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.

Conclusion

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