Spring Auto-Wiring Beans With @Autowired Annotation

Posted on March 12, 2010 ,     Last modified : August 30, 2012

By mkyong

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
 
}

 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">
 
  id="CustomerBean" class="com.mkyong.common.Customer">
   name="action" value="buy" />
   name="type" value="1" />
 

  id="PersonBean" class="com.mkyong.common.Person"> name="name" value="mkyong" /> name="address" value="address 123" /> name="age" value="28" />  

2. Register AutowiredAnnotationBeanPostProcessor

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

1. Include

Add Spring context and in bean configuration file.

 
 //...
 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">
 //...
 
  />
 //...

Full example,

 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">
 
  />
 
  id="CustomerBean" class="com.mkyong.common.Customer">
   name="action" value="buy" />
   name="type" value="1" />
 

  id="PersonBean" class="com.mkyong.common.Person"> name="name" value="mkyong" /> name="address" value="address ABC" /> name="age" value="29" />  

2. Include AutowiredAnnotationBeanPostProcessor

Include ‘AutowiredAnnotationBeanPostProcessor’ directly in bean configuration file.

 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">
 
 
class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor"/>
 
  id="CustomerBean" class="com.mkyong.common.Customer">
   name="action" value="buy" />
   name="type" value="1" />
 

  id="PersonBean" class="com.mkyong.common.Person"> name="name" value="mkyong" /> name="address" value="address ABC" /> name="age" value="29" />  

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.

 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">
 
  />
 
  id="CustomerBean" class="com.mkyong.common.Customer">
   name="action" value="buy" />
   name="type" value="1" />
 

  id="PersonBean1" class="com.mkyong.common.Person"> name="name" value="mkyong1" /> name="address" value="address 1" /> name="age" value="28" />   id="PersonBean2" class="com.mkyong.common.Person"> name="name" value="mkyong2" /> name="address" value="address 2" /> name="age" value="28" />  

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