对Spring Ioc的理解

1. 新建一个maven项目，工程目录如下

2. 添加接口MessageService及其实现类MessageServiceImpl

MessageService.java

package com.peppa.service;

public interface MessageService {
    String getMessage();
}

MessageServiceImpl.java

package com.peppa.serviceImpl;

import com.peppa.service.MessageService;

public class MessageServiceImpl implements MessageService {
    @Override
    public String getMessage() {
        return "hello world!";
    }
}

pom文件中添加spring-context依赖

<!-- https://mvnrepository.com/artifact/org.springframework/spring-context -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>5.1.5.RELEASE</version>
        </dependency>

在resources目录下添加application.xml文件，内容如下

<?xml version="1.0" encoding="UTF-8" ?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd" default-autowire="byName">
    <bean id="messageService" class="com.peppa.serviceImpl.MessageServiceImpl"></bean>
</beans>

添加Application.java，将MessageService从context中取出来，而不是使用 new MessageServiceImpl() 获取

package com;

import com.peppa.service.MessageService;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Application {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("classpath:application.xml");
        System.out.println("start success");
        MessageService messageService = context.getBean(MessageService.class);
        System.out.println("will print hello world");
        System.out.println(messageService.getMessage());
    }
}

通过配置文件来启动Spring的ApplicationContext，也就是IoC的核心。ApplicationContext启动过程中，会负责创建实例Bean，往各个Bean中注入依赖等。