Spring自动装配Bean的两种方法（二）_springcontextutil.autowirebean-优快云博客

本文介绍了Spring框架中@Autowired注解的基本用法，包括通过构造器、setter方法及字段自动装配Bean，并展示了如何使用@Qualifier指定具体装配哪个Bean。

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使用@Autowired注解装配bean

上一篇我们说到了在xml配置文件中装配bean,这种方法相对繁琐，灵活性不高，接下来我们看一下如何利用注解来快速装配bean。
@Autowired注解可以通过构造器、setter方法以及字段（属性）自动装配bean.
首先创建一个People类作为将要被装配的bean,以及一个装配People的类Company如下所示：

people

public class People {
    private String name;

    public People(String name) {
        this.name = name;
    }

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

    public String getName() {
        return name;
    }
}

Company

public class Company {

    private People people;

    public void setPeople(People people) {
        this.people = people;
    }

    public People getPeople() {
        return people;
    }
}

设置xml文件，包含了一个people bean和一个company bean，注意，其中添加了一句话，，这一句表示启动注解功能。

这里写图片描述
红色标注的地方也发生了变化，注意一下。

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">
    <context:annotation-config />
    <bean id="company" class="com.yiwen.test5.Company"></bean>
    <bean id="people1" class="com.yiwen.test5.People">
        <!--这里通过property和constructor-arg同时设置一个属性name，最终name的值是property1而不是constructor1，注意一下-->
        <property name="name" value="property1"></property>
        <constructor-arg name="name" value="constructor1"></constructor-arg>

    </bean>

</beans>

接下来就是利用注解的方式，将people bean注解到company中了。

通过setter方法注入
这里我们修改一下company的类，在setter上面写上@Autowired即可将对应的相同类型的bean（即people）注入到setter中

public class Company {

    private People people;

    @Autowired
    public void setPeople(People people) {
        this.people = people;
    }

    public People getPeople() {
        return people;
    }
}

测试结果
这里我们创建一个测试类，测试是否装配成功

    ```
    public class Test5 {
        public static void main(String[] args) {
            ApplicationContext context = new ClassPathXmlApplicationContext("test5.xml");
            Company company=(Company)context.getBean("company") ;

            System.out.print( company.getPeople().getName());
        }
    }

    ```

运行，输入结果为property1，表示已经将people bean装备进去了。

通过字段装配
这里还是修改了一下company类，在字段上加入@Autowired，可以用上一个测试类测试，结果相同。

public class Company {
    @Autowired
    private People people;

    public void setPeople(People people) {
        this.people = people;
    }

    public People getPeople() {
        return people;
    }
}

通过构造器装配
在构造器上加入@Autowired即可，可用上面的测试方法测试。

public class Company {

    private People people;
    @Autowired
    public Company(People people) {
        this.people = people;
    }

    public void setPeople(People people) {
        this.people = people;
    }

    public People getPeople() {
        return people;
    }
}

如果People在xml中有两个bean，那如何判断装配的是哪一个bean呢？如下所示：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">
    <context:annotation-config />
    <bean id="company" class="com.yiwen.test5.Company"></bean>
    <bean id="people1" class="com.yiwen.test5.People">
        <property name="name" value="property1"></property>
        <constructor-arg name="name" value="constructor1"></constructor-arg>

    </bean>
    <bean id="people2" class="com.yiwen.test5.People">
        <property name="name" value="property2"></property>
        <constructor-arg name="name" value="constructor2"></constructor-arg>

    </bean>
</beans>

这时候，在利用@Autowired时就会报错，因为spring也不知道到底注入哪一个people的bean,所以这里我们就用添加一个新的注解@Qualifier ，xml文件如下所示：

public class Company {

    private People people;

    public Company(People people) {
        this.people = people;
    }
    @Autowired
    @Qualifier("people1")
    public void setPeople(People people) {
        this.people = people;
    }

    public People getPeople() {
        return people;
    }
}

这里我们在setter上添加了一个新注解@Qualifier(“people1”)，表示使用的id为people1的people bean。

@Qualifier运用在构造方法上时，是注解在参数的前面，而不是构造方法之上，如下所示：

public class Company {

    private People people;
    @Autowired(required = false)
    public Company( @Qualifier("people1") People people ) {
        this.people = people;
    }

    public void setPeople(People people) {
        this.people = people;
    }

    public People getPeople() {
        return people;
    }
}

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。如下所示：

public class Company {

    private People people;

    public Company(People people) {
        this.people = people;
    }
    @Autowired(required = false)
    @Qualifier("people1")
    public void setPeople(People people) {
        this.people = people;
    }

    public People getPeople() {
        return people;
    }
}