This post on the JavaRanch Big Moose Saloon led me to fire up Eclipse and write some test code. The question is basically how can you use the ability to map a parent bean as part of a bean definition in Spring.

Let’s look at three classes and see how they can be mapped using Spring.

public abstract class SuperBean {
	private int id;
	private String name;
 
	public int getId() {
		return id;
	}
 
	public void setId(int id) {
		this.id = id;
	}
 
	public String getName() {
		return name;
	}
 
	public void setName(String name) {
		this.name = name;
	}
 
}
 
public class SubBean extends SuperBean {
 
	private String newProperty;
 
	public String getNewProperty() {
		return newProperty;
	}
 
	public void setNewProperty(String newProperty) {
		this.newProperty = newProperty;
	}
 
}
 
public class DifferentBean {
	private int id;
	private String name;
	private String myProperty;
 
	public int getId() {
		return id;
	}
 
	public void setId(int id) {
		this.id = id;
	}
 
	public String getName() {
		return name;
	}
 
	public void setName(String name) {
		this.name = name;
	}
 
	public String getMyProperty() {
		return myProperty;
	}
 
	public void setMyProperty(String myProperty) {
		this.myProperty = myProperty;
	}
 
}

now let’s map them in Spring:

<?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-2.0.xsd">
 
    <bean id="superBean" abstract="true"
        class="com.stevideter.spring.SuperBean">
        <property name="id" value="1" />
        <property name="name" value="superBean" />
    </bean>
 
    <bean id="subBean" class="com.stevideter.spring.SubBean"
        parent="superBean">
        <property name="id" value="2" />
        <property name="newProperty" value="subBean" />
    </bean>
 
    <bean id="differentBean" class="com.stevideter.spring.DifferentBean"
        parent="superBean">
        <property name="id" value="3" />
        <property name="myProperty" value="differentBean" />
    </bean>
</beans>

And now let’s write a simple app to show what happens with these mappings:

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
 
public class TestClass {
 
    public static void main(String[] args) {
 
        ApplicationContext context = new ClassPathXmlApplicationContext(
                new String[] {"applicationContext.xml"});
 
        SubBean subBean = (SubBean)context.getBean("subBean");
        System.out.printf("ID: %d, Name: %s, Property: %s\n",
                                subBean.getId(), subBean.getName(), subBean.getNewProperty());
 
        DifferentBean differentBean = (DifferentBean)context.getBean("differentBean");
        System.out.printf("ID: %d, Name: %s, Property: %s\n",
                                differentBean.getId(), differentBean.getName(),
                                differentBean.getMyProperty());
    }
}

If we run this class, we get the following results:

ID: 2, Name: superBean, Property: subBean
ID: 3, Name: superBean, Property: differentBean

What we learn from this is that you can use the parent attribute to map to a parent bean definition that your bean class doesn’t inherit from. What’s necessary is that the classes are compatible, which means that the child class has all the properties that are defined in the parent class. In fact, the documentation notes you don’t have to declare a class on the parent definition at all.

I find it more useful to think of the parent definition as a template, to avoid the trap of thinking it is an object-oriented style of inheritance. Another way to think of it is as a parent interface for the child bean; you must implement these properties in order to use this definition.

Posted: 22 Mar, 2008 |
Categories: Java , spring | Tags: , , , , | By: stevi.