@Required Annotation in Spring

The @Required annotation is used to specify that the value of a bean property is required to be dependency injected. That means, an error is caused if a value is not specified for that property.
The @Required annotation applies to bean property setter methods and it indicates that the affected bean property must be populated in XML configuration file at configuration time otherwise the container throws a BeanInitializationException exception. Below is an example to show the use of @Required annotation.

Simply apply the @Required annotation will not enforce the property checking, you have to register an RequiredAnnotationBeanPostProcessor to aware of the @Required annotation in bean configuration file. Simply applying @Required annotion is not enough to enforce the property checking, you also have to register RequiredAnnotationBeanPostProcessor in bean configuration file to do so. This configuration is done in two ways one by Include and another way is Include RequiredAnnotationBeanPostProcessor in bean configuration file. which is shown in this example’s spring.xml file.

In this example Circle class has the center property as required field declare with the @Required annotation. The @Required when written on top of setCenter() method it make sure that center property must    have    be    set    else   it   will   give   compile time error message that org.springframework.beans.factory.BeanInitializationException: Property ‘center ‘ is required for bean ‘circle’ which is clearly shown in this example.

Circle.java

package com.dineshonjava.sdnext.annotation.tutorial;

import org.springframework.beans.factory.annotation.Required;

public class Circle
{
 private Point center;

 /**
  * @param center the center to set
         * Here center is the required property for circle
  */
 @Required
 public void setCenter(Point center) 
 {
     this.center = center;
 }

 public void draw() 
 {
System.out.println("Circle is drawn of center ("+center.getX()+", "+center.getY()+")");
 }
}


Point.java

package com.dineshonjava.sdnext.annotation.tutorial;

public class Point
{
 private int x;
 private int y;
 /**
  * @return the x
  */
 public int getX() {
  return x;
 }
 /**
  * @param x the x to set
  */
 public void setX(int x) {
  this.x = x;
 }
 /**
  * @return the y
  */
 public int getY() {
  return y;
 }
 /**
  * @param y the y to set
  */
 public void setY(int y) {
  this.y = y;
 }
}


DrawingApp.java

package com.dineshonjava.sdnext.annotation.tutorial;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

/**
 * @author Dinesh Rajput
 *
 */
public class DrawingApp
{
 /**
  * @param args
  */
 public static void main(String[] args) 
 {
ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
Circle circle = (Circle) context.getBean("circle");
circle.draw();
 }
}

spring.xml

<beans xmlns:aop="http://www.springframework.org/schema/aop" xmlns:context="http://www.springframework.org/schema/context" xmlns:p="http://www.springframework.org/schema/p" xmlns:security="http://www.springframework.org/schema/security" xmlns:tx="http://www.springframework.org/schema/tx" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.springframework.org/schema/beans" xsi:schemalocation="http://www.springframework.org/schema/beans 
        http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
        http://www.springframework.org/schema/context 
        http://www.springframework.org/schema/context/spring-context-2.5.xsd
 http://www.springframework.org/schema/security 
        http://www.springframework.org/schema/security/spring-security-2.0.4.xsd 
        http://www.springframework.org/schema/aop 
        http://www.springframework.org/schema/aop/spring-aop-2.5.xsd 
        http://www.springframework.org/schema/tx 
        http://www.springframework.org/schema/tx/spring-tx-2.5.xsd">
  
<context:annotation-config></context:annotation-config>

<bean class="com.dineshonjava.sdnext.annotation.tutorial.Circle" id="circle">
  <!-- try without passing center and check the result -->
  <!--property name="center" ref="center"></property-->
</bean>
  
<bean class="com.dineshonjava.sdnext.annotation.tutorial.Point" id="center">
 <property name="x" value="10"></property>
 <property name="y" value="10"></property>
</bean>
</beans>
If we are using the element <context:annotation-config/> then there are no need to add the bean org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcess

<bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcess"></bean>
Actually element <context:annotation-config/> add all the beans AnnotationBeanPostProcess included in the spring container. there are no need to add individually for @required, @autowired etc.

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 raise BeanInitializationException exception and print the following error along with other log messages:

Output:
Jul 10, 2012 5:04:09 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@758fc9: startup date [Tue Jul 10 17:04:09 IST 2012]; root of context hierarchy
Jul 10, 2012 5:04:09 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [spring.xml]
Jul 10, 2012 5:04:10 PM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@19b5393: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,circle,center,org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor#0,org.springframework.context.annotation.ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor#0]; root of factory hierarchy
Jul 10, 2012 5:04:10 PM org.springframework.beans.factory.support.DefaultSingletonBeanRegistry destroySingletons
INFO: Destroying singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@19b5393: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,circle,center,org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor#0,org.springframework.context.annotation.ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor#0]; root of factory hierarchy

Exception in thread “main” org.springframework.beans.factory.BeanCreationException: Error creating bean with name ‘circle’ defined in class path resource [spring.xml]: Initialization of bean failed; nested exception is org.springframework.beans.factory.BeanInitializationException: Property ‘center’ is required for bean ‘circle’
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:527)

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:456)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:294)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:225)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:291)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:193)

at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:609)

at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:918)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:469)

at org.springframework.context.support.ClassPathXmlApplicationContext.(ClassPathXmlApplicationContext.java:139)
at org.springframework.context.support.ClassPathXmlApplicationContext.(ClassPathXmlApplicationContext.java:83)
at com.sdnext.annotation.tutorial.DrawingApp.main(DrawingApp.java:19)
Caused by: org.springframework.beans.factory.BeanInitializationException: Property ‘center’ is required for bean ‘circle’
at org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor.postProcessPropertyValues(RequiredAnnotationBeanPostProcessor.java:149)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1106)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:517)
… 11 more

Next, we can try above example after removing comment from ‘center’ property as follows:

spring.xml

<beans xmlns:aop="http://www.springframework.org/schema/aop" xmlns:context="http://www.springframework.org/schema/context" xmlns:p="http://www.springframework.org/schema/p" xmlns:security="http://www.springframework.org/schema/security" xmlns:tx="http://www.springframework.org/schema/tx" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.springframework.org/schema/beans" xsi:schemalocation="http://www.springframework.org/schema/beans 
        http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
        http://www.springframework.org/schema/context 
        http://www.springframework.org/schema/context/spring-context-2.5.xsd
 http://www.springframework.org/schema/security 
        http://www.springframework.org/schema/security/spring-security-2.0.4.xsd 
        http://www.springframework.org/schema/aop 
        http://www.springframework.org/schema/aop/spring-aop-2.5.xsd 
        http://www.springframework.org/schema/tx 
        http://www.springframework.org/schema/tx/spring-tx-2.5.xsd">
  
<context:annotation-config></context:annotation-config>

<bean class="com.sdnext.annotation.tutorial.Circle" id="circle">
  <!-- try with passing center and check the result -->
  <property name="center" ref="center"></property>
</bean>
  
<bean class="com.sdnext.annotation.tutorial.Point" id="center">
 <property name="x" value="10"></property>
 <property name="y" value="10"></property>
</bean>
</beans>

Now above example will produce following result:

Output:
Jul 10, 2012 5:38:54 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@ab50cd: startup date [Tue Jul 10 17:38:54 IST 2012]; root of context hierarchy
Jul 10, 2012 5:38:54 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [spring.xml]
Jul 10, 2012 5:38:54 PM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@19b5393: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,circle,center,org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor#0,org.springframework.context.annotation.ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor#0]; root of factory hierarchy

Circle is drawn of center (10, 10)

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