The Life Cycles of Enterprise Beans

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 23-4
illustrates the stages that a session bean passes through during its
lifetime. The client initiates the life cycle by invoking the

create

method. The EJB container instantiates the bean and then invokes the

setSessionContext

and

ejbCreate

methods in the session bean. The bean is now ready to have its business methods invoked.



Figure 23-4 Life Cycle of a Stateful Session Bean

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

ejbPassivate

method 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, calls the bean's

ejbActivate

method, and then moves it to the ready stage.

At the end of the life cycle, the client invokes the

remove

method, and the EJB container calls the bean's

ejbRemove

method. The bean's instance is ready for garbage collection.

Your code controls the invocation of only two life-cycle methods: the

create

and

remove

methods in the client. All other methods in Figure 23-4 are invoked by the EJB container. The

ejbCreate

method, for example, is inside the bean class, allowing you to perform
certain operations right after the bean is instantiated. For example,
you might wish to connect to a database in the

ejbCreate

method. See Chapter 31 for more information.

The Life Cycle of a Stateless Session Bean

Because a stateless session bean is never passivated, its life cycle
has only two stages: nonexistent and ready for the invocation of
business methods. Figure 23-5 illustrates the stages of a stateless session bean.



Figure 23-5 Life Cycle of a Stateless Session Bean

The Life Cycle of an Entity Bean

Figure 23-6
shows the stages that an entity bean passes through during its
lifetime. After the EJB container creates the instance, it calls the

setEntityContext

method of the entity bean class. The

setEntityContext

method 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

create

method, causing the EJB container to call the

ejbCreate

and

ejbPostCreate

methods. On the second path, the EJB container invokes the

ejbActivate

method. While an entity bean is in the ready stage, an it's business methods can be invoked.

There are also two paths from the ready stage to the pooled stage. First, a client can invoke the

remove

method, which causes the EJB container to call the

ejbRemove

method. Second, the EJB container can invoke the

ejbPassivate

method.



Figure 23-6 Life Cycle of an Entity Bean

At the end of the life cycle, the EJB container removes the instance from the pool and invokes the

unsetEntityContext

method.

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

ejbCreate

and

ejbActivate

methods must assign a value to the primary key. If the primary key is incorrect, the

ejbLoad

and

ejbStore

methods cannot synchronize the instance variables with the database. In the section The SavingsAccountBean Example, the

ejbCreate

method assigns the primary key from one of the input parameters. The

ejbActivate

method 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

null

in the

ejbPassivate

method.

The Life Cycle of a Message-Driven Bean

Figure 23-7 illustrates the stages in the 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

setMessageDrivenContext

method to pass the context object to the instance.
It calls the instance's

ejbCreate

method.



Figure 23-7 Life Cycle of a Message-Driven Bean

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

ejbRemove

method. The bean's instance is then ready for garbage collection.

From: http://java.sun.com/j2ee/1.4/docs/tutorial/doc/EJBConcepts9.html