IOC:Inversion of Control 控制反转，是一种设计原则，spring 中通过DI（dependency Injection）来具体实现。

比如原本对象的实例化，是通过程序主动New出来，IOC中的对象实例交给Spring框架来实例化，程序使用时直接通过spring获取即可。

1、IOC容器实例化对象的方式--构造方法

实体类：

package com.text.entity;

public class Student {
    private String name;
    private int age;

    public Student() {
        System.out.println("无参构造方法:" + this.hashCode());
    }

    public Student(String name, int age) {
        System.out.println("有参构造方法:" + this.hashCode());
        this.name = name;
        this.age = age;
    }
}

applicationContext.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"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="s1" class="com.text.entity.Student"></bean>
    <bean id="s2" class="com.text.entity.Student">
        <constructor-arg name="name" value="张三"/>
        <constructor-arg name="age" value="18"/>
    </bean>
</beans>

测试类：运行后输出 

无参构造方法:6018
有参构造方法:6018
null
张三

package com.text.entity;

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

public class Test {
    public static void main(String[] args) {
        String configLocation = "classpath:applicationContext.xml";
        //spring IOC容器
        ApplicationContext context = new ClassPathXmlApplicationContext(configLocation);
        Student s1 = context.getBean("s1", Student.class);
        Student s2 = context.getBean("s2", Student.class);
        System.out.println(s1.getName());
        System.out.println(s2.getName());
    }
}

其中：ClassPathXmlApplicationContext 加载的路径文件classpath:applicationContext.xml称作路径表达式，具体形式参考如下表格示例

对于本项目的classpath 就是下图target/classes文件夹：

2、对象依赖注入（DI)

示例：将课程对象注入到学生对象中

配置文件：applicationContext.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"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="course" class="com.text.entity.Course">
        <constructor-arg name="name" value="语文"/>
    </bean>
    <bean id="student" class="com.text.entity.Student">
        <property name="name" value="张三"/>
        <property name="age" value="20"/>
        <property name="course" ref="course"/>
    </bean>
</beans>

实体类：

package com.text.entity;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Course {
    private String name;//课程名称
}

package com.text.entity;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {
    private String name;
    private int age;
    private Course course;
}

测试类：

package com.text.entity;

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

public class Test {
    public static void main(String[] args) {
        String configLocation = "classpath:applicationContext.xml";
        //spring IOC容器
        ApplicationContext context = new ClassPathXmlApplicationContext(configLocation);
        Course course = context.getBean("course",Course.class);
        Student student = context.getBean("student",Student.class);
        System.out.println(student);//Student(name=张三, age=20, course=Course(name=语文))
        System.out.println(course);//Course(name=语文)
        System.out.println(student.getCourse() == course); //true
    }
}

3、查询IOC容器对象示例详情

通过ApplicationContext.getBeanDefinitionNames 方法获取IOC容器中的已经实例化的对象名

package com.text.entity;

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

public class Test {
    public static void main(String[] args) {
        String configLocation = "classpath:applicationContext.xml";
        //spring IOC容器
        ApplicationContext context = new ClassPathXmlApplicationContext(configLocation);
        Course course = context.getBean("course",Course.class);
        String[] beanDefinitionNames = context.getBeanDefinitionNames();
        for(String beanDefinitionName:beanDefinitionNames) {
            System.out.print(beanDefinitionName + "\t");//course	student	
        }
    }
}