一、前言

        最近学习了SpringBoot分页查询的两种写法，一种是手动实现，另一种是使用框架实现。现在我将具体的实现流程分享一下。

二、手动实现分页查询

        先复习一下，SQL中的limit关键字，下面一行sql语句的意思是从第二个数据开始查，查询出两条数据

SELECT * FROM map limit 1,2;

        使用limit前一个参数pageNum是从第几个数据开始查，后一个参数pageSize是查询多少条数据，注意数据库查询pageNum=0代表第一个数据。

那么在Springboot中该如何写呢？

三、了解@RequestParam

1.什么是@RequestParam

• @RequestParam：将请求参数绑定到你控制器的方法参数上（是springmvc中接收普通参数的注解）

 2.语法

语法：@RequestParam(value=”参数名”,required=”true/false”,defaultValue=””)
 
value：参数名
 
required：是否包含该参数，默认为true，表示该请求路径中必须包含该参数，如果不包含就报错。
 
defaultValue：默认参数值，如果设置了该值，required=true将失效，自动为false,如果没有传该参数，就使用默认值

3.测试环境

环境：jdk1.8 Tomcat8.5  idea2018  manven父工程子模块
 
步骤：
1、创建web工程、引入依赖
2、配置SpringMvc入口文件 --DispatcherServlet--为总调度、web.xml里配置
3、创建Springmvc.xml文件--理解为：适配器(这里不需要自已指定适配、springmvc会自动指定)--视图解析器
4、创建 业务处理器 Controller类
5、测试
 

四、了解QueryWrapper

1.QueryWrapper是什么？

        QueryWrapper就是在使用Mybatis-plus中真实用到的一种技术，也叫作构造器，能简化sql的操作。

2.常用方法总结

1、单表操作

代码如下（示例）：我要查询姓名、班级、年龄符合前端传过来参数的数据并进行排序。

@GetMapping("/list")
public TableDataInfo list(Student student){
	LambdaQueryWrapper<Student> lqw = new LambdaQueryWrapper<Student>();
	lqw.eq(Student::getName, student.getName());
	lqw.like(Student::getClass,student.getClass());
	lqw.between("age",student.getAge1(),student.getAge2());
	lqw.orderByAsc("age");
	List<Student> list = studentService.list(lqw);
}

以上代码对应的sql为：

select * from student where name = '?' and class like '%?%' and age between '?' and '?' order by '?' asc

        由此可以看出，QueryWrapper其实可以理解成一个放查询条件的盒子，我们把查询条件放在里面，他就会自动按照对应的条件进行查询数据。

        根据不同的查询要求，有不同的用法，常用到的比如：eq、like、and、or、isNull、isNotNull、ne、likeRight、between等；使用方法及说明见下图。

2、多表操作

//Controller
@GetMapping("/listAndClass")
public TableDataInfo listAndClass(Student student)
{
    QueryWrapper<Student > qw = new QueryWrapper<Student >();
    if(StringUtils.isNotBlank(student.getName())){
        qw.eq("s.name",student.getName());
    }
    if(StringUtils.isNotBlank(student.getClassName())){
        qw.like("c.name",student.getClassName());
    }
    startPage();
    List<Student > list = studentService.listAndClass(qw);
    return getDataTable(list);
}

//Service
 List<Student> listAndClass(QueryWrapper<Student> qw);

//Service impl
@Override
public List<Student> listAndClass(QueryWrapper<Student> qw) {
    return this.getBaseMapper().listAndClass(qw);
}

//Mapper
@Select("select s.*,c.name from student s left join class c on s.id = c.student_id "+
        "${ew.customSqlSegment}")
List<YwSpaqjgDj> listAndClass(@Param(Constants.WRAPPER) QueryWrapper<Student> qw);

