Spring Auto-Wiring Beans with @Autowired annotation

In last Spring auto-wiring in XML example, it will autowired the matched property of any bean in current Spring container. In most cases, you may need autowired property in a particular bean only.

In Spring, you can use @Autowired annotation to auto wire bean on the setter method, constructor or a field. Moreover, it can autowired property in a particular bean.

Note
The @Autowired annotation is auto wire the bean by matching data type.

See following full example to demonstrate the use of @Autowired.

1. Beans

A customer bean, and declared in bean configuration file. Later, you will use “@Autowired” to auto wire a person bean.

package com.mkyong.common;   public class Customer  { 	//you want autowired this field. 	private Person person;   	private int type; 	private String action;   	//getter and setter method   }
<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="CustomerBean" class="com.mkyong.common.Customer"> 		<property name="action" value="buy" /> 		<property name="type" value="1" /> 	</bean>   	<bean id="PersonBean" class="com.mkyong.common.Person"> 		<property name="name" value="mkyong" /> 		<property name="address" value="address 123" /> 		<property name="age" value="28" /> 	</bean>   </beans>

2. Register AutowiredAnnotationBeanPostProcessor

To enable @Autowired, you have to register ‘AutowiredAnnotationBeanPostProcessor‘, and you can do it in two ways :

1. Include <context:annotation-config />

Add Spring context and <context:annotation-config /> in bean configuration file.

<beans  	//... 	xmlns:context="http://www.springframework.org/schema/context" 	//... 	http://www.springframework.org/schema/context 	http://www.springframework.org/schema/context/spring-context-2.5.xsd"> 	//...   	<context:annotation-config /> 	//... </beans>

Full example,

<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">   	<context:annotation-config />   	<bean id="CustomerBean" class="com.mkyong.common.Customer"> 		<property name="action" value="buy" /> 		<property name="type" value="1" /> 	</bean>   	<bean id="PersonBean" class="com.mkyong.common.Person"> 		<property name="name" value="mkyong" /> 		<property name="address" value="address ABC" /> 		<property name="age" value="29" /> 	</bean>   </beans>
2. Include AutowiredAnnotationBeanPostProcessor

Include ‘AutowiredAnnotationBeanPostProcessor’ directly in 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  class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor"/>   	<bean id="CustomerBean" class="com.mkyong.common.Customer"> 		<property name="action" value="buy" /> 		<property name="type" value="1" /> 	</bean>   	<bean id="PersonBean" class="com.mkyong.common.Person"> 		<property name="name" value="mkyong" /> 		<property name="address" value="address ABC" /> 		<property name="age" value="29" /> 	</bean>   </beans>

3. @Autowired Examples

Now, you can autowired bean via @Autowired, and it can be applied on setter method, constructor or a field.

1. @Autowired setter method
package com.mkyong.common;   import org.springframework.beans.factory.annotation.Autowired;   public class Customer  { 	private Person person; 	private int type; 	private String action; 	//getter and setter methods   	@Autowired 	public void setPerson(Person person) { 		this.person = person; 	} }
2. @Autowired construtor
package com.mkyong.common;   import org.springframework.beans.factory.annotation.Autowired;   public class Customer  { 	private Person person; 	private int type; 	private String action; 	//getter and setter methods   	@Autowired 	public Customer(Person person) { 		this.person = person; 	} }
3. @Autowired field
package com.mkyong.common;   import org.springframework.beans.factory.annotation.Autowired;   public class Customer  { 	@Autowired 	private Person person; 	private int type; 	private String action; 	//getter and setter methods }

The above example will autowired ‘PersonBean’ into Customer’s person property.

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(new String[] {"SpringBeans.xml"});       	Customer cust = (Customer)context.getBean("CustomerBean");     	System.out.println(cust);       } }

Output

Customer [action=buy, type=1,  person=Person [address=address 123, age=28, name=mkyong]]

Dependency checking

By default, the @Autowired will perform the dependency checking to make sure the property has been wired properly. When Spring can’t find a matching bean to wire, it will throw an exception. To fix it, you can disable this checking feature by setting the “required” attribute of @Autowired to false.

package com.mkyong.common;   import org.springframework.beans.factory.annotation.Autowired;   public class Customer  { 	@Autowired(required=false) 	private Person person; 	private int type; 	private String action; 	//getter and setter methods }

In the above example, if the Spring can’t find a matching bean, it will leave the person property unset.

@Qualifier

The @Qualifier annotation us used to control which bean should be autowire on a field. For example, bean configuration file with two similar person beans.

<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">   	<context:annotation-config />   	<bean id="CustomerBean" class="com.mkyong.common.Customer"> 		<property name="action" value="buy" /> 		<property name="type" value="1" /> 	</bean>   	<bean id="PersonBean1" class="com.mkyong.common.Person"> 		<property name="name" value="mkyong1" /> 		<property name="address" value="address 1" /> 		<property name="age" value="28" /> 	</bean>   	<bean id="PersonBean2" class="com.mkyong.common.Person"> 		<property name="name" value="mkyong2" /> 		<property name="address" value="address 2" /> 		<property name="age" value="28" /> 	</bean>   </beans>

Will Spring know which bean should wire?

To fix it, you can use @Qualifier to auto wire a particular bean, for example,

package com.mkyong.common;   import org.springframework.beans.factory.annotation.Autowired; import org.springframework.beans.factory.annotation.Qualifier;   public class Customer  { 	@Autowired 	@Qualifier("PersonBean1") 	private Person person; 	private int type; 	private String action; 	//getter and setter methods }

It means, bean “PersonBean1″ is autowired into the Customer’s person property. Read this full example – Spring Autowiring @Qualifier example

Conclusion

This @Autowired annotation is highly flexible and powerful, and definitely better than “autowire” attribute in bean configuration file.

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