Question about writeable static fields in EJB 3.1

[Ajay Dhar]

This is a question regarding code in the first example in Ivan Krizsan's OCP Java EE 6 EJB Developer Study Notes on page 22. But first on page 13 of the study notes, and in the EJB 3.1 specification (chapter 21, section 2), it states that one EJB programming restriction is that an EJB must not use writable static fields. The motivation being to ensure consistency in a distributed environment.

Here is the code of a stateful session bean taken from the study notes on page 22:

package com.ivan.scbcd6;

import java.util.Date;
import java.util.concurrent.TimeUnit;

import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;
import javax.ejb.LocalBean;
import javax.ejb.PostActivate;
import javax.ejb.PrePassivate;
import javax.ejb.Remove;
import javax.ejb.Stateful;
import javax.ejb.StatefulTimeout;
import javax.ejb.TransactionAttribute;
import javax.ejb.TransactionAttributeType;

/**
 * Session Bean implementation class StatefulSession1Bean
 */
@Stateful
@LocalBean
@StatefulTimeout(value=5, unit=TimeUnit.SECONDS)
@TransactionAttribute(TransactionAttributeType.NEVER)
public class StatefulSession1Bean {
	
	private static int sCurrentInstanceNumber = 1;
	private int mInstanceNumber;
	
	// default constructor
    public StatefulSession1Bean() { }

    @PostConstruct
    public void initialize() {
    	mInstanceNumber = sCurrentInstanceNumber++;
    	System.out.println("*** StatefulSession1Bean " + mInstanceNumber + " created.");
    }
    
    @PreDestroy
    public void destroy() {
    	System.out.println("*** StatefulSession1Bean " + mInstanceNumber + " destroyed.");
    }
    
    @PrePassivate
    public void prepassivate() {
    	System.out.println("*** StatefulSession1Bean " + mInstanceNumber + " passivated.");
    }
    
    @PostActivate
    public void postactivate() {
    	System.out.println("*** StatefulSession1Bean " + mInstanceNumber + " activated.");
    }
    
    @Remove
    public void remove() {
    	System.out.println("*** StatefulSession1Bean " + mInstanceNumber + " remove.");
    }
    
    /**
     * Creates a greeting to the person with the supplied name.
     * 
     * @param inName - Name of person to greet.
     * @return greeting.
     */
    public String greeting(final String inName) {
    	Date currentTime = new Date();
    	
    	String message = "Hello " + inName + ", I am stateful session bean " + mInstanceNumber + 
    			". The time is now: " + currentTime;
    	return message;
    }
    
}

If you look at line number 26, there is a static int declared as an instance variable. This static variable is updated in the @PostConstruct method. Doesn't this violate the restriction that an EJB must not use writeable static fields?

Thanks,
Ajay

[Dieter Quickfend]

Yes, it is not conform to the spec and there is no way to predict the value in a distributed environment.

[Frits Walraven]

Yes, the examples that are handled by Ivan are just for testing purposes. In a distributed production environment these examples should not be used.

I guess an extra note explaining this exception would be helpful.

Regards,
Frits

[Ivan Krizsan]

Hello!
The simplest way that I could think of to keep track of the number of instances of the session bean was a writeable static field, as seen in the example.
This was done in order to show when a certain instance of the session bean is instantiated and destroyed and how many instances of the session bean are created.
There is nothing in the instructions that suggests using the example in a distributed environment.
I have included a comment on the reason for using a writable static field in the latest version of the study notes.
Thanks for bringing this to my attention!
Best wishes!
Ivan