Activation and Passivation: What Are They All About?

You may understand the concepts of activation and passivation -- the

acts of turning a bean "on" and "off", respectively. What the terms,

and to a lesser extent the associated methods in the

javax.ejb.SessionBean and javax.ejb.EntityBean interfaces (ejbActivate

() and ejbPassivate()), actually mean depends on whether they're

applied to a stateless session bean, an entity bean, or a stateful

session bean.

Entity beans and stateless beans, under most application servers, never

really die. Instead, instances of such beans are "pooled" and made to

wait for another request that requires their services. When they're

pooled, entity beans are not associated with any EJBObject (which can

usually be read as "unique record in a database").

Instances of stateful beans, on the other hand, are usually destroyed --

removed from the volatile memory controlled by the container that's

running the beans -- but not eliminated completely. Their serializable

properties -- those fields that are specific to the state of the

session bean -- are written to disk by the container. Because the

serializable portions of the object are stored persistently, a new

stateful bean can be created and populated with the appropriate session-

specific information when the session comes back to life.

In most cases, the issue of when passivation happens is up to the

container and usually depends on how much demand there is for the

memory the bean instances are tying up. The container can choose to

passivate a bean when it sees that the use of that bean has slowed, and

it figures -- by whatever optimization algorithm it employs -- that it

would be better off passivating and possibly re-activating the bean

than leaving it active but unused.

If the bean server manages activation and passivation, then what are

ejbActivate() and ejbPassivate() for? They can almost be thought of as

event handlers. The first method tells the bean that it's just been

activated; the second tells the bean it's about to be passivated. The

bean's code can react to calls to these methods however it likes.

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