29Spring_Autowriter的一些疑惑（很重要）

我用一个Autowriter去注解一个属性，而且我没有在Spring的配置文件中的bean.xml中注册bean(<bean id=""...);那么这个注解有用吗？答案是不行。也就是说要用Autowriter注解时，其实必须要保证在bean容器中注册过这个bean.

bean在bean容器中的注册分为两种：

1.手动：就是在spring.xml配置文件中，写上

<bean id="configa" class="com.guiugu.shen.bean2.HelloService">
    </bean>

2.自动注册，那就是要用自动扫描了。在spring.xml配置文件中写上：

<context:component-scan base-package="com.guiugu.shen.bean2"/>

自动扫描时：会去com.guiugu.shen.bean2这个包下面去寻找有没有被

@Service用于标注业务层组件

@Controller用于标注控制层组件

@Repository用于标注数据访问组件，即DAO组件

@Component泛指组件，当组件不好归类的时候，我们可以使用这个注解进行标注。

这些注解修饰过，有的话就会被自动注册到bean容器中。

只有用以上其中一个方法注册过才会用@Autowriter是有用。

案例如下：

案例结构：

先给出错误案例：

我最先的想法是：只要在applicationContext.xml中配一个自动扫描包，就可以了。如下

applicationContext.xml：

<?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:context="http://www.springframework.org/schema/context"
    xmlns:aop="http://www.springframework.org/schema/aop"
    xmlns:tx="http://www.springframework.org/schema/tx"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context-3.0.xsd
           http://www.springframework.org/schema/aop
           http://www.springframework.org/schema/aop/spring-aop-3.0.xsd
           http://www.springframework.org/schema/tx 
           http://www.springframework.org/schema/tx/spring-tx-3.0.xsd"
    default-lazy-init="true">
<context:component-scan base-package="com.guiugu.shen.bean2"/>

</beans>

HelloService.java:

package com.guiugu.shen.bean2;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import org.springframework.stereotype.Service;



public class HelloService {
    
    public void sayHello()
    {
        System.out.print("hello");
        
    }

}

TestJavaSpring.java代码：

package com.guiugu.shen.bean2;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = "classpath:applicationContextt.xml")

public class TestJavaSpring {
    @Autowired
    private HelloService helloService;
    @Test
    public void testSayhell()
    {
        helloService.sayHello();
        
    }
    

}

运行结果：

Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No matching bean of type [com.guiugu.shen.bean2.HelloService] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}

很明显：上面的结果是找不到对应的bean.因为没有在bean容器中注册bean.

两个解决办法：1.手动在applicationContext.xml注册对应的bean.

<?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:context="http://www.springframework.org/schema/context"
    xmlns:aop="http://www.springframework.org/schema/aop"
    xmlns:tx="http://www.springframework.org/schema/tx"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context-3.0.xsd
           http://www.springframework.org/schema/aop
           http://www.springframework.org/schema/aop/spring-aop-3.0.xsd
           http://www.springframework.org/schema/tx 
           http://www.springframework.org/schema/tx/spring-tx-3.0.xsd"
    default-lazy-init="true">
<!--  手动注册这个bean  >
<bean id="configa" class="com.guiugu.shen.bean2.HelloService">
     
    </bean>
</beans>

运行结果:正确。输出为hello。因为手动注册过了这个bean。

第二种解决办法：在applicationContext.xml用自动扫描器去扫描包。并且在HelloService上用@Component，因为这样自动扫描包才能识别。自动扫描包能识别的注解是：

@Service@Controller@Repository@Component。

ApplicationContext.xml代码:

<?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:context="http://www.springframework.org/schema/context"
    xmlns:aop="http://www.springframework.org/schema/aop"
    xmlns:tx="http://www.springframework.org/schema/tx"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context-3.0.xsd
           http://www.springframework.org/schema/aop
           http://www.springframework.org/schema/aop/spring-aop-3.0.xsd
           http://www.springframework.org/schema/tx 
           http://www.springframework.org/schema/tx/spring-tx-3.0.xsd"
    default-lazy-init="true">
<--扫描到之后会自动会去注册bean-->

<context:component-scan base-package="com.guiugu.shen.bean2"/>

</beans>

HelloService代码：

package com.guiugu.shen.bean2;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import org.springframework.stereotype.Service;


@Component
public class HelloService {
    
    public void sayHello()
    {
        System.out.print("hello");
        
    }

}

TestJavaSpring.java:

package com.guiugu.shen.bean2;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = "classpath:applicationContextt.xml")

public class TestJavaSpring {
    @Autowired
    private HelloService helloService;
    @Test
    public void testSayhell()
    {
        helloService.sayHello();
        
    }
    

}

运行结果正确。

上面的执行流程是：

Spring启动，然后根据<context:component-scan base-package="com.guiugu.shen.bean2"/>去这个包下面去寻找被@Service@Controller@Repository@Component注册类，找到的话就把他自动注册到bean容器中。

这样当我们执行

@Autowired

private HelloService helloService;

就会从bean容器中把对应的类取出来实例化（因为之前已经注册进去了）