@Configuration 和 @Bean 讲解

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原文链接：https://blog.youkuaiyun.com/javaloveiphone/article/details/52182899

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本文深入解析Spring框架中@Bean与@Configuration注解的使用方法及其等效的XML配置。介绍了如何利用这些注解来替代XML配置，实现bean的注册与管理，以及不同作用域的设置。

转自
https://blog.youkuaiyun.com/javaloveiphone/article/details/52182899

1、@Configuration标注在类上，相当于把该类（配置类）作为spring的xml配置文件中的< beans>，作用为：配置spring容器(应用上下文)

package com.test.spring.support.configuration;

@Configuration
public class TestConfiguration {
    public TestConfiguration(){
        System.out.println("spring容器启动初始化。。。");
    }
}

相当于：

<?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:jdbc="http://www.springframework.org/schema/jdbc"  
    xmlns:jee="http://www.springframework.org/schema/jee" xmlns:tx="http://www.springframework.org/schema/tx"
    xmlns:util="http://www.springframework.org/schema/util" xmlns:task="http://www.springframework.org/schema/task" xsi:schemaLocation="
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.0.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.0.xsd
        http://www.springframework.org/schema/jdbc http://www.springframework.org/schema/jdbc/spring-jdbc-4.0.xsd
        http://www.springframework.org/schema/jee http://www.springframework.org/schema/jee/spring-jee-4.0.xsd
        http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-4.0.xsd
        http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-4.0.xsd
        http://www.springframework.org/schema/task http://www.springframework.org/schema/task/spring-task-4.0.xsd" default-lazy-init="false">


</beans>

主方法进行测试：

package com.test.spring.support.configuration;

public class TestMain {
    public static void main(String[] args) {

        //@Configuration注解的spring容器加载方式，用AnnotationConfigApplicationContext替换ClassPathXmlApplicationContext
        ApplicationContext context = new AnnotationConfigApplicationContext(TestConfiguration.class);

        //如果加载spring-context.xml文件：
        //ApplicationContext context = new ClassPathXmlApplicationContext("spring-context.xml");
    }
}

从运行主方法结果可以看出，spring容器已经启动了：

八月 11, 2016 12:04:11 下午 org.springframework.context.annotation.AnnotationConfigApplicationContext prepareRefresh
信息: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@203e25d3: startup date [Thu Aug 11 12:04:11 CST 2016]; root of context hierarchy
spring容器启动初始化。。。

2、@Bean标注在方法上(返回某个实例的方法)，等价于spring的xml配置文件中的< bean>，作用为：注册bean对象

bean类：

package com.test.spring.support.configuration;

public class TestBean {

    public void sayHello(){
        System.out.println("TestBean sayHello...");
    }

    public String toString(){
        return "username:"+this.username+",url:"+this.url+",password:"+this.password;
    }

    public void start(){
        System.out.println("TestBean 初始化。。。");
    }

    public void cleanUp(){
        System.out.println("TestBean 销毁。。。");
    }
}

配置类：

package com.test.spring.support.configuration;

@Configuration
public class TestConfiguration {
        public TestConfiguration(){
            System.out.println("spring容器启动初始化。。。");
        }

    //@Bean注解注册bean,同时可以指定初始化和销毁方法
    //@Bean(name="testNean",initMethod="start",destroyMethod="cleanUp")
    @Bean
    @Scope("prototype")
    public TestBean testBean() {
        return new TestBean();
    }
}

主方法测试类：

package com.test.spring.support.configuration;

public class TestMain {
    public static void main(String[] args) {
        ApplicationContext context = new AnnotationConfigApplicationContext(TestConfiguration.class);
        //获取bean
        TestBean tb = context.getBean("testBean");
        tb.sayHello();
    }
}

注：
(1)、@Bean注解在返回实例的方法上，如果未通过@Bean指定bean的名称，则默认与标注的方法名相同；

(2)、@Bean注解默认作用域为单例singleton作用域，可通过@Scope(“prototype”)设置为原型作用域；

(3)、既然@Bean的作用是注册bean对象，那么完全可以使用@Component、@Controller、@Service、@Ripository等注解注册bean，当然需要配置@ComponentScan注解进行自动扫描。

bean类：

package com.test.spring.support.configuration;

//添加注册bean的注解
@Component
public class TestBean {

    public void sayHello(){
        System.out.println("TestBean sayHello...");
    }

    public String toString(){
        return "username:"+this.username+",url:"+this.url+",password:"+this.password;
    }
}

配置类：

@Configuration

//添加自动扫描注解，basePackages为TestBean包路径
@ComponentScan(basePackages = "com.test.spring.support.configuration")
public class TestConfiguration {
    public TestConfiguration(){
        System.out.println("spring容器启动初始化。。。");
    }

    //取消@Bean注解注册bean的方式
    //@Bean
    //@Scope("prototype")
    //public TestBean testBean() {
    //  return new TestBean();
    //}
}

主方法测试获取bean对象：

public class TestMain {
    public static void main(String[] args) {
        ApplicationContext context = new AnnotationConfigApplicationContext(TestConfiguration.class);
        //获取bean
        TestBean tb = context.getBean("testBean");
        tb.sayHello();
    }
}

sayHello()方法都被正常调用。