Spring 注解<context:annotation-config> 和 <context:component-scan>的作用与区别

<context:annotation-config> 是用于激活那些已经在spring容器里注册过的bean（无论是通过xml的方式还是通过packagesanning的方式）上面的注解。(激活@Resource和@Autowired注解)

<context:component-scan>除了具有<context:annotation-config>的功能之外，<context:component-scan>还可以在指定的package下扫描以及注册javabean 。(激活@Resource和@Autowired注解，同时可以配置扫描的包以激活@Service、@Controller等注解)

1.<context:component-scan>的作用:(开发中用这一个足以)

　　一方面可以配置扫描注解的包的路径，另一方面具有<context:annotation-config>的作用，也就是可以实现注解注入(Autowired与Resource等注解)。

例如:

目录结构:

ApplicationContext-test.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"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">
<!--配置扫描注解的包-->
<context:component-scan base-package="cn.qlq.test" ></context:component-scan>
</beans>

package cn.qlq.test.dao;

import org.springframework.stereotype.Repository;

/**
* @Author: qlq
* @Description
* @Date: 22:49 2018/9/28
*/
@Repository
public class UserDao {
public void saveUser() {
System.out.println("save user");
}
}

package cn.qlq.test.service;

import cn.qlq.test.dao.UserDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

/**
* @Author: qlq
* @Description
* @Date: 22:52 2018/9/28
*/
@Service
public class UserService {
@Autowired
private UserDao userDao;

public void saveUser() {
userDao.saveUser();
}
}

 测试类:

package cn.qlq.test;

import cn.qlq.test.service.UserService;
import org.apache.xbean.spring.context.ClassPathXmlApplicationContext;

/**
* @Author: qlq
* @Description
* @Date: 22:54 2018/9/28
*/
public class TestApp {
public static void main(String[] args) {
ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("ApplicationContext-test.xml");
//第一种:用beanId获取bean
//        UserService userService = (UserService) applicationContext.getBean(UserService.class);
//第二种:用bean的class获取bean
UserService userService = (UserService) applicationContext.getBean(UserService.class);
userService.saveUser();
}
}

结果:

 save user

 补充:

　　<context:component-scan....可以扫描多个包，逗号隔开就行，而且扫描包的时候会自动扫描子包。如下也是正确的:

<?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"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">
<!--配置扫描注解的包-->
<context:component-scan base-package="cn.qlq.test.dao,cn.qlq.test.service" ></context:component-scan>
</beans>

2.<context:annotation-config>的作用与测试

　　<context:annotation-config> 是用于激活那些已经在spring容器里注册过的bean（无论是通过xml的方式还是通过package sanning的方式）上面的注解。也就是激活@Autowired和@Resource注解。但是不会扫描@Service等注解配置的bean。

如下:

<?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: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/context http://www.springframework.org/schema/context/spring-context.xsd">
<context:annotation-config/>

<bean id="userDao" class="cn.qlq.test.dao.UserDao"/>
<bean id="userService" class="cn.qlq.test.service.UserService"/>
</beans>

package cn.qlq.test.dao;

import org.springframework.stereotype.Repository;

/**
* @Author: qlq
* @Description
* @Date: 22:49 2018/9/28
*/
public class UserDao {
public void saveUser() {
System.out.println("save user");
}
}

package cn.qlq.test.service;

import cn.qlq.test.dao.UserDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

/**
* @Author: qlq
* @Description
* @Date: 22:52 2018/9/28
*/
public class UserService {
@Autowired
private UserDao userDao;

public void saveUser() {
userDao.saveUser();
}
}

测试代码同上，可以正常运行。

如果我们修改xml为下面:(去掉bean定义)

<?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"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">
<!--配置扫描注解的包-->
<context:annotation-config/>
</beans>

结果报错没有找到bean:

Exception in thread "main" org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [cn.qlq.test.service.UserService] is defined
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:371)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:331)
at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:972)
at cn.qlq.test.TestApp.main(TestApp.java:17)

修改xml为如下:(去掉annotation-config标签)

<?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"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">
<bean id="userDao" class="cn.qlq.test.dao.UserDao"/>
<bean id="userService" class="cn.qlq.test.service.UserService"/>
</beans>

结果会报空指针异常，service中没有注入dao:

Exception in thread "main" java.lang.NullPointerException
at cn.qlq.test.service.UserService.saveUser(UserService.java:17)
at cn.qlq.test.TestApp.main(TestApp.java:18)