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Tags/Keywords : Spring Singleton, Spring Container, Spring, Singleton,Questions, Example, Code, Tutorial, Article
Author : ISHTEK
Date (Year/Month/Date): 2009-09-25
Example showing Spring's singleton scope Bean Instance

Please be informed that NONE of the design/code from this
page is claiming to be some sort of best practices and we DO NOT expect
any of our visitor/reader of this page to assume this as some sort of
best practice for any context and should not be using this 
as it is without appropriate evaluation to their, so to say, 
specific programming context.

This page intends only to provide bit and piece of known ways  for
doing some sort of example and may not be fit for any other purpose.


Explaining difference between GOF Singleton pattern and
SpringFramework Singleton pattern for the Bean.

As we may be knowing that default scope for the bean from
Spring container using Spring's ApplicationContext and BeanFactory
is singleton.

Thus we may have bean instance from the BeanFactory as singleton
in nature, but is quite different from the GOF Singleton Pattern.



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As GOF Singleton Pattern looking at an instance that is per class
and per classloader level.

But SpringFramework Bean singleton scope is at per SpringFramework
container level.

So for a perticular Spring ApplicationContext and BeanFactory, there
could be a single bean instance for a bean id.

So to show this with the help of following example where I am first
creating a single application context and BeanFactory using 
ClassPathXmlApplicationContext and passing test.xml configuration file
as argument parameter.
I am retrieving a Bean instance form the BeanFactory using a bean id 
and then setting value for the instance variable of this Bean instance.
Then I am again retrieving another Bean instance for the second time
but for the same bean id as that of bean id used in the first case.

So if the Spring container manages singleton scope for this Bean, then
I should expect the value set in this first case, 
to be retained in the second case as well.

 
import org.springframework.beans.factory.*;
import org.springframework.context.*;
import org.springframework.context.support.*;

public class Testclient{
  public Testclient() {
    try{
        ApplicationContext appContext = 
            new ClassPathXmlApplicationContext("test.xml");
        BeanFactory factory = appContext;
        //Retrieving bean from the bean factory.
        //Bean id as "one"
        One one1 = (One)factory.getBean("one");
        one1.setName("test name");

        //Bean id as "one"
        One one2 = (One)factory.getBean("one");
        System.out.println("Name from the singleton bean :"+one2.getName());
    } catch(Exception ex) {
        ex.printStackTrace();
    }
  }
  public static void main(String args[]) {
      new Testclient();
  }
}


The bold red color text above, shows the way Bean is retrieved
from Spring container and the an example name value is set.

While the bold blue colored text above, shows the way another 
bean instance is retrieved from Spring container, but 
using the same bean id.

And the value from the second bean instance will be able to
return the value that is already set in the earlier step.
This shows that both these instances are the same, thus
can be treated as singleton from Spring's singleton scope.

This example is then slightly modified to include another
instance of SpringFramework's ApplicationContext and BeanFactory
instances again, and then the same ways , value for name field
is set in first case, and then the value is retrieved from the
another returned Bean instance from Spring Container.

But now there is a difference, that is observed, and it is that
the returned value is null, not the original value that is 
set in first case.

 
import org.springframework.beans.factory.*;
import org.springframework.context.*;
import org.springframework.context.support.*;

public class Testclient{
  public Testclient() {
    try{
        ApplicationContext appContext = 
            new ClassPathXmlApplicationContext("test.xml");
        BeanFactory factory = appContext;
        //Retrieving bean from the bean factory.
        One one1 = (One)factory.getBean("one");
        one1.setName("test name");

        ApplicationContext appContextNew = 
            new ClassPathXmlApplicationContext("test.xml");
        BeanFactory factoryNew = appContextNew;
        One one2 = (One)factoryNew.getBean("one");
        System.out.println("Name from the singleton bean :"+one2.getName());
    } catch(Exception ex) {
        ex.printStackTrace();
    }
  }
  public static void main(String args[]) {
      new Testclient();
  }
}


This shows that in the above modified code, we are dealing with
two different Spring Context and so we have two different bean
instances with one taking the setter value and the other returning null
value.

Code for the Bean in this example, is as follows:

 
public class One
{
  private String name;
  public void setName(String arg) {
	  name = arg;
  }
  public String getName() {
	  return name;
  }
}


and the Spring configuration file as follows:

 
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" 
  "http://www.springframework.org/dtd/spring-beans.dtd">
<beans>
  <bean id="one" class="One">
  </bean>
</beans>




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