Thursday Nov 06, 2008

Singleton EJB support in GlassFish V3 Prelude

The GlassFish Enterprise Server V3 Prelude has been released. To get a sneak preview of some of the EJB 3.1 features in V3 Prelude, you need to install ejb container by following the instructions from my earlier blog

In this blog, I will describe one of the EJB 3.1 features available in V3 Prelude: Singleton EJBs.

One of the frequent problems that ejb developers face is to figure out how to share data between multiple components of an application. Singleton session beans are meant for this purpose. A Singleton EJB is a session bean component that is instantiated once per application. A Singleton session bean is intended to be shared by concurrent requests. In other words, all requests to the singleton will be routed to the single instance of the Singleton bean instance.

Let me demonstrate this by using a simple application. Lets say that we want to count the number of times a HelloServlet was accessed. Though there are other ways to do this, lets use a Singleton bean to accomplish the task. Note that, we can not maintain a counter inside a Stateless session bean as the container might maintain a pool of these stateless bean instances (and each will have their own counters). The Singleton session bean (called CounterBean) keeps a counter. It will have a incrementAndGetHitCount() method that increments the counter and will be called from the SimpleServletServlet.

Defining a Singleton bean

A singleton bean can be defined by using the @Singleton annotation. Note that Singleton beans can have business interfaces and interceptors just like Stateless and Stateful session beans. Also, remember that EJB 3.1 now allows optional interface view. More can be read about optional interface view in my earlier blog.

package com.sun.ejb31.test;

import javax.ejb.Singleton;
@Singleton
public class CounterBean {
    private int hitCount;

    //Note the use of synchronized keyword
    public synchronized int incrementAndGetHitCount() {
        return hitCount++;
    }

}

Handling concurrency in a Singleton

Since there is only one instance of the Singleton bean, the container routes all the requests (method invocations) to the single instance. This means that concurrent invocations are possible. The EJB 3.1 specification allows two choices to handle concurrency issues:

Container Managed Concurrency (CMC): With Container Managed Concurrency, the container is responsible for controlling concurrent access to the bean instance based on method-level locking metadata. Each business method method can be annotated with either a Read(shared) lock or Write (exclusive) lock Note: GlassFish V3 Prelude supports only BMC.

Bean Managed Concurrency (BMC): With Bean Managed Concurrency demarcation, the container allows full concurrent access to the Singleton bean instance. It is the responsibility of the bean developer to guard its state as necessary against synchronization errors due to concurrent access.

Obtaining a reference to the Singleton bean

Obtaining a reference to a Singleton bean is no different from obtaining a reference to other types of beans. In our example, we will inject a reference to the Singleton bean into our Servlet (though a JNDI lookup would have worked as well too).
package com.sun.ejb31.test;

...
import javax.ejb.EJB;

public class SimpleServlet extends HttpServlet {
   
    @EJB CounterBean counterBean;
    
    protected void processRequest(
        HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        try {
            out.println("<html>");
            out.println("<head>");
            out.println("<title>Servlet SimpleServlet</title>");  
            out.println("</head>");
            out.println("<body>");
            out.println("<h1>Number of times this servlet is accessed: "
                    + counterBean.incrementAndGetHitCount());
            out.println("</body>");
            out.println("</html>");
        } finally { 
            out.close();
        }
    }

    ...
    ... 

}

How to deploy and run

Assuming that you already have installed the ejb container in V3 Prelude, (if not follow these instructions)

To start the server: java -jar <install_dir>/modules/glassfish.jar

To deploy: <install_dir>/bin/asadmin deploy SimpleCounter.war

To redeploy: <install_dir>/bin/asadmin deploy --force=true SimpleCounter.war

URL to access the servlet: http://localhost:8080/SingletonCounter/SimpleServlet

Resources

EJB 3.1 public draft

SimpleCounter.war

Source files: CounterBean.java and SimpleServlet.java

EJB 3.1 Container in GlassFish V3 Prelude

The GlassFish Enterprise Server V3 Prelude has been released. In this blog, I will describe the steps to install the EJB container to have a sneak preview of some of the EJB 3.1 features.
  • First download the V3 prelude bundle
  • Then start the server by doing: java -jar <install_dir>/glassfish/modules/glassfish.jar
  • Open a browser window and access the V3 administration GUI by accessing the url: http://localhost:4848
  • From the left panel, select the uptate tool
  • Select glassfish-ejb component and click install to install the ejb-container module.
  • Restart the server and you are now ready to deploy ejb applications to the V3 prelude application server
  • Note:

    Only Stateless Session beans and Singleton beans with local interfaces, no interface view are supported. Stateful, Message driven and EJB 2.x entity beans are not supported. Remote interfaces and Remote business interfaces for any of the bean type are not supported yet. Timer Service is supported. See Marina's blog about timer service.

    EJB container implementation in V3 Prelude gives you an early look at some of the EJB 3.1 features. It is not a full, feature-complete container implementation and is not suitable for production deployments. It is suitable for experimentation and exploration. Experiment, take a look at the new approach being taken in GlassFish v3, and then let us know what you think

    For a full list of what is planned in EJB3.1 please refer to Ken's blog

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    Mahesh Kannan

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