In Spring, the inheritance is supported in bean configuration for a bean to share common values, properties or configurations.
A child bean or inherited bean can inherit its parent bean configurations, properties and some attributes. In additional, the child beans are allow to override the inherited value.
See following full example to show you how bean configuration inheritance works in Spring.
package com.mkyong.common;
public class Customer {
private int type;
private String action;
private String Country;
//...
}
Bean configuration file
<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-2.5.xsd">
<bean id="BaseCustomerMalaysia" class="com.mkyong.common.Customer">
<property name="country" value="Malaysia" />
</bean>
<bean id="CustomerBean" parent="BaseCustomerMalaysia">
<property name="action" value="buy" />
<property name="type" value="1" />
</bean>
</beans>
Above is a ‘BaseCustomerMalaysia’ bean contains a ‘Malaysia’ value for country property, and the ‘CustomerBean’ bean inherited this value from its parent (‘BaseCustomerMalaysia’).
Run it
package com.mkyong.common;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class App
{
public static void main( String[] args )
{
ApplicationContext context =
new ClassPathXmlApplicationContext("SpringBeans.xml");
Customer cust = (Customer)context.getBean("CustomerBean");
System.out.println(cust);
}
}
output
Customer [type=1, action=buy, Country=Malaysia]
The ‘CustomerBean’ bean just inherited the country property from its parent (‘BaseCustomerMalaysia’).
Inheritance with abstract
In above example, the ‘BaseCustomerMalaysia’ is still able to instantiate, for example,
Customer cust = (Customer)context.getBean("BaseCustomerMalaysia");
If you want to make this base bean as a template and not allow others to instantiate it, you can add an ‘abstract‘ attribute in the <bean> element. For example
<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-2.5.xsd">
<bean id="BaseCustomerMalaysia" class="com.mkyong.common.Customer" abstract="true">
<property name="country" value="Malaysia" />
</bean>
<bean id="CustomerBean" parent="BaseCustomerMalaysia">
<property name="action" value="buy" />
<property name="type" value="1" />
</bean>
</beans>
Now, the ‘BaseCustomerMalaysia’ bean is a pure template, for bean to inherit it only, if you try to instantiate it, you will encounter the following error message.
Customer cust = (Customer)context.getBean("BaseCustomerMalaysia");
org.springframework.beans.factory.BeanIsAbstractException:
Error creating bean with name 'BaseCustomerMalaysia':
Bean definition is abstract
Pure Inheritance Template
Actually, parent bean is not necessary to define class attribute, often times, you may just need a common property for sharing. Here’s is an example
<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-2.5.xsd">
<bean id="BaseCustomerMalaysia" abstract="true">
<property name="country" value="Malaysia" />
</bean>
<bean id="CustomerBean" parent="BaseCustomerMalaysia"
class="com.mkyong.common.Customer">
<property name="action" value="buy" />
<property name="type" value="1" />
</bean>
</beans>
In this case, the ‘BaseCustomerMalaysia’ bean is a pure template, to share its ‘country’ property only.
Overrride it
However, you are still allow to override the inherited value by specify the new value in the child bean. Let’s see this example
<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-2.5.xsd">
<bean id="BaseCustomerMalaysia" class="com.mkyong.common.Customer" abstract="true">
<property name="country" value="Malaysia" />
</bean>
<bean id="CustomerBean" parent="BaseCustomerMalaysia">
<property name="country" value="Japan" />
<property name="action" value="buy" />
<property name="type" value="1" />
</bean>
</beans>
The ‘CustomerBean’ bean is just override the parent (‘BaseCustomerMalaysia’) country property, from ‘Malaysia’ to ‘Japan’.
Customer [Country=Japan, action=buy, type=1]
Conclusion
The Spring bean configuration inheritance is very useful to avoid the repeated common value or configurations for multiple beans.
Could the spring bean configuration inheritance be done for constructor arguments?
HI,
i have a simple question,
apart from restricting the parent bean to be instantiated, is there any other advantage of making parent bean abstract or is it the only reason..
if I have some class like this
public abstract class Action{
public abstract void execute();
}
and other class is
public class Play{
@Override
public void execute(){
//somework
}
}
than what should i do
nicely explained thank you Mkyong…..
There is a typo in paragraph title:
“Overrride it”
I wonder how come private properties of Customer Class are inherited and available to child class? In Java it does not that way right? It will be helpful if you can provide some more information on that.
– Thanks.
If you notice properly, we are inheriting the class, but it is inheritance on bean level. Therefore, a child bean can inherit values of parent bean. When you say childBean.getAttribute1() you actually are calling the getAttribute1() on parentBean (assuming attribute1 beaing an attribute in the class)
ur telling correct but we are not inheriting the class,we are inheriting bean dependency object ,so child bean copy of properties in parent bean .
That is what I meant. Nitpicking huh? 🙂
yes..its true..i applied the same in my real-time project.. its so helpful and avoiding to rewrite the same kind of properties in multiple beans. Explained simply nd clearly. Thank You.
Thanks! This helped me out of some hair pulling. For some reason specifying the class for the parent class was causing problems for me.
You should write a book
Thanks very clearly explained.
i have a class B that extends class A..
is that necessary/compulsory to give the relationship in spring configuration file while initializing like this
if we don’t give what will happen??
not required in inheritence,,,,, if u r going with composition & u don’t want to create object of dependent class then u have to provide information about ur classes to IOC
Hi mkyong,
Superb tutorial….! I learn a lot many things here…Thanks for this tutorial…!
Hi,
1. Do i need to declare like
‘public abstract class BaseCustomerMalaysia’ in bean file ?
2. Do i need to extend BaseCustomerMalaysia to CustomerBean ?
No, download the attachment to view the full source code.
The bean (POJO) is “Customer” only, “BaseCustomerMalaysia” is the bean is, that represent Customer class.
where is the class for CustomerBean ?