The Life Cycle of a Stateless Session Bean (Enterprise JavaBeans)

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.[5]Figure 7-1 illustrates the states and transitions that a stateless session bean instance goes through in its lifetime.

[5] Some vendors do not pool stateless instances, but may instead create and destroy instances with each method invocation. This is an implementation-specific decision that shouldn't impact the specified life cycle of the stateless bean instance.

Figure 7-1. Stateless session bean life cycle

7.2.1. Does Not Exist

When a bean instance is in the Does Not Exist state, it is not an instance in the memory of the system. In other words, it has not been instantiated yet.

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.

7.2.2.1. 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.

WARNING

Bean classes, entity and session alike, must never define constructors. Take care of initialization needs within ejbCreate() and other callback methods available through the bean class's EnterpriseBean interface (EntityBean or SessionBean). The container instantiates instances of the bean class using Class.newInstance(), which requires a no-argument constructor.

NOTE

Although the life cycle of a bean instance is defined by the specification, the actual implementation by EJB vendors need only support the specified life cycle as perceived by the bean class and the client. For this reason, a bean developer must only depend on behavior described by the specification. The specification does not describe the behavior of Java language constructors; it only describes the behavior of the create and callback methods in the 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.[6]

[6] The EJB 1.0 specification is not clear on whether an EJBContext should be stored in a transient or nontransient field in the bean instance. The recommended approach is to use nontransient, but some vendors will preserve an EJBContext reference's binding to the instance only if it is set as a transient field. Consult your vendor for its implementation-specific requirements. EJB 1.1 explicitly states that the EJBContext must not be referenced in a transient field.

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.[7] 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.

[7] The duration of a stateless bean instance's life is assumed to be very long. However, some EJB servers may actually destroy and create instances with every method invocation, making this strategy less attractive. Consult your vendor's documentation for details on how your EJB server handles stateless instances.

7.2.2.2. 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.)

Clients that need a remote reference to a stateless session bean begin by invoking the create() method on the bean's EJB home:

// 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.

7.2.2.3. 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.