以上笔记来源：
尚硅谷Spring零基础入门到进阶，一套搞定spring6全套视频教程（源码级讲解）https://www.bilibili.com/video/BV1kR4y1b7Qc

目录

19.1  @Autowired注入

①场景一：属性注入

19.1.1创建UserDao接口

19.1.2  创建UserDaoImpl实现类

19.1.3  创建UserService接口

19.1.4  创建UserServiceImpl实现类

19.1.5  创建Controller类

19.1.6  创建测试类

19.1.7  运行结果

②场景二：set注入

19.2.1  修改UserServiceImpl实现类

19.2.2 修改Controller类

③场景三：构造方法注入

19.3.1  修改UserServiceImpl实现类

19.3.2  修改Controller类

④场景四：形参上注入

19.4.1  修改UserServiceImpl类

19.4.2  修改Controller类

⑤场景五：只有一个构造函数，无注解

19.5.1 修改UserServiceImpl类

⑥  场景六：@Autowired注解和@Qualifier注解联合

19.6.1  添加dao层实现

19.6.2  修改UserServiceImpl类

19.7  总结

---

19.1  @Autowired注入

package org.springframework.beans.factory.annotation;​
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;import java.lang.annotation.Target;​@Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD,ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documentedpublic 
@interface Autowired
 {    
boolean required() default true;
}

源码中有两处需要注意：

• 第一处：该注解可以标注在哪里？

• • 构造方法上

  • 方法上

  • 形参上

  • 属性上

  • 注解上

第二处：该注解有一个required属性，默认值是true，表示在注入的时候要求被注入的Bean必须是存在的，如果不存在则报错。如果required属性设置为false，表示注入的Bean存在或者不存在都没关系，存在的话就注入，不存在的话，也不报错。

①场景一：属性注入

19.1.1创建UserDao接口

package com.atguigu.spring6.autowired.dao;

import com.atguigu.spring6.bean.User;

public interface UserDao {
    public void UserDaoadd();
}

19.1.2  创建UserDaoImpl实现类

package com.atguigu.spring6.autowired.dao;

import com.atguigu.spring6.autowired.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Repository;
import org.springframework.stereotype.Service;

@Repository
public class UserDaoImpl implements UserDao {

    @Override
    public void UserDaoadd() {
        System.out.println("addDao被调用了");
    }
}

19.1.3  创建UserService接口

package com.atguigu.spring6.autowired.service;

public interface UserService {
    public void UserServiceadd();
}

19.1.4  创建UserServiceImpl实现类

package com.atguigu.spring6.autowired.service;

import com.atguigu.spring6.autowired.dao.UserDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class UserServiceImpl implements UserService {

    @Autowired
    UserDao userDao;
    @Override
    public void UserServiceadd() {
        System.out.println("Service方法被调用了");
        userDao.UserDaoadd();
    }
}

19.1.5  创建Controller类

package com.atguigu.spring6.autowired.controller;

import com.atguigu.spring6.autowired.service.UserService;
import com.atguigu.spring6.autowired.service.UserServiceImpl;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import org.springframework.stereotype.Controller;

@Controller
public class controller {
    @Autowired
    UserService userService;

    public void addcontroller(){
        System.out.println("controller调用了");
        userService.UserServiceadd();
    }

}

19.1.6  创建测试类

package com.atguigu.spring6.autowired;
import com.atguigu.spring6.autowired.controller.controller;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Test {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("bean.xml");
        controller controller = context.getBean(controller.class);
        controller.addcontroller();
    }
}

19.1.7  运行结果

②场景二：set注入

19.2.1  修改UserServiceImpl实现类

package com.atguigu.spring6.autowired.service;

import com.atguigu.spring6.autowired.dao.UserDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class UserServiceImpl implements UserService {

//    @Autowired
    private UserDao userDao;

    @Autowired
    public void setUserDao(UserDao userDao) {
        this.userDao = userDao;
    }
    @Override
    public void UserServiceadd() {
        System.out.println("Service方法被调用了");
        userDao.UserDaoadd();
    }

}

19.2.2 修改Controller类

package com.atguigu.spring6.autowired.controller;

import com.atguigu.spring6.autowired.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;

