@Required Annotation in spring

The @Required annotation applies to bean property setter methods, as in the following example:

package org.arpit.javapostsforlearning;

public class Country {

    Capital capital; 

    @Required
    public void setCapital(Capital capital) {
        this.capital = capital;
    }
}

This annotation simply indicates that the affected bean property must be populated at configuration time: either through an explicit property value in a bean definition or through autowiring. The container will throw an exception if the affected bean property has not been populated; this allows for eager and explicit failure, avoiding NullPointerExceptions or the like later on.

Suppose you have very large application and you get   NullPointerExceptions because required dependency has not been injected then it is very hard to find out what goes wrong.So this annotation helps us in debugging the code.

Example:

For configuring spring in your eclipse ide please refer  hello world example

1.Country.java:

This is simple pojo class having some attributes so here country has name and object of Capital class.

Create Country.java under package org.arpit.javapostsforlearning.Copy following content into Country.java.

package org.arpit.javapostsforlearning;
import org.springframework.beans.factory.annotation.Required;
public class Country {

    String countryName;
    Capital capital;
    public String getCountryName() {
        return countryName;
    }
    public void setCountryName(String countryName) {
        this.countryName = countryName;
    }
    public Capital getCapital() {
        return capital;
    }

    @Required
    public void setCapital(Capital capital) {
        this.capital = capital;
    }
}

2.Capital.java

This is also simple pojo class having one attribute called "capitalName".

Create Capital.java under package org.arpit.javapostsforlearning.java.Above Country class contains object of this class.Copy following content into Capital.java

package org.arpit.javapostsforlearning;

public class Capital {

    String capitalName;

    public String getCapitalName() {
        return capitalName;
    }

    public void setCapitalName(String capitalName) {
        this.capitalName = capitalName;
    }
}

3.RequiredAnnotationInSpringMain.java

This class contains main function.Create RequiredAnnotationInSpringMain.java under package org.arpit.javapostsforlearning.Copy following content into RequiredAnnotationInSpringMain.java

 package org.arpit.javapostsforlearning;

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

public class RequiredAnnotationInSpringMain{

    public static void main(String[] args) {
        ApplicationContext appContext = new ClassPathXmlApplicationContext("ApplicationContext.xml");

        Country countryObj = (Country) appContext.getBean("CountryBean");
        String countryName=countryObj.getCountryName();
        Capital capital=countryObj.getCapital();
        String capitalName=capital.getCapitalName();
        System.out.println(capitalName+" is capital of "+countryName);
       
    }
}

4.ApplicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-3.0.xsd">
 <context:annotation-config/>
 <bean id="CountryBean" class="org.arpit.javapostsforlearning.Country">
  <property name="countryName" value="India" />
 </bean>
 <bean id="CaptialBean" class="org.arpit.javapostsforlearning.Capital">
  <property name="capitalName" value="Delhi" />
 </bean>
</beans>

Now you can note here that we have not injected capital property into country class but we have used it for getting capitalName in RequiredAnnotationInSpringMain class.

5.Run it:

When you will run above application,you will get following as output.

Sep 01, 2012 1:43:16 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@17a1767: startup date [Sat Sep 01 13:43:16 IST 2012]; root of context hierarchy
Sep 01, 2012 1:43:17 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [ApplicationContext.xml]
Sep 01, 2012 1:43:17 PM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@54446d: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,CountryBean,CaptialBean,org.springframework.context.annotation.ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor#0]; root of factory hierarchy
Sep 01, 2012 1:43:17 PM org.springframework.beans.factory.support.DefaultSingletonBeanRegistry destroySingletons
INFO: Destroying singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@54446d: defining beans [org.springframework.context.annotation.internalConfigurationAnnotationProcessor,org.springframework.context.annotation.internalAutowiredAnnotationProcessor,org.springframework.context.annotation.internalRequiredAnnotationProcessor,org.springframework.context.annotation.internalCommonAnnotationProcessor,CountryBean,CaptialBean,org.springframework.context.annotation.ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor#0]; root of factory hierarchy
Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'CountryBean' defined in class path resource [ApplicationContext.xml]: Initialization of bean failed; nested exception is org.springframework.beans.factory.BeanInitializationException: Property 'capital' is required for bean 'CountryBean'
    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:585)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:913)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:464)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
    at org.arpit.javapostsforlearning.SetterInjectionMain.main(SetterInjectionMain.java:11)
Caused by: org.springframework.beans.factory.BeanInitializationException: Property 'capital' is required for bean 'CountryBean'
    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

When you run it,you will get exception complaining "Property 'capital' is required for bean 'CountryBean'"