Can different threads access different methods of a specific instance of a stateless EJB?

Question

@Stateless
public class MyBean1 {

    pulic void method1() {
        //method implementation
    }

    pulic void method2() {
        //method implementation
    } 
}

Consider a specific instance of MyBean1. Then we know that method1() or method2() cannot be accessed by multiple threads at the same time. But, while method1() is being accessed by a thread, can method2() be accessed by another thread?

Answer 1

I think that secion 4.3.14 of the ejb 3.1 spec gives the answer.

4.3.14 Serializing Session Bean Methods
The following requirements apply to Stateless and Stateful session beans. See Section 4.8.5 for Singleton session bean concurrency requirements.

The container serializes calls to each stateful and stateless session bean instance. Most containers will support many instances of a session bean executing concurrently; however, each instance sees only a serialized sequence of method calls. Therefore, a stateful or stateless session bean does not have to be coded as reentrant.

The container must serialize all the container-invoked callbacks (that is, the business method interceptor methods, lifecycle callback interceptor methods, timeout callback methods, beforeCompletion, and so on), and it must serialize these callbacks with the client-invoked business method calls.

....

As far as I understand the EJB spec you should use Singletons if you want a fine-grained control over concurrency (bean managed, container managed).

Answer 2

Let's modify your example a bit

@Stateless
public class MyBean1 {
    @Resource
    private SessionContext sessionContext;

    pulic void method1() {
        // method implementation
        //   As a side-effect, something is written into a database
        //   using an XA data source,
        //   and a message is sent using XA JMS
        //   (both under control of an XA transaction)
    }

    pulic int method2(int i) {
        return i * i;
    } 
}

For example, the session context is used to get the UserTransaction and getCallerPrincipal. They are not necessarily always the same (when two clients all the EJB). As for the UserTransaction: This one is bound to the current thread (see Javadoc).

As the session context is stored in a field (and not passed an argument to each individual method), the same EJB instance cannot be accessed by two different clients.

Therefore the specification requires the container to serialize calls to the same instance.

If you look at method2, a purely functional implementation without any side-effect, there is no need for EJBs.