Overview

float: right; margin-left: 20px;INLINE

<object width="400" height="250"><param name="movie" value="http://www.youtube.com/v/aLx2jta96xU?fs=1&amp;hl=en_US"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/aLx2jta96xU?fs=1&amp;hl=en_US" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="400" height="250"></embed></object>

This example shows how to create a Stateless session EJB using annotations. As stated in the "JSR 220: Enterprise JavaBeansTM,Version 3.0 - EJB Core Contracts and Requirements",

What this means is quite simply that stateless beans are shared. They do in fact have state as you can assign values to the variables, etc. in the bean instance. The only catch is there are a pool of identical instances and you are not guaranteed to get the exact same instance on every call. For each call, you get whatever instance happens to be available. This is identical to checking out a book from the library or renting a movie from the video store. You are essentially checking out or renting a new bean instance on each method call.

With EJB 3.0, it's now possible to write stateless session bean without specifying a deployment descriptor; you basically have to write just a remote or local business interface, which is a plain-old-java-interface, annotated with the @Remote or @Local annotation the stateless session bean implementation, a plain-old-java-object which implements the remote or the local business interface and is annotated with the @Stateless annotation

This example is the "simple-stateless" example located in the openejb-examples.zip available on the download page.

The Code

In this example we develop a simple EJB 3 Stateless session EJB. Every stateless session bean implementation must be annotated using the annotation @Stateless or marked that way in the ejb-jar.xml file.

Our Stateless bean has 2 business interfaces: CalculatorRemote, a remote business interface, and CalculatorLocal, a local business interface. A minimum of one business interface is required.

Bean

In EJB 3.0 session beans do not need to implement the javax.ejb.SessionBean interface. You can simply annotate it as @Stateless if you want it to be a stateless session bean.

Users of EJB 2.x may notice the bean actually implements the business interfaces! In the prior version of EJB implementing the remote interface (which derives from javax.ejb.EJBObject) in your bean was just not allowed. Now there is no javax.ejb.EJBObject requirement, so implementing the business interfaces is standard practice for EJB 3.0.

Local business interface

Local interfaces in EJB are pass-by-reference interfaces. Meaning that normal java semantics are used for passing arguments, return values and exceptions. A business local interface can be any plain java interface. There are no restrictions on the method arguments, return types, or throws clauses.

Unless specified otherwise, every interface your bean implements (and it's parent class implements and so on) is considered to be a local business interface. You can use the @Local annotation to explicitly state that an interface is a local interface, but this is not required.

You'll notice that in EJB 3.0 the Local Business Interface of a stateless session bean does not need to extend from javax.ejb.EJBLocalObject and does not need a javax.ejb.EJBLocalHome interface as they did in EJB 2.x and prior. Per the vocabulary of the EJB spec, interfaces that implement javax.ejb.EJBLocalObject or javax.ejb.EJBLocalHome are considered Component Interfaces and the plain java interface above is considered a Business Interface.

Remote business interface

Remote interfaces are pass-by-value interfaces. Meaning that all method parameters, return values, and exceptions are serialized on every call. The result is that you get a copy of the original object and not the original object. The advantage is of course that Remote interfaces can be used to invoke an EJB across a network in a client-server fashion. There are no restrictions on the Remote interface itself, but there are on the data passed in and out of the remote interface. The values passed into a method or returned from a method of a Remote interface must be serializable. It is fine for the method signature to be, for example, "public Object myMethod(Object myParam)" as long as the value passed in and returned implements java.io.Serializable.

As stated above, the Remote Business Interface of a bean can be any plain old interface. It does not need to extend javax.ejb.EJBObject, it does not need a javax.ejb.EJBHome, the methods do not need to throw javax.rmi.RemoteException, and the bean class can implement it!

At minimum the interface must be annotated with @Remote either in the interface itself or in the bean class, or the interface must be declared via <business-remote> in the ejb-jar.xml.

Writing a unit test for the example

Writing an unit test for the stateless session EJb is quite simple. We need just to write a setup method to create and initialize the InitialContext, and then write our test methods

setUp

Test the local business interface

Test the remote business interface

Running

Running the example is fairly simple. In the "simple-stateless" directory of the examples zip, just run:

$ mvn clean install

Which should create output like the following.