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@Autowiredannotation 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.