[转载]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. 

<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;
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.