Spring Bean Definition Inheritance Example

Spring Bean definition inheritance-. A bean definition potentially contains a large amount of configuration information, including container specific information and constructor arguments and property values. A child bean definition inherits configuration data from a parent definition. The child definition can override some values, or add others, as needed. Using parent and child bean definitions can save a lot of typing. Effectively, this is a form of templating.
Spring Bean Definition Inheritance
@ImageSource-Slideshare.net

If you work with an ApplicationContext interface programmatically, child bean definitions are represented by the ChildBeanDefinition class. Most users do not work with them on this level, instead configuring bean definitions declaratively in something like the ClassPathXmlApplicationContext

When you use XML-based configuration metadata, you indicate a child bean definition by using the parent attribute, specifying the parent bean as the value of this attribute.

Spring Bean definition inheritance has nothing to do with Java class inheritance but inheritance concept is same. You can define a parent bean definition as a template and other child beans can inherit required configuration from the parent bean.

Now the following example illustrate the bean definition inheritance with triangle classes and its parents classes.
Triangle.java
package com.dineshonjava.sdnext.beanDefInherit.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.beanDefInherit.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;
 }
}
Following is the configuration file spring.xml where we defined "parentTriangle" bean which has one property "pointA". Next there are two beans "triangle1" bean and "triangle2" bean has been defined as a child of "parentTriangle" bean by using parent attribute. The child beans inherits "pointA" property as is, and overrides "pointA" property and introduces two more properties "pointB" and "pointC.
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 class="com.dineshonjava.sdnext.beanDefInherit.tutorial.Triangle" id="parentTriangle">
    <property name="pointA" ref="pointA"></property>
</bean>
<bean class="com.dineshonjava.sdnext.beanDefInherit.tutorial.Triangle" id="triangle1" parent="parentTriangle">
   <property name="pointB" ref="pointB"></property>
   <property name="pointC" ref="pointC"></property>
</bean>
  
<bean class="com.dineshonjava.sdnext.beanDefInherit.tutorial.Triangle" id="triangle2" parent="parentTriangle">
   <property name="pointB" ref="pointB"></property>
</bean>
  
<bean class="com.dineshonjava.sdnext.beanDefInherit.tutorial.Point" id="pointA">
  <property name="x" value="0"></property>
  <property name="y" value="0"></property>
</bean>
  
<bean class="com.dineshonjava.sdnext.beanDefInherit.tutorial.Point" id="pointB">
  <property name="x" value="-20"></property>
  <property name="y" value="0"></property>
</bean>
  
<bean class="com.dineshonjava.sdnext.beanDefInherit.tutorial.Point" id="pointC">
  <property name="x" value="20"></property>
  <property name="y" value="0"></property>
</bean>
</beans>
Once you are done with creating source and bean configuration files, let us run the following application. If everything is fine with your application, this will print the following message:
Point.java
package com.dineshonjava.sdnext.beanDefInherit.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("triangle1");
triangle.draw();
 }
}
Now we get the following message:
Output:
Jul 1, 2012 1:43:35 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@ab50cd: startup date [Sun Jul 01 01:43:35 IST 2012]; root of context hierarchy
Jul 1, 2012 1:43:35 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [spring.xml]
Jul 1, 2012 1:43:36 AM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@14c1103: defining beans [parentTriangle,triangle1,triangle2,pointA,pointB,pointC]; root of factory hierarchy
PointA is (0, 0)
PointB is (-20, 0)
PointC is (20, 0)


If you observed here, we did not pass "pointA" while creating "triangle1" bean, but it got passed because of Bean Definition Inheritance.



In Next Chapter we will discuss about Bean Lifecycle and Callbacks in Spring.
                                          
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