new和@Autowired区别

@Autowired注入的对象在注入之前就已经实例化，是从ioc容器中获取已经初始化的对象

new实例化一个对象，new对象不能注入其他对象，因为new出来的对象生命周期不受ioc容器管控，自然无法完成属性的注入

 

实例：

package com.example.SpringBootStudy.controller;

import com.example.SpringBootStudy.service.TestService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

/**
 * @Author jyy
 * @Description {}
 * @Date 2018/12/19 18:28
 */
@RestController
@RequestMapping(value = "/test")
public class TestController {

    @Autowired
    private TestService testService;

    @RequestMapping(value = "/print",method = RequestMethod.GET)
    public void test() {
        testService.test();
    }
}

package com.example.SpringBootStudy.service;

import com.example.SpringBootStudy.dao.TestDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

/**
 * @Author jyy
 * @Description {}
 * @Date 2018/12/19 18:26
 */
@Service
public class TestService {

    @Autowired
    private TestDao testDao;

    public void test() {
        testDao.test();
    }
}

package com.example.SpringBootStudy.dao;

import org.springframework.stereotype.Repository;

/**
 * @Author jyy
 * @Description {}
 * @Date 2018/12/19 18:26
 */
@Repository
public class TestDao {

    public void test() {
        System.out.println("调用成功！");
    }
}

输出结果：

调用成功！

package com.example.SpringBootStudy.controller;

import com.example.SpringBootStudy.service.TestService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

/**
 * @Author jyy
 * @Description {}
 * @Date 2018/12/19 18:28
 */
@RestController
@RequestMapping(value = "/test")
public class TestController {

    //@Autowired
    //private TestService testService;
    private TestService testService = new TestService();

    @RequestMapping(value = "/print", method = RequestMethod.GET)
    public void test() {
        testService.test();
    }
}

输出结果：

报出空指针异常，跟踪发现Service中的testDao未正确赋值

总结：@Autowired是从IOC容器中获取已经初始化的对象，此对象中@Autowired的属性也已经通过容器完成了注入，整个生命周期都交由容器管控。然而通过new出来的对象，生命周期不受容器管控，自然也无法完成属性的自动注入(上例中，TestDao就是testService的属性，由于testService是new出来的，无法在完成testDao属性注入)