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
Source files: CounterBean.java and SimpleServlet.java
