Why stateful session bean

Stateful session beans can be useful, especially where the number of concurrent users is not too large- i. To help with understanding the difference that a stateless vs.

In this situation there was an entire set of services in the application that looked-up data for users for use in maintenance tasks, or placing orders, etc. As it paged through the list, if it needed to get more data than was on the client at that time, it called the server again. When it did this it actually passed a row number to the server to say, starting at this row number, return back the next set of rows. This made the queries very complex, as the queries themselves had to scroll through the data and return the correct set.

Of course the user could page down through, but also back up again, and this process was repeated many times in certain cases. So, the design was stateless, but the load it actually exacted on the database was extraordinarily high, and the response time of the services themselves suffered.

Users complained of poor response times as well. To solve this issue the services were made stateful with one instance per client , the paging logic was removed from the queries themselves, and scrolled through the data in memory as the client paged through the rows.

Each of the business methods models a step in the account creation workflow and incrementally populates the state instance variables. The workflow is terminated when the client invokes either of the Remove annotated methods Q. Specifying stateful bean business interfaces works in almost exactly the same way as it does for stateless beans—with a couple of exceptions.

Stateful session beans support local and remote invocation through the Local and Remote annotations. However, a stateful session bean cannot have a web service endpoint interface. This is because SOAP-based web services are inherently stateless in nature. The reason for this will become clear as we discuss the stateful bean lifecycle next.

As we mentioned in section 3. In this section, we explain this concept in more depth. The container follows these steps:. Essentially, passivation means that the bean is moved out of active memory, serialized, and stored in temporary storage. A stateful bean maintains client state and cannot be pooled. It may be passivated when the client is not using it and must be activated when the client needs it again.

Like a stateless session bean, the stateful session bean has lifecycle callback methods, or callbacks, that are invoked when the PostConstruct event occurs as an instance is created and when the PreDestroy event occurs before the instance is destroyed. But now we have two new callback events for which we can have callbacks: PrePassivate and PostActivate, which are part of the passivation process.

Just as in listing 3. Also as in listing 3. However, you should note that we invoke the very same method for both the PreDestroy and PrePassivate callbacks:.

Similarly, the exact same action is taken for both the PostConstruct and Post-Activate callbacks:. For a large number of beans, this could easily make the machine run out of memory. The container employs the technique of passivation to save memory when possible. NOTE Passivation essentially means saving a bean instance into disk instead of holding it in memory. The container accomplishes this task by serializing the entire bean instance and moving it into permanent storage like a file or the database.

Activation is the opposite of passivation and is done when the bean instance is needed again. The container activates a bean instance by retrieving it from permanent storage, deserializing it, and moving it back into memory.

This means that all bean instance variables that you care about and should be saved into permanent storage must either be a Java primitive or implement the java. Serializable interface. The point of the PrePassivate callback is to give the bean a chance to prepare for serialization. This may include copying nonserializable variable values into Serializable variables and clearing unneeded data out of those variables to save total disk space needed to store the bean.

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