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.