@Required
Incase the bean value is not defined using required will display the spring error message instead of null pointer exception for the bean whose value is not defined.

spring.xml

<bean class="com.mugil.shapes.Circle" id="circleId">		
</bean>

Note above the pointA is undefined for above circle bean

Spring Error without @Required annotation

Exception in thread "main" java.lang.NullPointerException
	at com.mugil.shapes.Circle.drawShape(Circle.java:11)
	at com.mugil.shapes.DrawingApp.main(DrawingApp.java:17)

circle.java

public class Circle implements Shape{
	private Point center;
	
	@Override
	public void drawShape() {
		System.out.println("Shape of Circle ");
		System.out.println("Center of Cirlce "+ getCenter().getX() + " - " + getCenter().getY());
	}

	public Point getCenter() {
		return center;
	}

	@Required
	public void setCenter(Point center) {
		this.center = center;
	}		
}

spring.xml

<bean class="com.mugil.shapes.Circle" id="circleId">		
</bean>
	
<bean class="org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor"/>

Spring Error is Thrown

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'circleId' defined in class path resource [spring1.xml]: Initialization of bean failed; nested exception is org.springframework.beans.factory.BeanInitializationException: Property 'center' is required for bean 'circleId'

Autowired Annotation

1. @Autowired works by taking into consideration type,name,qualifier
2. If type not found name will be considered

spring.xml(by Type)

<beans>
	<bean id="PointA" class="com.mugil.shapes.Point">
		<property name="x" value="0"/>
		<property name="y" value="20"/>
	</bean>	
	<bean class="com.mugil.shapes.Circle" id="circleId"></bean>	
	<bean class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor"/>
</beans>

spring.xml(by Name)

<beans>
	<bean id="center" class="com.mugil.shapes.Point">
		<property name="x" value="0"/>
		<property name="y" value="20"/>
	</bean>	
	<bean class="com.mugil.shapes.Circle" id="circleId"></bean>	
	<bean class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor"/>
</beans>

In above xml center is the name of instance variable in circle class
spring.xml(by Qualifier)

<?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-2.5.xsd
		http://www.springframework.org/schema/context 
		http://www.springframework.org/schema/context/spring-context-2.5.xsd">		
	<bean id="pointA" class="com.mugil.shapes.Point">
		<qualifier value="circle"/>			
		<property name="x" value="20"/>
		<property name="y" value="0"/>
	</bean>	
	<bean class="com.mugil.shapes.Circle" id="circleId"></bean>	
	<bean class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor"/>
</beans>

circle.java

public class Circle implements Shape{
	@Autowired
	@Qualifier("circle")
	private Point center;
	
	@Override
	public void drawShape() {
		System.out.println("Shape of Circle ");
		System.out.println("Center of Cirlce "+ getCenter().getX() + " - " + getCenter().getY());
	}

	public Point getCenter() {
		return center;
	}
	
	public void setCenter(Point center) {
		this.center = center;
	}		
}