April 2008 Archives

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Hi, I'm French and I've been working for over seven years with Java EE. I describe myself as something between an experimented developer, a junior architect and a BEA evangelist :) I have decided to share my knowledge, built from my experience and the issues I face while working for my clients. Feel free to drop me a line, in French or English!

Reminder on how to use EJB3 with Weblogic 10

First of all, I'd like to point out the excellent tutorial by Greg Mally on using EJB3 within Workshop :

http://dev2dev.bea.com/blog/gmally/archive/2007/11/using_workshop.html

How to define an EJB3 ?

Well, it's quite easy. According to the specification, you only have to use annotations to define your EJB.

For instance, let's take the following EJB :

@Stateless(
	name = "PdfRetriever", 
	mappedName = "ejb/stateless/PdfRetriever", 
	description = "Gets a PDF file from the server")
@Local ({PdfRetriever.class})
@Remote ({PdfRetrieverRemote.class})
public class PdfRetrieverBean implements 
			PdfRetriever, PdfRetrieverRemote {
[...]
}

For specifying the interfaces (local or remote), you can choose to use only the annotations or the implementation declarations.

The use of annotations is maybe a bit quicker to read, but the implement declaration prevents you from missing an interface method. I wrote both so that you can make your choice.

As for the deployment descriptor, the ejb-jar.xml must have the correct declaration, that is to say :

<ejb-jar xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/ejb-jar_3_0.xsd"
version="3.0">

How to call it with remote interface ?

Here's the code to call the EJB :

Properties props = new Properties();
props.put(Context.INITIAL_CONTEXT_FACTORY,
	"weblogic.jndi.WLInitialContextFactory");
props.put(Context.PROVIDER_URL, 
	"t3://localhost:8001");
InitialContext ctx = new InitialContext(props);
PdfRetrieverRemote ejb = (PdfRetrieverRemote) 
	ctx.lookup("ejb.stateless.PdfRetriever#sample.ejb.PdfRetrieverRemote");
ejb.getPdfFileFromServer("myFile.pdf");

Be aware that the JNDI name is not exactly the same as entered in the EJB definition ("ejb/stateless/PdfRetriever").

It has been postfixed with a # and the full-qualified name of the Remote interface.

How to call it with local interface ?

Note : "Local" doesn't refer to the same JVM but the same EAR.

The easiest way to call your EJB is through a Servlet thanks to an annotation.

As explained above, your servlet must reside in the same EAR than the EJB3 module.

You don't have to use JNDI to reach your EJB, just reference it that way :

@EJB
private PdfRetriever pdfRetriever;

public void myMethod () {
    byte[] myPdfFile = pdfRetriever.getPdfFileFromServer("myFile.pdf");
}

Note : Due to lots of messages asking me how to call an EJB3 from a pojo, I will write some details about that, but know that this is not the way it is sensed to be used.

According to the specification, an EJB3 is to be called via an annotation in a Servlet or another EJB.

This solution lies on the "java:comp/env" mechanism.

EJB within a web application

First part : declaring the EJB

Taking the example quoted above, you will have to add an annotation to at least one servlet.

Example :

@EJB(beanInterface=sample.ejb.PdfRetriever.class, name = "ejb/PdfRetriever")
  public class EjbInvoker extends javax.servlet.http.HttpServlet implements         javax.servlet.Servlet {

This annotation replaces the EJB declaration in the web.xml file. It makes the EJB available for all the War context.

Else you can declare your EJB in the web.xml :

<ejb-local-ref>
  <ejb-ref-name>ejb/PdfRetriever</ejb-ref-name>
  <local>sample.ejb.PdfRetriever</local>
</ejb-local-ref>

http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd

As you can see, the attributes are different :

Note : With the annotation, the beanInterface attribute expects a class.

Second part : calling the EJB from the POJO

Now the EJB has been made visible within the whole web application, the code to call the EJB from the POJO is quite simple :

InitialContext ctx = new InitialContext(); 
PdfRetriever ejb = (PdfRetriever) ctx.lookup("java:comp/env/ejb/PdfRetriever"); 

Note : The lookup has been made on the name specified in "name" attribute for the annotation and "ejb-ref-name" for the DD, that is to say "ejb/PdfRetriever"

You can then manipulate your local EJB.

EJB within an EJB module

The scenario is the same than for the web application, except that the element you've got to add the EJB declaration to is now an EJB and the DD to modify is not the web.xml but the ejb-jar.xml

May 2008

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