一般的Spring Boot项目有四个大包
- pojo：是与数据库表相关的实体类
- controller：接收前端传过来的参数进行操作，之后交给业务层
- service：业务层，不直接对接数据库，只是在这层写接口，有接口之后还需要写接口实现，实现里调用mapper（数据）层
- mapper：写逻辑，对数据库操作

业务需求: 输入用户id，返回用户信息

1、知道数据库表是怎样的，写对应的实体类(pojo层)

就要创建pojo包，在pojo包下创建实体类

package com.walker.pojo;

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

@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
    private Integer id;
    private String name;
    private Integer age;
    private Integer gender;
    private String phone;
}

2、写Mapper层，具体实现逻辑或者sql语句

创建mapper包，在mapper包下写该业务mapper

package com.walker.mapper;

import com.walker.pojo.User;
import org.apache.ibatis.annotations.Mapper;
import org.apache.ibatis.annotations.Select;

@Mapper
public interface UserMapper  {

    @Select("select * from user where id = ${id}")
    User findById(Integer id);
}

3、写service接口

创建service包，在service包下写该业务接口

package com.walker.service;

import com.walker.pojo.User;

public interface UserService {

    User findById(Integer id);
}

4、实现service接口，并且调用mapper逻辑

在service包下创建实现接口的包（impl），在impl包下写该业务的接口的实现类

package com.walker.service.impl;

import com.walker.mapper.UserMapper;
import com.walker.pojo.User;
import com.walker.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class UserServiceImpl implements UserService {

    @Autowired
    private UserMapper userMapper;

    @Override
    public User findById(Integer id) {
        return userMapper.findById(id);
    }
}

5、写controller，接收前端传的参数，调用service（业务）层

创建controller包，在controller包下写该业务

package com.walker.controller;

import com.walker.pojo.User;
import com.walker.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class UserController {

    @Autowired
    private UserService userService;

    @RequestMapping("/find")
    public User findById(@RequestParam(name="id")  Integer id) {
        return userService.findById(id);
    }
}