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BeanPostProcessor in Spring

In this tutorial we discuss about the BeanPostProcessor and its need. BeanPostProcessor is interface that tells Spring to do some processing after initialization some beans.
BeanPostProcessor is interface that tells Spring to do some processing after initialization some beans.This allows you to add some custom logic before and after spring bean creation.
BeanPostProcessor
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BeanPostProcessor in Spring
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Many processes in the IoC container were made to be extensible. A specific extensible process can be referred to as an extension point. One extension point, the BeanPostProcessor interface, allows the modification of a bean instance before and after the properties are set. Another extension point is the BeanFactoryPostProcessor interface which allows direct modification of bean definitions before a bean is instantiated.
An ApplicationContext will automatically register and process a bean that implements either of these interfaces (BeanPostProcessor , BeanFactoryPostProcessor ), but a BeanFactory would have to have a BeanPostProcessor or BeanFactoryPostProcessor registered with it programatically as given below.
.....
// create BeanFactory
ConfigurableBeanFactory  factory = new XmlBeanFactory(new FileSystemResource("spring.xml"));
// now register some beans
// now register any needed BeanPostProcessors
DisplayNameBeanPostProcessor postProcessor = new DisplayNameBeanPostProcessor(); 
factory.addBeanPostProcessor(postProcessor);
.....
// now start using the factory
Since this manual registration step is not convenient, and ApplictionContexts are functionally supersets of BeanFactories, it is generally recommended that ApplicationContext variants are used when bean post-processors are needed.
BeanPostProcessor: A BeanPostProcessor gives you a chance to process an instance of a bean created by the IoC container after it's instantiation and then again after the initialization life cycle event has occurred on the instance. You could use this to process fields that were set, perform validation on a bean, or even look up values from a remote resource to set on the bean as defaults.

Spring's different AOP proxies for caching, transactions, etc. are all applied by BeanPostProcessor .
Lets see the following example which describe the BeanPostProcessor activity...
DisplayNameBeanPostProcessor.java
package com.dineshonjava.sdnext.beanPostProcessor.tutorial;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor;

public class DisplayNameBeanPostProcessor implements BeanPostProcessor 
{  
@Override
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException 
{
System.out.println("In After bean Initialization method. Bean name is "+beanName);
return bean;
}

@Override
public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException 
{
System.out.println("In Before bean Initialization method. Bean name is "+beanName);
return bean;
}
}
BeanPostProcessor interface has two method...
1. postProcessAfterInitialization(Object bean, String beanName) execute after initialization of each beans in the Spring IoC Container.
2. postProcessBeforeInitialization(Object bean, String beanName) execute before initialization of each beans in the Spring IoC Container.
Triangle.java
package com.dineshonjava.sdnext.beanPostProcessor.tutorial;

public class Triangle
{
 private Point pointA;
 private Point pointB;
 private Point pointC;
 /**
  * @param pointA the pointA to set
  */
 public void setPointA(Point pointA) {
  this.pointA = pointA;
 }

 /**
  * @param pointB the pointB to set
  */
 public void setPointB(Point pointB) {
  this.pointB = pointB;
 }

 /**
  * @param pointC the pointC to set
  */
 public void setPointC(Point pointC) {
  this.pointC = pointC;
 }

 public void draw()
 {
System.out.println("PointA is ("+pointA.getX()+", "+pointA.getY()+")");
System.out.println("PointB is ("+pointB.getX()+", "+pointB.getY()+")");
System.out.println("PointC is ("+pointC.getX()+", "+pointC.getY()+")");
 }
}
Point.java
package com.dineshonjava.sdnext.beanPostProcessor.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;
 }
}
DisplayNameBeanPostProcessor bean in the Spring IoC Container is automatically called when before and after any beans (triangle, pointA, pointB, pointC) creation. In the below configuration file has the DisplayNameBeanPostProcessor bean
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">
  
<bean autowire="byName" class="com.dineshonjava.sdnext.beanPostProcessor.tutorial.Triangle" id="triangle"></bean>
  
<bean class="com.dineshonjava.sdnext.beanPostProcessor.tutorial.Point" id="pointA">
  <property name="x" value="0"></property>
  <property name="y" value="0"></property>
</bean>
  
<bean class="com.dineshonjava.sdnext.beanPostProcessor.tutorial.Point" id="pointB">
  <property name="x" value="-20"></property>
  <property name="y" value="0"></property>
</bean>
  
<bean class="com.dineshonjava.sdnext.beanPostProcessor.tutorial.Point" id="pointC">
   <property name="x" value="20"></property>
   <property name="y" value="0"></property>
</bean>
  
<bean class="com.dineshonjava.sdnext.beanPostProcessor.tutorial.DisplayNameBeanPostProcessor"></bean>
</beans>
If every thing is OK now run the following class file and see the result on console.
DrawingApp.java
package com.dineshonjava.sdnext.beanPostProcessor.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");
Triangle triangle = (Triangle) context.getBean("triangle");
triangle.draw();
 }
}

Output:
Jul 2, 2012 11:28:23 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@ab50cd: startup date [Mon Jul 02 23:28:23 IST 2012]; root of context hierarchy
Jul 2, 2012 11:28:23 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [spring.xml]
Jul 2, 2012 11:28:23 PM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@14c1103: defining beans [triangle,pointA,pointB,pointC,com.sdnext.beanPostProcessor.tutorial.DisplayNameBeanPostProcessor#0]; root of factory hierarchy
In Before bean Initialization method. Bean name is pointA
In After bean Initialization method. Bean name is pointA
In Before bean Initialization method. Bean name is pointB
In After bean Initialization method. Bean name is pointB
In Before bean Initialization method. Bean name is pointC
In After bean Initialization method. Bean name is pointC
In Before bean Initialization method. Bean name is triangle
In After bean Initialization method. Bean name is triangle
PointA is (0, 0)
PointB is (-20, 0)
PointC is (20, 0)

There are four beans (triangle, pointA, pointB, pointC) in the Spring IoC Container and both methods of the BeanPostProcessor interface is executes four times.

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In Next Chapter we will discuss about BeanFactoryPostProcessor in Spring in Application.
                                          
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