Spring使用@Autowired注解自动装配

它会匹配当前Spring容器任何bean的属性自动装配。在大多数情况下,你可能只需要在特定的 bean 自动装配属性。

在Spring中,可以使用 @Autowired 注解通过setter方法,构造函数或字段自动装配Bean。此外,它可以在一个特定的bean属性自动装配。
注 @Autowired注解是通过匹配数据类型自动装配Bean。

请参见下面的完整的例子来演示如何使用@Autowired。

  1. Beans
    一个 Customer bean 在bean配置文件中声明。稍后,您将使用 “@Autowired” 来自动装配一个Person bean。
    package com.yiibai.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.yiibai.common.Customer"> <property name="action" value="buy" /> <property name="type" value="1" /> </bean> <bean id="PersonBean" class="com.yiibai.common.Person"> <property name="name" value="yiibai" /> <property name="address" value="address 123" /> <property name="age" value="28" /> </bean> </beans>
  2. 注册AutowiredAnnotationBeanPostProcessor
    要启用@Autowired,必须注册“AutowiredAnnotationBeanPostProcessor',你可以用两种方式做到这一点:
  3. Include <context:annotation-config />
    添加 Spring 上下文和<context:annotation-config />在bean配置文件中。
    <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>
    下面是完整的实例
    <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.yiibai.common.Customer"> <property name="action" value="buy" /> <property name="type" value="1" /> </bean> <bean id="PersonBean" class="com.yiibai.common.Person"> <property name="name" value="yiibai" /> <property name="address" value="address ABC" /> <property name="age" value="29" /> </bean> </beans>
  4. 包含 AutowiredAnnotationBeanPostProcessor
    直接在bean配置文件包含“AutowiredAnnotationBeanPostProcessor”。
    <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.yiibai.common.Customer"> <property name="action" value="buy" /> <property name="type" value="1" /> </bean> <bean id="PersonBean" class="com.yiibai.common.Person"> <property name="name" value="yiibai" /> <property name="address" value="address ABC" /> <property name="age" value="29" /> </bean> </beans>
  5. @Autowired示例
    现在,你可以通过 @Autowired 自动装配 bean,它可以在 setter 方法,构造函数或字段中使用。
  6. @Autowired setter 方法
    package com.yiibai.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; }}
  7. @Autowired 构造方法
    package com.yiibai.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; }}
  8. @Autowired 字段
    package com.yiibai.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}
    上面的例子会自动装配“PersonBean”到Customer的person属性。
    执行它
    package com.yiibai.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[] {"applicationContext.xml"}); Customer cust = (Customer)context.getBean("CustomerBean"); System.out.println(cust); }}
    输出
    Customer [person=Person [name=YiibaiA], type=1, action=buy]
    依赖检查

默认情况下,@Autowired将执行相关检查,以确保属性已经装配正常。当Spring无法找到匹配的Bean装配,它会抛出异常。要解决这个问题,可以通过 @Autowired 的“required”属性设置为false来禁用此检查功能。
package com.yiibai.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}
在上面的例子中,如果Spring不能找到一个匹配的Bean,person属性将不设定。
@Qualifier
@Qualifier注解我们用来控制bean应在字段上自动装配。例如,具有两个类似的 person bean 配置文件。
<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.yiibai.common.Customer"> <property name="action" value="buy" /> <property name="type" value="1" /> </bean> <bean id="PersonBean1" class="com.yiibai.common.Person"> <property name="name" value="yiibai-1" /> <property name="address" value="address-1" /> <property name="age" value="29" /> </bean> <bean id="PersonBean2" class="com.yiibai.common.Person"> <property name="name" value="yiibai-2" /> <property name="address" value="address-2" /> <property name="age" value="28" /> </bean> </beans>
Spring知道哪个 bean 应当装配?
为了解决这个问题,可以使用 @Qualifier 自动装配一个特定的 bean,例如,
package com.yiibai.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}
这意味着,“PersonBean1” bean被自动装配到customer的person属性。阅读下面完整的例子 – Spring自动装配@Qualifier实例
总结
这@Autowired注解非常灵活,功能强大,绝对比bean配置文件的“autowire”属性要更好。

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