Singleton Bean Scope in Spring

Singleton Bean Scope: Scopes a single bean definition to a single object instance per Spring IoC container. This is the default behavior of the spring container

When a bean is a singleton, only one shared instance of the bean will be managed, and all requests for beans with an id or ids matching that bean definition will result in that one specific bean instance being returned by the Spring container.

We can say another way, when you define a bean definition and it is scoped as a singleton, then the Spring IoC container will create exactly one instance of the object defined by that bean definition. This single instance will be stored in a cache of such singleton beans, and all 

Lets see with example which show the singleton scope of the ‘zeroPoint’ bean in the spring container.


Point.java

package com.dineshonjava.sdnext.beanscope.tutorial;

public class Point
{
 private int x;
 private int y;
 /**
  * @return the x
  */
 public int getX() {
  return x;
 }
 /**
  * @param x the x to set
  */
 public void setX(int x) {
  this.x = x;
 }
 /**
  * @return the y
  */
 public int getY() {
  return y;
 }
 /**
  * @param y the y to set
  */
 public void setY(int y) {
  this.y = y;
 }
}


spring.xml

<beans xmlns:aop="http://www.springframework.org/schema/aop" xmlns:p="http://www.springframework.org/schema/p" xmlns:security="http://www.springframework.org/schema/security" xmlns:tx="http://www.springframework.org/schema/tx" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.springframework.org/schema/beans" xsi:schemalocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd
     http://www.springframework.org/schema/security http://www.springframework.org/schema/security/spring-security-2.0.4.xsd 
        http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd 
  http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.5.xsd">
  
   
  <bean class="com.dineshonjava.sdnext.beanscope.tutorial.Point" id="zeroPoint" scope="singleton">
   <property name="x" value="0"></property>
   <property name="y" value="0"></property>
  </bean>
  
</beans>


DrawingApp.java

package com.dineshonjava.sdnext.beanscope.tutorial;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

/**
 * @author Dinesh Rajput
 *
 */
public class DrawingApp 
{
 /**
  * @param args
  */
 public static void main(String[] args) 
 {
ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");

Point point1 = (Point) context.getBean("zeroPoint");
Point point2 = (Point) context.getBean("zeroPoint");
if(point1 == point2)
 System.out.println("In Singleton Demo >> Both are same objects values are.." +
     "n 1. point1= "+point1+" n 2. point2= "+point2);
else
  System.out.println("In Protptype Demo >> Both are different objects values are.." +
     "n 1. point1= "+point1+" n 2. point2= "+point2);
 }
}
Output:
Jun 28, 2012 9:10:46 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@ab50cd: startup date [Thu Jun 28 21:10:46 IST 2012]; root of context hierarchy
Jun 28, 2012 9:10:46 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [spring.xml]
Jun 28, 2012 9:10:46 PM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@1543c88: defining beans [zeroPoint]; root of factory hierarchy

In Singleton Demo >> Both are same objects values are..
1. point1= com.dineshonjava.sdnext.beanscope.tutorial.Point@1198891
2. point2= com.dineshonjava.sdnext.beanscope.tutorial.Point@1198891

Once you are done with creating source and bean configuration files, let us run the application. If everything is fine with your application, this will print the above message: See the both address value of the object is same.
Bean scopes using annotation:


spring.xml

<beans xmlns:aop="http://www.springframework.org/schema/aop" xmlns:context="http://www.springframework.org/schema/context" xmlns:p="http://www.springframework.org/schema/p" xmlns:security="http://www.springframework.org/schema/security" xmlns:tx="http://www.springframework.org/schema/tx" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.springframework.org/schema/beans" xsi:schemalocation="http://www.springframework.org/schema/beans">
  
<context:component-scan base-package="com.dineshonjava.sdnext.beanscope.tutorial">
</context:component-scan></beans>
Here component-scan element has attribute ‘base-package‘. base-package contain the base package of the Bean class.

base-package = “com.dineshonjava.sdnext.beanscope.tutorial”


Ponit.java

package com.dineshonjava.sdnext.beanscope.tutorial;

import org.springframework.context.annotation.Scope;
import org.springframework.stereotype.Service;

@Service
@Scope("singleton")
public class Point
{
 private int x;
 private int y;
 /**
  * @return the x
  */
 public int getX() {
  return x;
 }
 /**
  * @param x the x to set
  */
 public void setX(int x) {
  this.x = x;
 }
 /**
  * @return the y
  */
 public int getY() {
  return y;
 }
 /**
  * @param y the y to set
  */
 public void setY(int y) {
  this.y = y;
 }
}

Annotation @Service:
Target : Class
Description:
Here @Service is streotype annotation which declared above the class name that means this class is a bean of the spring container.
In Spring container this bean recognized by the id “point” means that same as the class name except first letter of the class name is the small letter.
for example: class ‘Point‘ get as ‘point‘ bean
                    class ‘Circle‘ get as ‘circle‘ bean

Annotation @Scope:
Target : Class
Description:
Here @Scope is annotation which declared above the class name that define the scope the bean

 @Scope(“singleton”)
 @Scope(“prototype”)

DrawingApp.java

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.dineshonjava.sdnext.beanscope.tutorial.Point;
/**
 * @author Dinesh Rajput
 *
 */
public class DrawingApp 
{
 /**
  * @param args
  */
 public static void main(String[] args) 
 {
ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
Point point1 = (Point) context.getBean("point");
Point point2 = (Point) context.getBean("point");
if(point1 == point2)
  System.out.println("In Singleton Demo >> Both are same objects values are.." +
  "n 1. point1= "+point1+" n 2. point2= "+point2);
else
  System.out.println("In Protptype Demo >> Both are different objects values are.." +
  "n 1. point1= "+point1+" n 2. point2= "+point2);
 }
}
Output:
Jun 28, 2012 10:22:04 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@145d068: startup date [Thu Jun 28 22:22:04 IST 2012]; root of context hierarchy
Jun 28, 2012 10:22:04 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [spring.xml]
Jun 28, 2012 10:22:04 PM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@1cbfe9d: defining beans [point,org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,org.springframework.context.annotation.ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor#0]; root of factory hierarchy

In Singleton Demo >> Both are same objects values are..
1. point1= com.dineshonjava.sdnext.beanscope.tutorial.Point@84da23
2. point2= com.dineshonjava.sdnext.beanscope.tutorial.Point@84da23

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                              Bean Scope in The Spring