Question

I am new to EJBs and recently started working on EJBs (3.0). I have been using Java for 6 years but never worked with EJB’s before. I am overwhelmed with the complexity of the whole EJB business to say the least. I fail to understand where I can practically apply some of the concepts.

One question which comes to my mind after understanding the Stateless session bean is, Can you not always replace a stateless session bean with a shared instance of a class with no local members (effectively making it stateless)? I read about instance pooling being done for stateless session beans. If there is no state, can you not simply use one instance?

I am deploying my samples on OpenEJB and one place where I had to use stateless session bean was to interact with the EntityManager. I am not sure if I can get handle to the EntityManager in an arbitrary class. Apart from that, still puzzled what problem stateless session bean tries to solve.

Answer 1

A stateless session bean may have state. But it may not have conversational state. So it’s perfectly acceptable that a session bean method does the following (although bad practice):

public void foo() {
    this.someVar = bar();
    this.someOtherVar = baz();
    zing();
}

Besides pooling, the EJB container provides several services with stateless beans:

• dependency injection
• declarative transaction demarcation
• declarative security
• access to the EJB context
• etc.

So a stateless session bean is much more useful than a simple stateless singleton.