本次代码结构都是通过 mybatis plus 代码生成器直接生成

因为 mybatis plus 在 service 层和在 mapper 层中封装的方法有部分差异，所以这里特别分开进行罗列

一、mybatis plus 在 service 中封装的常用的方法

package com.wxw.springboot_mybatisplus.sys.controller;


import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.baomidou.mybatisplus.core.metadata.IPage;
import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
import com.wxw.springboot_mybatisplus.sys.entity.SysUser;
import com.wxw.springboot_mybatisplus.sys.service.SysUserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;

import org.springframework.web.bind.annotation.RestController;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * <p>
 * 用户表 前端控制器
 * </p>
 *
 * @author WXW
 * @since 2020-10-23
 */
@RestController
@RequestMapping("/sys/sys-user")
public class SysUserController {

    @Autowired
    SysUserService sysUserService;


    //分页查询
    public void page() {
        IPage<SysUser> page = sysUserService.page(
                new Page<>(0, 5),
                new QueryWrapper<SysUser>().lambda()// lambda 表达式格式的条件构造器
                .eq(SysUser::getUserName,"admin")//查询名字等于 admin
                .ne(SysUser::getUserName,"admin")//查询名字不等于 admin
        );
        page.getRecords();//对象集合
        page.getTotal();//总条数
        page.getCurrent();//当前页
        page.getPages();//总页数
        page.getSize();//每页条数
    }

    //list 查询
    public void list() {
        sysUserService.list(); //查询所有
        sysUserService.list(new QueryWrapper<SysUser>().lambda() //查询list 带条件的
                //这些条件都可以加判断，比如接收前端传入参数，可能为空，可能不为空
                // .eq(判断条件,对象属性,参数)
                .eq(1 == 1,SysUser::getUserName,"admin")//查询名字等于 admin
                .ne(1 == 1,SysUser::getUserName,"admin")//查询名字不等于 admin
        );

        //根据 map 查询
        Map<String,Object> map = new HashMap<>();
        map.put("user_name","admin");
        sysUserService.listByMap(map);
    }

    //插入
    public void insert(SysUser sysUser) {
        List<SysUser> list = new ArrayList<>();
        sysUserService.save(sysUser);//插入单个对象
        sysUserService.saveBatch(list);//批量插入
        sysUserService.saveOrUpdate(sysUser);//插入或者修改单个对象
        sysUserService.saveOrUpdateBatch(list);//批量插入或者修改单个对象
    }

    //修改
    public void update(SysUser sysUser) {
        List<SysUser> list = new ArrayList<>();
        //根据条件修改
        sysUserService.update(new QueryWrapper<SysUser>().lambda()
                .eq(SysUser::getUserName,"admin")
        );
        sysUserService.updateById(sysUser);//根据 id 修改
    }

    //修改
    public void delete(SysUser sysUser) {
        //根据条件删除
        sysUserService.remove(new QueryWrapper<SysUser>().lambda()
                .eq(SysUser::getUserName,"admin")
        );

        sysUserService.removeById(sysUser);//根据 id 删除

        List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        sysUserService.removeByIds(list);//根据多个 id 删除

        //根据map 条件 删除
        Map<String,Object> map = new HashMap<>();
        map.put("user_name","admin");
        sysUserService.removeByMap(map);
    }

    //获取单个对象
    public void getOne() {
        //根据条件获取单个对象
        sysUserService.getOne(new QueryWrapper<SysUser>().lambda()
                .eq(SysUser::getUserName,"admin"));
    }

    //根据id 查询
    public void selectById(SysUser sysUser) {
        //根据条件获取单个对象
        sysUserService.getById(sysUser.getId());
        sysUserService.getById(sysUser);
    }

    //count
    public void count(SysUser sysUser) {
        //获取count
        sysUserService.count();
        //根据条件获取 count
        sysUserService.count(new QueryWrapper<SysUser>().lambda()
                .eq(SysUser::getUserName,"admin"));
    }