@Controller
public class controller {
//    @Autowired
    private UserService userService;
    @Autowired
    public void setUserService(UserService userService) {
        this.userService = userService;
    }

        public void addcontroller(){
        System.out.println("controller调用了");
        userService.UserServiceadd();
    }

}

③场景三：构造方法注入

19.3.1  修改UserServiceImpl实现类

package com.atguigu.spring6.autowired.service;

import com.atguigu.spring6.autowired.dao.UserDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class UserServiceImpl implements UserService {

    private UserDao userDao;

    @Autowired
    public UserServiceImpl(UserDao userDao) {
        this.userDao = userDao;
    }

    @Override
    public void UserServiceadd() {
        System.out.println("Service方法被调用了");
        userDao.UserDaoadd();
    }

}

19.3.2  修改Controller类

package com.atguigu.spring6.autowired.controller;

import com.atguigu.spring6.autowired.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;

@Controller
public class controller {
    private UserDao userDao;

    public void addcontroller(){
        System.out.println("controller调用了");
        userService.UserServiceadd();
    }
    @Autowired
    public controller(UserService userService) {
       this.userService = userService;
    }
}

④场景四：形参上注入

19.4.1  修改UserServiceImpl类

package com.atguigu.spring6.autowired.service;

import com.atguigu.spring6.autowired.dao.UserDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class UserServiceImpl implements UserService {

    public UserServiceImpl(@Autowired UserDao userDao) {
        this.userDao = userDao;
    }

    @Override
    public void UserServiceadd() {
        System.out.println("Service方法被调用了");
        userDao.UserDaoadd();
    }

}

19.4.2  修改Controller类

package com.atguigu.spring6.autowired.controller;

import com.atguigu.spring6.autowired.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;

/**
 * @package: com.atguigu.spring6.autowired
 * @className: controller
 * @Description:
 * @author: haozihua
 * @date: 2024/10/22 11:37
 */
@Controller
public class controller {

        public void addcontroller(){
        System.out.println("controller调用了");
        userService.UserServiceadd();
    }
    public controller(@Autowired UserService userService) {
       this.userService = userService;
    }
}

⑤场景五：只有一个构造函数，无注解

19.5.1 修改UserServiceImpl类

package com.atguigu.spring6.autowired.service;

import com.atguigu.spring6.autowired.dao.UserDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class UserServiceImpl implements UserService {

    private UserDao userDao;

    public UserServiceImpl(@Autowired UserDao userDao) {
        this.userDao = userDao;
    }

    @Override
    public void UserServiceadd() {
        System.out.println("Service方法被调用了");
        userDao.UserDaoadd();
    }

}

注意：

当有参数的构造方法只有一个时，@Autowired注解可以省略。

说明：有多个构造方法时呢？大家可以测试（再添加一个无参构造函数），测试报错

⑥  场景六：@Autowired注解和@Qualifier注解联合

19.6.1  添加dao层实现

package com.atguigu.spring6.autowired.service;s

import com.atguigu.spring6.autowired.dao.UserDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Service;

@Service
public class UserServiceImpl implements UserService {

    @Autowired
    @Qualifier("userDaoImpl")
    private UserDao userDao;


    @Override
    public void UserServiceadd() {
        System.out.println("Service方法被调用了");
        userDao.UserDaoadd();
    }

}

测试：测试异常

错误信息中说：不能装配，UserDao这个Bean的数量等于2

怎么解决这个问题呢？当然要byName，根据名称进行装配了。

19.6.2  修改UserServiceImpl类

package com.atguigu.spring6.service.impl;

import com.atguigu.spring6.dao.UserDao;
import com.atguigu.spring6.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class UserServiceImpl implements UserService {

    @Autowired
    @Qualifier("userDaoImpl") // 指定bean的名字
    private UserDao userDao;

    @Override
    public void out() {
        userDao.print();
        System.out.println("Service层执行结束");
    }
}

19.7  总结

• @Autowired注解可以出现在：属性上、构造方法上、构造方法的参数上、setter方法上。

• 当带参数的构造方法只有一个，@Autowired注解可以省略。（）

• @Autowired注解默认根据类型注入。如果要根据名称注入的话，需要配合@Qualifier注解一起使用。