Question on Singleton Session Bean Instantiation

[Mohit G Gupta]

Source:OCP JavaEE 6 EJB Developer Study Notes by Ivan A Krizsan,Version: April 8, 2012

Hi All,

Below is the Code for Singleton Session Bean and Client Servlet that accesses the Bean.

Singleton Session Bean

package com.ivan.scbcd6;
import java.util.Date;
import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;
import javax.ejb.LocalBean;
import javax.ejb.Singleton;
/**
* This class implements a singleton session bean with the ability
* to hold state.
*/
@Singleton
@LocalBean
public class SingletonSessionBeanA
{
/* Constant(s): */
private final static String BEAN_NAME = "SingletonSessionBeanA";
/* Instance variable(s): */
private String mStoredMessage = "[no message set]";

public SingletonSessionBeanA(){
	System.out.println("*** " + BEAN_NAME + " - Created");
}
@PostConstruct
public void intialize()
{
System.out.println("*** " + BEAN_NAME + " - Initialized");
}
@PreDestroy
public void cleanUp()
{
System.out.println("*** " + BEAN_NAME + " - Destroyed");

}
public String retrieveMessage()
{
Date theCurrentTime = new Date();
return "Message from " + BEAN_NAME + " - " + mStoredMessage + " "
+ theCurrentTime;
}
public void storeMessage(final String inStoredMessage)
{
mStoredMessage = inStoredMessage;
}
}

Client Servlet

package com.ivan.scbcd6.client;
import java.io.IOException;
import java.io.PrintWriter;
import javax.ejb.EJB;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.ivan.scbcd6.SingletonSessionBeanA;
/**
* Servlet acting as a client of the singleton session bean.
*/
@WebServlet(name = "SingletonClientServlet", urlPatterns = "/test.do")
public class SingletonClientServlet extends HttpServlet
{
/* Constant(s): */
private static final long serialVersionUID = 1L;
private final static String STORE_ACTION = "store";
private final static String CLEAR_ACTION = "clear";
/* Instance variable(s): */
@EJB
private SingletonSessionBeanA mSingletonBeanA;
@Override
protected void doGet(HttpServletRequest inRequest,
HttpServletResponse inResponse) throws ServletException, IOException
{
System.out.println("**** Entering SingletonClientServlet");
String theRequestNameParam = inRequest.getParameter("name");
String theRequestActionParam = inRequest.getParameter("action");
/* Set default name if none provided. */
if (theRequestNameParam == null || theRequestNameParam.equals(""))
{
theRequestNameParam = "Anonymous Coward";
}
/* Generate output from servlet using session beans. */
PrintWriter theResponseWriter = inResponse.getWriter();
String theMessage;
theMessage = mSingletonBeanA.retrieveMessage();
theResponseWriter.println(theMessage);
/* 
* Store or clear data in one of the singleton session beans
* if the supplied action so specifies.
*/
if (theRequestActionParam != null)

{
if (STORE_ACTION.equals(theRequestActionParam))
{
mSingletonBeanA.storeMessage(theRequestNameParam);
}
if (CLEAR_ACTION.equals(theRequestActionParam))
{
mSingletonBeanA.storeMessage("[CLEARED]");
}
}
System.out.println("**** Exiting SingletonClientServlet");
theResponseWriter.println("Finished invoking singleton session beans!");
}
}

Below is the messages logged server.log:

INFO: Virtual server server loaded system default web module
INFO: Virtual server server loaded system default web module
INFO: Virtual server server loaded system default web module

INFO: Portable JNDI names for EJB SingletonSessionBeanA : [java:global/BusinessMethodInterceptors/SingletonSessionBeanA, java:global/BusinessMethodInterceptors/SingletonSessionBeanA!com.ivan.scbcd6.SingletonSessionBeanA]
INFO: *** SingletonSessionBeanA - Created
INFO: Loading application BusinessMethodInterceptors at /BusinessMethodInterceptors
INFO: Loading application BusinessMethodInterceptors at /BusinessMethodInterceptors
INFO: Loading application BusinessMethodInterceptors at /BusinessMethodInterceptors
INFO: Hibernate Validator bean-validator-3.0-JBoss-4.0.2
INFO: Instantiated an instance of org.hibernate.validator.engine.resolver.JPATraversableResolver.
INFO: BusinessMethodInterceptors was successfully deployed in 2,759 milliseconds.

INFO: Updating configuration from org.apache.felix.fileinstall-autodeploy-bundles.cfg
INFO: Installed E:\Applications\GlassFish\glassfishv3\glassfish\modules\autostart\org.apache.felix.fileinstall-autodeploy-bundles.cfg
INFO: {felix.fileinstall.poll (ms) = 5000, felix.fileinstall.dir = E:\Applications\GlassFish\glassfishv3\glassfish\domains\domain1\autodeploy\bundles, felix.fileinstall.debug = 1, felix.fileinstall.bundles.new.start = true, felix.fileinstall.tmpdir = C:\Users\MOHITG~1\AppData\Local\Temp\fileinstall-9049558674289836899, felix.fileinstall.filter = null}

INFO: **** Entering SingletonClientServlet
INFO: *** SingletonSessionBeanA - Created
INFO: *** SingletonSessionBeanA - Initialized
INFO: **** Exiting SingletonClientServlet

INFO: Server shutdown initiated
INFO: *** SingletonSessionBeanA - Destroyed

INFO: JMXStartupService: Stopped JMXConnectorServer: service:jmx:rmi://MOHITGUPTA-PC:8686/jndi/rmi://MOHITGUPTA-PC:8686/jmxrmi
INFO: JMXStartupService and JMXConnectors have been shut down.
INFO: Shutdown procedure finished

Request Issued:

http://localhost:8080/BusinessMethodInterceptors/
http://localhost:8080/BusinessMethodInterceptors/test.do

Here,BusinessMethodInterceptors is the name of Web Project.

As per my understanding,Singletion Session Bean is instantiated once per application per class.
However, in the server.log I see that it is Instantiated(Created) twice in application (See message "INFO: *** SingletonSessionBeanA - Created" is displayed twice)
Please advise if my understanding is incorrect.

[Frits Walraven]

Hi Mohit,

This is what I also noticed when I was preparing for the exam: the constructor of a Singleton is called twice (!). This is an implementation specific thing not mandated (or explained) by the specifications. The PostConstruct method is only called once luckily...

Regards,
Frits

[Mohit G Gupta]

Thanks again Frits