The Life Cycles of Enterprise Beans
An enterprise bean goes through various stages during its lifetime, or life cycle. Each type of enterprise bean--session, entity, or message-driven--has a different life cycle.
The descriptions that follow refer to methods that are explained along with the code examples in the next two chapters. If you are new to enterprise beans, you should skip this section and try out the code examples first.
The Life Cycle of a Stateful Session Bean
Figure 3-3 illustrates the stages that a session bean passes through during its lifetime. The client initiates the life cycle by invoking the
createmethod. The EJB container instantiates the bean and then invokes the
ejbCreatemethods in the session bean. The bean is now ready to have its business methods invoked.
While in the ready stage, the EJB container may decide to deactivate, or passivate, the bean by moving it from memory to secondary storage. (Typically, the EJB container uses a least-recently-used algorithm to select a bean for passivation.) The EJB container invokes the bean's
ejbPassivatemethod immediately before passivating it. If a client invokes a business method on the bean while it is in the passive stage, the EJB container activates the bean, moving it back to the ready stage, and then calls the bean's
Figure 3-3 Life Cycle of a Stateful Session Bean
At the end of the life cycle, the client invokes the
removemethod and the EJB container calls the bean's
ejbRemovemethod. The bean's instance is ready for garbage collection.
Your code controls the invocation of only two life-cycle methods--the
removemethods in the client. All other methods in Figure 3-3 are invoked by the EJB container. The
ejbCreatemethod, for example, is inside the bean class, allowing you to perform certain operations right after the bean is instantiated. For instance, you may wish to connect to a database in the
ejbCreatemethod. See Chapter 16 for more information.
The Life Cycle of a Stateless Session Bean
Because a stateless session bean is never passivated, its life cycle has just two stages: nonexistent and ready for the invocation of business methods. Figure 3-4 illustrates the stages of a stateless session bean.
Figure 3-4 Life Cycle of a Stateless Session Bean
The Life Cycle of an Entity Bean
Figure 3-5 shows the stages that an entity bean passes through during its lifetime. After the EJB container creates the instance, it calls the
setEntityContextmethod of the entity bean class. The
setEntityContextmethod passes the entity context to the bean.
After instantiation, the entity bean moves to a pool of available instances. While in the pooled stage, the instance is not associated with any particular EJB object identity. All instances in the pool are identical. The EJB container assigns an identity to an instance when moving it to the ready stage.
There are two paths from the pooled stage to the ready stage. On the first path, the client invokes the
createmethod, causing the EJB container to call the
ejbPostCreatemethods. On the second path, the EJB container invokes the
ejbActivatemethod. While in the ready stage, an entity bean's business methods may be invoked.
There are also two paths from the ready stage to the pooled stage. First, a client may invoke the
removemethod, which causes the EJB container to call the
ejbRemovemethod. Second, the EJB container may invoke the
At the end of the life cycle, the EJB container removes the instance from the pool and invokes the
Figure 3-5 Life Cycle of an Entity Bean
In the pooled state, an instance is not associated with any particular EJB object identity. With bean-managed persistence, when the EJB container moves an instance from the pooled state to the ready state, it does not automatically set the primary key. Therefore, the
ejbActivatemethods must set the primary key. If the primary key is incorrect, the
ejbStoremethods cannot synchronize the instance variables with the database. In the The SavingsAccountEJB Example, the
ejbCreatemethod assigns the primary key from one of the input parameters. The
ejbActivatemethod sets the primary key (
id) as follows:id = (String)context.getPrimaryKey();
In the pooled state, the values of the instance variables are not needed. You can make these instance variables eligible for garbage collection by setting them to
The Life Cycle of a Message-Driven Bean
Figure 3-6 illustrates the stages in the life cycle of a message-driven bean.
Figure 3-6 Life Cycle of a Message-Driven Bean
The EJB container usually creates a pool of message-driven bean instances. For each instance, the EJB container instantiates the bean and performs these tasks:
- It calls the
setMessageDrivenContextmethod to pass the context object to the instance.
- It calls the instance's
Like a stateless session bean, a message-driven bean is never passivated, and it has only two states: nonexistent and ready to receive messages.
At the end of the life cycle, the container calls the
ejbRemovemethod. The bean's instance is then ready for garbage collection.