between stateless and stateful session beans; stateless beans define instance pooling in their life cycle and stateful beans do not.Figure 7-1 illustrates the states and transitions that a stateless session bean instance goes through in its lifetime.
Figure 7-1. Stateless session bean life cycle
7.2.1. Does Not Exist
7.2.2. The Method-Ready Pool
Stateless bean instances enter the Method-Ready Pool as the container needs them. When the EJB server is first started, it will probably create a number of stateless bean instances and enter them into the Method-Ready Pool. (The actual behavior of the server depends on the implementation.) When the number of stateless instances servicing client requests is insufficient, more can be created and added to the pool.
22.214.171.124. Transitioning to the Method-Ready Pool
When an instance transitions from the Does Not Exist state to the Method-Ready Pool, three operations are performed on it. First, the bean instance is instantiated by invoking the Class.newInstance() method on the stateless bean class.
Second, the SessionBean.setSessionContext(SessionContextcontext) method is invoked on the bean instance. This is when the instance receives its reference to the EJBContext for its lifetime. The SessionContext reference may be stored in a nontransient instance field of the stateless session bean.
Finally, the no-argument ejbCreate() method is invoked on the bean instance. Remember that a stateless session bean only has one ejbCreate() method, which takes no arguments. The ejbCreate() method is invoked only once in the life cycle of the stateless session bean; when the client invokes the create() method on the EJB home, it is not delegated to the bean instance. This is significantly different from both entity beans and stateful session beans.
Stateless session beans are not subject to activation, so they can maintain open connections to resources for their entire life cycle. The ejbRemove() method should close any open resources before the stateless session bean is evicted from memory at the end of its life cycle. More about ejbRemove() later in this section.
126.96.36.199. Life in the Method-Ready Pool
Once an instance is in the Method-Ready Pool, it is ready to service client requests. When a client invokes a business method on an EJB object, the method call is delegated to any available instance in the Method-Ready Pool. While the instance is executing the request, it is unavailable for use by other EJB objects. Once the instance has finished, it is immediately available to any EJB object that needs it. This is slightly different from the instance pool for entity beans, described in Chapter 6, "Entity Beans". In the entity instance pool, a bean instance might be swapped in to service an EJB object for several method invocations. Stateless session instances are only dedicated to an EJB object for the duration of the method.
Although vendors can choose different strategies to support stateless session beans, it's likely that vendors will use an instance-swapping strategy similar to that used for entity beans (the strategy utilized by entity beans is described in Chapter 6, "Entity Beans"). However, the swap is very brief, lasting only as long as the business method needs to execute. When an instance is swapped in, its SessionContext changes to reflect the context of its EJB object and the client invoking the method. The bean instance may be included in the transactional scope of the client's request, and it may access SessionContext information specific to the client request, for example, the security and transactional methods. Once the instance has finished servicing the client, it is disassociated from the EJB object and returned to the Method-Ready Pool.
Stateless session beans are not subject to activation and never have their ejbActivate() or ejbPassivate() callback methods invoked. The reason is simple: stateless instances have no conversational state that needs to preserved. (Stateful session beans do depend on activation, as we'll see later.)
// EJB 1.0: Use native cast Object ref = jndiConnection.lookup("ProcessPaymentHome"); ProcessPaymentHome home = (ProcessPaymentHome) PortableRemoteObject.narrow(ref,ProcessPaymentHome.class); ProcessPayment pp = home.create();
Unlike the entity bean and stateful session bean, invoking the create() method does not result in a call to the bean's ejbCreate() method. In stateless session beans, calling the EJB home's create() method results in the creation of an EJB object for the client, but that is all. The ejbCreate() method of a stateless session bean is only invoked once in the life cycle of an instance--when it is transitioning from the Does Not Exist state to the Method-Ready Pool. It isn't reinvoked every time a client requests a remote reference to the bean.
188.8.131.52. Transitioning out of the Method-Ready Pool: The death of a stateless bean instance
Bean instances leave the Method-Ready Pool for the Does Not Exist state when the server no longer needs the instance. This occurs when the server decides to reduce the total size of the Method-Ready Pool by evicting one or more instances from memory. The process begins by invoking the ejbRemove() method on the instance. At this time, the bean instance should perform any cleanup operations, like closing open resources. The ejbRemove() method is only invoked once in the life cycle of a stateless session bean's instance--when it is about to transition to the Does Not Exist state. When a client invokes one of the remove() methods on a stateless session bean's remote or home interface, it is not delegated to the bean instance. The client's invocations of this method simply invalidates the stub and releases the EJB object; it notifies the container that the client no longer needs the bean. The container itself invokes the ejbRemove() method on the stateless instance, but only at the end of the instance's life cycle. Again, this is very different from both stateful session beans and entity beans, which suffer more destructive consequences when the client invokes a remove method. During the ejbRemove() method, the SessionContext (and JNDI ENC for EJB 1.1) is still available to the bean instance. Following the execution of the ejbRemove() method, the bean is dereferenced and eventually garbage collected.
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