Spring 自动检测和注册 Bean

[JasonWu73]

像Spring 基于 Java 代码的 IoC 容器配置一文件中，我们是通过手动指定某个 Bean 加载到 IoC 容器中，那如果现在有几十个 Bean，难道我们还要每个指定吗？

Spring 为我们提供了自动检测并注册 Bean 的机制。以下是几种比较常见的组件扫描配置方式：

通过 XML 启用组件扫描

1、在 resources 目录中创建 spring-context.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 https://www.springframework.org/schema/context/spring-context.xsd">

  <context:component-scan base-package="net.wuxianjie.demo.spring"/>

</beans>

2、然后修改主类 MySpring.java：

package net.wuxianjie.demo.spring;

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

public class MySpring {

  public static void main(String[] args) throws Exception {

    ApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring-context.xml");

    UserService userService = applicationContext.getBean("userService", UserService.class);
    userService.userLogin("Jason", "123");
  }
}

通过 Java 注解启用组件扫描

1、创建配置类 SpringContextConfig.java：

package net.wuxianjie.demo.spring;

import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@Configuration
@ComponentScan(basePackages = "net.wuxianjie.demo.spring")
public class SpringContextConfig {

}

2、然后修改主类 MySpring.java：

package net.wuxianjie.demo.spring;

import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class MySpring {

  public static void main(String[] args) throws Exception {

    ApplicationContext applicationContext = new AnnotationConfigApplicationContext(SpringContextConfig.class);

    UserService userService = applicationContext.getBean("userService", UserService.class);
    userService.userLogin("Jason", "123");
  }
}

通过 scan(String…​) 启用组件扫描

此方法，我们只需要修改主类 MySpring.java 即可：

package net.wuxianjie.demo.spring;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class MySpring {

  public static void main(String[] args) throws Exception {

    AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
    applicationContext.scan("net.wuxianjie.demo.spring");
    applicationContext.refresh();

    UserService userService = applicationContext.getBean("userService", UserService.class);
    userService.userLogin("Jason", "123");
  }
}