}

二、mybatis plus 在 mapper 中封装的常用方法

package com.wxw.springboot_mybatisplus.sys.service.impl;

import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.baomidou.mybatisplus.core.metadata.IPage;
import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
import com.wxw.springboot_mybatisplus.sys.entity.SysUser;
import com.wxw.springboot_mybatisplus.sys.mapper.SysUserMapper;
import com.wxw.springboot_mybatisplus.sys.service.SysUserService;
import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
import org.springframework.stereotype.Service;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * <p>
 * 用户表 服务实现类
 * </p>
 *
 * @author WXW
 * @since 2020-10-23
 */
@Service
public class SysUserServiceImpl extends ServiceImpl<SysUserMapper, SysUser> implements SysUserService {


    //分页查询
    public void page() {
        IPage<SysUser> page = baseMapper.selectPage(
                new Page<>(0, 5),
                new QueryWrapper<SysUser>().lambda()// lambda 表达式格式的条件构造器
                        .eq(SysUser::getUserName,"admin")//查询名字等于 admin
                        .ne(SysUser::getUserName,"admin")//查询名字不等于 admin
        );
        page.getRecords();//对象集合
        page.getTotal();//总条数
        page.getCurrent();//当前页
        page.getPages();//总页数
        page.getSize();//每页条数
    }

    //list 查询
    public List<SysUser> list() {
        baseMapper.selectList(new QueryWrapper<SysUser>()); //查询所有

        //最大值查询（有时候我们需要自定义一些sql ，就可以使用这种写法）
        QueryWrapper<SysUser> userQueryWrapper = new QueryWrapper<>();
        userQueryWrapper.select("max(age) as total");
        List<Map<String, Object>> list = baseMapper.selectMaps(userQueryWrapper);
        list.get(0).get("total");


        baseMapper.selectList(new QueryWrapper<SysUser>().lambda() //查询list 带条件的
                //这些条件都可以加判断，比如接收前端传入参数，可能为空，可能不为空
                // .eq(判断条件,对象属性,参数)
                .eq(1 == 1,SysUser::getUserName,"admin")//查询名字等于 admin
                .ne(1 == 1,SysUser::getUserName,"admin")//查询名字不等于 admin
        );

        //根据 map 查询
        Map<String,Object> map = new HashMap<>();
        map.put("user_name","admin");
        baseMapper.selectByMap(map);

        return null;
    }

    //插入
    public void insert(SysUser sysUser) {
        baseMapper.insert(sysUser);//插入单个对象
    }

    //修改
    public void update(SysUser sysUser) {
        //根据条件修改
        baseMapper.update(sysUser,new QueryWrapper<SysUser>().lambda()
                .eq(SysUser::getUserName,"admin")
        );
        baseMapper.updateById(sysUser);//根据 id 修改
    }

    //修改
    public void delete(SysUser sysUser) {
        //根据条件删除
        baseMapper.delete(new QueryWrapper<SysUser>().lambda()
                .eq(SysUser::getUserName,"admin")
        );

        baseMapper.deleteById(sysUser);//根据 id 删除

        List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        baseMapper.deleteBatchIds(list);//根据多个 id 删除

        //根据map 条件 删除
        Map<String,Object> map = new HashMap<>();
        map.put("user_name","admin");
        baseMapper.deleteByMap(map);
    }

    //获取单个对象
    public void getOne() {
        //根据条件获取单个对象
        baseMapper.selectOne(new QueryWrapper<SysUser>().lambda()
                .eq(SysUser::getUserName,"admin"));
    }

    //根据id 查询
    public void selectById(SysUser sysUser) {
        //根据条件获取单个对象
        baseMapper.selectById(sysUser.getId());
        baseMapper.selectById(sysUser);
    }

    //count
    public void count(SysUser sysUser) {
        //获取count
        baseMapper.selectCount(new QueryWrapper<>());
        //根据条件获取 count
        baseMapper.selectCount(new QueryWrapper<SysUser>().lambda()
                .eq(SysUser::getUserName,"admin"));
    }
}

三、关于 mybatis plus 查询条件构造器

类似于上文用在查询语句中的 .eq 的方法

最后贴上 mybatis plus 官方地址

官方文档1

官方文档2

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