Spring Auto Wiring @Autowired

30 Aug 2015

Reading time ~1 minute

一、bean

Customer.java

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
    
    @Override
    public String toString() {
        return "Customer [person=" + person + ", type=" + type + ", action="
                + action + "]";
    }
}

Person.java

package com.mkyong.common;
    
public class Person {
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Person [name=" + name + "]";
    }
}

二、 AutowiredAnnotationBeanPostProcessor

　　使用@Autowired, 必须在SpringBeans.xml登记 AutowiredAnnotationBeanPostProcessor,下面有两种方式：

1、context:annotation-config

<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" />
    </bean>
    
</beans>

2、直接包含AutowiredAnnotationBeanPostProcessor

将<context:annotation-config />替换成

<bean class="org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor"/>

三、运行

在App.java中

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(
                "SpringBeans.xml");

        Customer cust = (Customer) context.getBean("customer");
        System.out.println(cust);
    }
}

四、运行结果

Customer [person=Person [name=mkyongA], type=1, action=buy]

五、Dependency checking

　　默认情况下，@Autowired是使用Dependency checking的，确保属性总是可以成功注入。如果Spring没有找到匹配的bean去wire，就会扔出异常。可以使用@Autowired(required=false)去关闭Dependency checking。

六、@Qualifier

　　@Qualifier用来控制哪个bean应该被autowire。例如，在下面的例子的SpringBeans.xml中配置了PersonBean1和PersonBean2.

<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" />
    </bean>
    
    <bean id="PersonBean2" class="com.mkyong.common.Person">
        <property name="name" value="mkyong2" />
    </bean>
    
</beans>

此时，在Customer.java中用@Qualifier("PersonBean1")就可以将PersonBean1注入到Customer的person属性中。

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
}