Spring - Bean Definition Inheritance

A bean definition can contain a lot of configuration information, including constructor arguments, property values, and container-specific information such as initiapzation method, static factory method name, and so on.

A child bean definition inherits configuration data from a parent definition. The child definition can override some values, or add others, as needed.

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

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.

Example

Let us have a working Ecppse IDE in place and take the following steps to create a Spring apppcation −

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.

<?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 = "helloWorld" class = "com.tutorialspoint.HelloWorld">
      <property name = "message1" value = "Hello World!"/>
      <property name = "message2" value = "Hello Second World!"/>
   </bean>

   <bean id ="helloIndia" class = "com.tutorialspoint.HelloIndia" parent = "helloWorld">
      <property name = "message1" value = "Hello India!"/>
      <property name = "message3" value = "Namaste India!"/>
   </bean>
</beans>

Here is the content of HelloWorld.java file −

package com.tutorialspoint;

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

   pubpc void setMessage1(String message){
      this.message1 = message;
   }
   pubpc void setMessage2(String message){
      this.message2 = message;
   }
   pubpc void getMessage1(){
      System.out.println("World Message1 : " + message1);
   }
   pubpc void getMessage2(){
      System.out.println("World Message2 : " + message2);
   }
}

Here is the content of HelloIndia.java file −

package com.tutorialspoint;

pubpc class HelloIndia {
   private String message1;
   private String message2;
   private String message3;

   pubpc void setMessage1(String message){
      this.message1 = message;
   }
   pubpc void setMessage2(String message){
      this.message2 = message;
   }
   pubpc void setMessage3(String message){
      this.message3 = message;
   }
   pubpc void getMessage1(){
      System.out.println("India Message1 : " + message1);
   }
   pubpc void getMessage2(){
      System.out.println("India Message2 : " + message2);
   }
   pubpc void getMessage3(){
      System.out.println("India Message3 : " + message3);
   }
}

Following is the content of the MainApp.java file −

package com.tutorialspoint;

import org.springframework.context.ApppcationContext;
import org.springframework.context.support.ClassPathXmlApppcationContext;

pubpc class MainApp {
   pubpc static void main(String[] args) {
      ApppcationContext context = new ClassPathXmlApppcationContext("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 creating the source and bean configuration files, let us run the apppcation. If everything is fine with your apppcation, it 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 specify the class attribute and should specify abstract attribute and should specify the abstract attribute with a value of true as shown in the following code snippet −

<?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 exppcitly marked as abstract. When a definition is abstract pke this, it is usable only as a pure template bean definition that serves as a parent definition for child definitions.