@Configuration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {

@Target(ElementType.TYPE)：注解作用在类上
@Retention(RetentionPolicy.RUNTIME)：保留策略 —> 运行时保留
@Documented:表明这个注解应该被 javadoc工具记录. 默认情况下,javadoc是不包括注解的.
@Component: 表明这个类也会被注入到spring容器中

我们再来看下注释对@Configuration的解释

Indicates that a class declares one or more {@link Bean @Bean} methods and
may be processed by the Spring container to generate bean definitions and
service requests for those beans at runtime

通过上述解释可以看出@Configuration注解的类成为一个配置类，在配置类里声明了@Bean的方法会交给spring容器管理。也就是说@Configuration标记的类等同于下面的Spring XML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.xsd
                    http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <!-- bean definitions here -->

</beans>

演示

新建一个实体类

package org.example.entity;

public class HelloWord {

    public void sayHello(){
        System.out.println("this is sayHello method");
    }
}

1.通过传统xml方式注册bean
配置文件：bean.xml

<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 definitions here -->


    <bean id="helloWord" class="org.example.entity.HelloWord"/>
</beans>

测试：

@Test
    public void testXmlConfig(){
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("bean.xml");
        HelloWord helloWord = (HelloWord) context.getBean("helloWord");
        helloWord.sayHello();
    }

结果：

this is sayHello method

2.通过注解的方式注册bean
配置类：

package org.example.config;

import org.example.entity.HelloWord;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class HelloWordConfig {

    @Bean
    public HelloWord helloWord(){
        return new HelloWord();
    }
}

测试：

@Test
    public void testConfigurationAnno(){
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(HelloWordConfig.class);
        HelloWord helloWord = (HelloWord) context.getBean("helloWord");
        helloWord.sayHello();
    }

结果：

this is sayHello method

注意：我们前面也分析到了@Configuration被@Component修饰，说明@Configuration注解的类也会加入到spring容器，我们来测试一下

@Test
    public void testConfigurationAnno(){
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(HelloWordConfig.class);
        Arrays.stream(context.getBeanDefinitionNames()).forEach(
                c->{
                    System.out.println(c);
                }
        );
    }

结果：

image.png

通过结果可以看出HelloWordConfig类和HelloWord类都被注册到Spring容器中了。