# Singleton Session Bean

By

Paul Tomoiu

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In 

Examples

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Published 2022-12-03

This tutorial explains to you what a Singleton Session Bean in Java is.

A Singleton, is a class which is instantiated once per application and exist for the lifecycle of the application. The Singleton Session Beans (from EJB 3.1) are business objects having a global shared state within a JVM. The concept of "Singleton" is old, but for EJB, the concept was implemented differently.

NOTES:

The @Startup annotation causes the bean to be instantiated by the container when the application starts.

The @PostConstruct annotation before a method will call that method when the Singleton is initialized.

Here is class created as singleton (example):

package singleton;
 
import javax.annotation.PostConstruct;
import javax.ejb.Singleton;
import javax.ejb.Startup;
 
@Startup
@Singleton
public class SingletonExample {
 
    private int counter;
     
    @PostConstruct
    public void postConstruct() {
        counter = 0;
        System.out.println("postConstruct() method is called !");  
         
    }
     
    public  int getCounter() {
        return counter;
    }
     
    public void setCounter(int newCounterValue) {
        counter = newCounterValue;
    }
}

When you deploy the EJB application the EJB container will initialize the Singleton and this could be seen in the logs: