什么是注解

注解就是代码特殊标记，格式:@注解名称(属性名称=属性值...)

使用注解，注解可以作用在类，方法，属性等

使用注解的目的

为了简化xml配置

Spring针对Bean管理中创建对象提供了四个注解

@Component

@Service

@Controller

@Repository

以上四个注解功能是一样的，都可以用来创建bean实例

基于注解的方式实现对象的创建

第一步 引入依赖

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>c</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-beans</artifactId>
            <version>5.3.14</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>5.3.14</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <version>5.3.14</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-expression</artifactId>
            <version>5.3.14</version>
        </dependency>
        <!-- 日志包 -->
        <dependency>
            <groupId>commons-logging</groupId>
            <artifactId>commons-logging</artifactId>
            <version>1.2</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.13.2</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>druid</artifactId>
            <version>1.2.8</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-aop</artifactId>
            <version>5.3.15</version>
        </dependency>


    </dependencies>
</project>

第二步 开启组件扫描

<?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">
    <!-- 
    开启组件扫描 
    如果要扫描多个包 
        方法一 多个包之间使用逗号隔开
        方法二 扫描包的上层目录
    -->
    <!--<context:component-scan base-package="com.test4.dao,com.test4.service"/>-->
    <context:component-scan base-package="com.test4"/>
</beans>

第三步 添加注解

package com.test4.service;

import org.springframework.stereotype.Component;

//在注解里面value属性值可以省略不写
//默认值是雷名称，首字母小写
@Component(value = "userService")
public class UserService {

    public void add(){
        System.out.println("userService add");
    }
}

第四步 测试

package com.test4.service;

import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import static org.junit.Assert.*;

public class UserServiceTest {

    @Test
    public void add() {
        ApplicationContext context = new ClassPathXmlApplicationContext("spring12.xml");
        UserService userService = context.getBean("userService", UserService.class);
        System.out.println("userService = " + userService);
        userService.add();
    }
}

开启组件扫描配置

<?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">
    <!--
    开启组件扫描
        如果要扫描多个包
            方法一 多个包之间使用逗号隔开
            方法二 扫描包的上层目录
    -->
    <!--<context:component-scan base-package="com.test4.dao,com.test4.service"/>-->
    <context:component-scan base-package="com.test4"/>

    <!--
    开启组件扫描配置 示例1
        use-default-filters="false" 表示现在不使用默认filter，使用自己配置的filter
        context:include-filter 扫描那些内容
    -->
    <context:component-scan base-package="com.test4" use-default-filters="false">
        <context:include-filter type="annotation" expression="org.springframework.stereotype.Controller"/>
    </context:component-scan>

    <!--
    开启组件扫描配置 示例2
        context:exclude-filter 不去扫描那些内容
    -->
    <context:component-scan base-package="com.test4">
        <context:exclude-filter type="annotation" expression="org.springframework.stereotype.Controller"/>
    </context:component-scan>
</beans>