@Required Annotation in Spring Framework

@Required annotation is placed in Spring API package org.springframework.beans.factory.annotation.* . This annotation is used for checking Spring Dependency. It checks whether properties of bean are set or not. @Required annotation is placed over the setter for the properties which is to be checked for Spring Dependency. 

A Spring Bean by the name "RequiredAnnotationBeanPostProcessor" checks if properties annotated with @Required have been set. Before a bean is initialized into a Spring Container this bean post processor checks for properties whether are set or not (marked as @Required).

Let's look at it by a simple example -

(WITHOUT @Required ANNOTATION)

1. Create a class say "Address.java"

package com.hubberspot.spring.ioc;

public class Address {

 private String street;
 private String city;
 
 public String getStreet() {
  return street;
 }
 public void setStreet(String street) {
  this.street = street;
 }
 public String getCity() {
  return city;
 }
 public void setCity(String city) {
  this.city = city;
 } 
 
}

2. Create a class say "Employee.java"

package com.hubberspot.spring.ioc;

public class Employee {

 private String firstName; 
 private Address address;

 public String getFirstName() {
  return firstName;
 }

 public void setFirstName(String firstName) {
  this.firstName = firstName;
 }

 public Address getAddress() {
  return address;
 }

 public void setAddress(Address address) {
  this.address = address;
 } 
 
}

3. Create a Spring Configuration file say "spring.xml"

<beans xmlns:context="http://www.springframework.org/schema/context" 
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-3.0.xsd
        http://www.springframework.org/schema/context
        http://www.springframework.org/schema/context/spring-context-3.0.xsd">
  
 <bean class="com.hubberspot.spring.ioc.Employee" id="employee">
  <property name="firstName" value="Dinesh"></property>
 </bean>

 <bean class="com.hubberspot.spring.ioc.Address" id="address">
     <property name="street" value="Park Street"></property>
     <property name="city" value="Pune"></property>
 </bean>

</beans>

4. Create a Test class say "Test.java"

package com.hubberspot.spring.ioc;

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

public class Test {

 public static void main(String[] args) {

  // ApplicationContext is a Spring interface which
  // provides with the configuration for an application.
  // It provides us with all the methods that BeanFactory
  // provides. It loads the file resources in a older
  // and generic manner. It helps us to publish events to the
  // listener registered to it. It also provides quick support
  // for internationalization. It provides us with the object
  // requested, it reads the configuration file and provides
  // us with the necessary object required.
  // We are using concrete implementation of ApplicationContext
  // here called as ClassPathXmlApplicationContext because this
  // bean factory reads the xml file placed in the classpath of
  // our application. We provide ClassPathXmlApplicationContext
  // with a configuration file called as spring.xml placed
  // at classpath of our application.
  ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");

  // In order to get a object instantiated for a particular bean
  // we call getBean() method of ClassPathXmlApplicationContext
  // passing it the id for which the object is to be needed.
  // Here getBean() returns an Object. We need to cast it back
  // to the Employee object. Without implementing new keyword we
  // have injected object of Employee just by reading an xml
  // configuration file.  
  Employee employee = (Employee) context.getBean("employee");

  System.out.println("FirstName : " + employee.getFirstName());

  System.out.println("Address : " + employee.getAddress().getStreet() + " , "
    + employee.getAddress().getCity());


 }

}

As we haven't provided Dependency Injection for Address bean in Employee bean. When we run the above Test class the output comes out to be as -

Exception in thread "main" FirstName : Dinesh
java.lang.NullPointerException
    at com.hubberspot.spring.ioc.Test.main(Test.java:36)

Its a null pointer exception because Address bean is not set in Employee bean. When we access the properties of Address bean through Address we get null pointer exception.

(WITH @Required ANNOTATION)

1. Create a class say "Address.java"

package com.hubberspot.spring.ioc;

public class Address {

 private String street;
 private String city;
 
 public String getStreet() {
  return street;
 }
 public void setStreet(String street) {
  this.street = street;
 }
 public String getCity() {
  return city;
 }
 public void setCity(String city) {
  this.city = city;
 } 
 
}

2. Create a class say "Employee.java" having @Required annotation on Address setter method.

package com.hubberspot.spring.ioc;

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

public class Employee {

 private String firstName; 
 private Address address;

 public String getFirstName() {
  return firstName;
 }

 public void setFirstName(String firstName) {
  this.firstName = firstName;
 }

 public Address getAddress() {
  return address;
 }

 @Required
 public void setAddress(Address address) {
  this.address = address;
 }  
 
}

3. Create a Spring Configuration file say "spring.xml" having a tag context:annotation-config for registering RequiredAnnotationBeanPostProcessor which checks if all the bean properties with the @Required annotation have been set.

<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="employee" class="com.hubberspot.spring.ioc.Employee">
  <property name="firstName" value="Dinesh"></property>
 </bean>

 <bean id="address" class="com.hubberspot.spring.ioc.Address">
     <property name="street" value="Park Street"></property>
     <property name="city" value="Pune"></property>
 </bean>

</beans>

4. Create a Test class say "Test.java"

package com.hubberspot.spring.ioc;

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

public class Test {

 public static void main(String[] args) {

  // ApplicationContext is a Spring interface which
  // provides with the configuration for an application.
  // It provides us with all the methods that BeanFactory
  // provides. It loads the file resources in a older
  // and generic manner. It helps us to publish events to the
  // listener registered to it. It also provides quick support
  // for internationalization. It provides us with the object
  // requested, it reads the configuration file and provides
  // us with the necessary object required.
  // We are using concrete implementation of ApplicationContext
  // here called as ClassPathXmlApplicationContext because this
  // bean factory reads the xml file placed in the classpath of
  // our application. We provide ClassPathXmlApplicationContext
  // with a configuration file called as spring.xml placed
  // at classpath of our application.
  ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");

  // In order to get a object instantiated for a particular bean
  // we call getBean() method of ClassPathXmlApplicationContext
  // passing it the id for which the object is to be needed.
  // Here getBean() returns an Object. We need to cast it back
  // to the Employee object. Without implementing new keyword we
  // have injected object of Employee just by reading an xml
  // configuration file.  
  Employee employee = (Employee) context.getBean("employee");

  System.out.println("FirstName : " + employee.getFirstName());

  System.out.println("Address : " + employee.getAddress().getStreet() + " , "
    + employee.getAddress().getCity());


 }

}

As we haven't provided Dependency Injection for Address bean in Employee bean but this time we have marked setter method of Address with @Required annotation. When we run the above Test class the output comes out to be as -

Exception in thread "main" org.springframework.beans.factory.BeanCreationException:
Error creating bean with name 'employee' defined in class path resource [spring.xml]:
Initialization of bean failed; nested exception is
org.springframework.beans.factory.BeanInitializationException:
Property 'address' is required for bean 'employee'


It checks this time before initialization of Employee bean that Address bean is not injected and exception is thrown.