Spring Bean Definition Inheritance

Following is the configuration file Beans.xml where we defined "helloWorld" bean which has two properties message1 and message2. Next "helloIndia" bean has been defined as a child of "helloWorld" bean by using parent attribute. The child bean inherits message2 property as is, and overrides message1 property and introduces one more property message3. 

 1 <beans xmlns="http://www.springframework.org/schema/beans"

 2 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

 3 xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-

 4 3.0.xsd">

 5 <bean id="helloWorld" class="com.tutorialspoint.HelloWorld"> <property name="message1" value="Hello World!"/>

 6 <property name="message2" value="Hello Second World!"/>

 7 </bean>

 8 <bean id="helloIndia" class="com.tutorialspoint.HelloIndia" parent="helloWorld">

 9 <property name="message1" value="Hello India!"/> <property name="message3" value="Namaste India!"/>

10    </bean>

11 </beans>

Here is the content of HelloWorld.java file: 

package com.tutorialspoint;

public class HelloWorld { private String message1; private String message2;

public void setMessage1(String message){ this.message1 = message;

}

public void setMessage2(String message){ this.message2 = message;

}

public void getMessage1(){

System.out.println("World Message1 : " + message1);

}

public void getMessage2(){

System.out.println("World Message2 : " + message2);

} }

Here is the content of HelloIndia.java file: 

package com.tutorialspoint;

public class HelloIndia { 

private String message1; 

private String message2; 

private String message3;

public void setMessage1(String message){ this.message1 = message;

}

public void setMessage2(String message){ this.message2 = message;

}

public void setMessage3(String message){ this.message3 = message;

}

public void getMessage1(){

System.out.println("India Message1 : " + message1);}

public void getMessage2(){

System.out.println("India Message2 : " + message2);}

public void getMessage3(){

System.out.println("India Message3 : " + message3);}

}

Following is the content of the MainApp.java file: 

import org.springframework.context.ApplicationContext;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class MainApp {

public static void main(String[] args) {

ApplicationContext context =

new ClassPathXmlApplicationContext("Beans.xml");

      HelloWorld objA = (HelloWorld) context.getBean("helloWorld");

      objA.getMessage1();

      objA.getMessage2();

      HelloIndia objB = (HelloIndia) context.getBean("helloIndia");

      objB.getMessage1();

      objB.getMessage2();

      objB.getMessage3();

} }



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 following message:

World Message1 : Hello World!

World Message2 : Hello Second World!

India Message1 : Hello India!

India Message2 : Hello Second World!

India Message3 : Namaste India!

If you observed here, we did not pass message2 while creating "helloIndia" bean, but it got passed because of Bean Definition Inheritance.

 

Bean Definition Template 

You can create a Bean definition template which can be used by other child bean definitions without putting much effort. While defining a Bean Definition Template, you should not specifyclassattribute and should specifyabstractattribute with a value oftrueas shown below: 

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns="http://www.springframework.org/schema/beans"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-

3.0.xsd">

<bean id="beanTeamplate" abstract="true">

<property name="message1" value="Hello World!"/> <property name="message2" value="Hello Second World!"/> <property name="message3" value="Namaste India!"/>

</bean>

<bean id="helloIndia" class="com.tutorialspoint.HelloIndia" parent="beanTeamplate">

<property name="message1" value="Hello India!"/> <property name="message3" value="Namaste India!"/>

   </bean>

</beans>

The parent bean cannot be instantiated on its own because it is incomplete, and it is also explicitly marked as abstract. When a definition is abstract like this, it is usable only as a pure template bean definition that serves as a parent definition for child definitions.