五.SpringBoot实现分页查询

1.创建UserController

import cn.hutool.core.util.StrUtil;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
import com.example.demo.common.Constants;
import com.example.demo.common.Result;
import com.example.demo.controller.dto.UserDTO;
import com.example.demo.entity.User;
import com.example.demo.service.IUserService;
import com.example.demo.utils.TokenUtils;
import org.springframework.web.bind.annotation.*;

import javax.annotation.Resource;
import java.util.List;

@CrossOrigin
@RestController
@RequestMapping("/user")
public class UserController {

    @Resource
    private IUserService userService;

    @PostMapping("/login")
    public Result login(@RequestBody UserDTO userDTO) {
        String username = userDTO.getUsername();
        String password = userDTO.getPassword();
        if (StrUtil.isBlank(username) || StrUtil.isBlank(password)) {
            return Result.error(Constants.CODE_400,"参数错误");
        }
        UserDTO dto = userService.login(userDTO);
        return Result.success(dto);
    }

    @PostMapping("/register")
    public Result register(@RequestBody UserDTO userDTO) {
        String username = userDTO.getUsername();
        String password = userDTO.getPassword();
        if (StrUtil.isBlank(username) || StrUtil.isBlank(password)) {
            return Result.error(Constants.CODE_400,"参数错误");
        }
        return Result.success(userService.register(userDTO));
    }

    //新增或者更新
    @PostMapping
    public Result save(@RequestBody User user) {
        String username = user.getUsername();
        if (StrUtil.isBlank(username)) {
            return Result.error(Constants.CODE_400, "参数错误");
        }
        if (user.getId() != null) {
            user.setPassword(null);
        } else {
            user.setNickname(user.getUsername());
            if (user.getPassword() == null) {
                user.setPassword("123456");
            }
        }
        return Result.success(userService.saveOrUpdate(user));
    }

    //删除
    @DeleteMapping("/{id}")
    public Result delete(@PathVariable Integer id) {
        return Result.success(userService.removeById(id));
    }

    @PostMapping("/del/batch")
    public Result deleteBatch(@RequestBody List<Integer> ids) {//批量删除
        return Result.success(userService.removeByIds(ids));
    }

    @GetMapping("/{id}")
    public Result findOne(@PathVariable Integer id) {
        return Result.success(userService.getById(id));
    }

    @GetMapping("/username/{username}")
    public Result findOne(@PathVariable String username) {
        QueryWrapper<User> queryWrapper = new QueryWrapper<>();
        queryWrapper.eq("username", username);
        return Result.success(userService.getOne(queryWrapper));
    }

}

并在Controller里面写好page接口

@GetMapping("/page")
    public Result findPage(@RequestParam Integer pageNum,
                           @RequestParam Integer pageSize,
                           @RequestParam(defaultValue = "") String username) {
        QueryWrapper<User> queryWrapper = new QueryWrapper<>();
        queryWrapper.orderByDesc("id");
        if (!"".equals(username)) {
            queryWrapper.like("username", username);
        }
        return Result.success(userService.page(new Page<>(pageNum, pageSize), queryWrapper));

    }

不难看出我们是根据数据库的id进行查询出数据库的数据

queryWrapper.orderByDesc("id")

public static void main(String[] args) {
    Object o = new Object();   
    Object o1 = o;
    Object o2 = o;
    System.out.println(o3.equals(o2));
}

代码输出：true

        所以我们是使用 equals来确定我们查询数据的对象，所以我们这里选择使用username来查询数据库里面的具体数据

if (!"".equals(username)) {
            queryWrapper.like("username", username);
        }

七、前端使用

1.前端技术栈

        对于前端，我们使用的是Vue+Element来进行功能实现，对于跨域的处理我使用的是axios进行处理，并且对axios进行了封装，具体步骤请查看：解决SpringBoot和前端Vue的跨域问题_慕言要努力的博客-CSDN博客

2.组件使用

我选用的是Element官网的组件来进行数据渲染

我们进行组件使用，并且设置分页的数据数量，数据可分为一页5条、10条以及15条 

<div style="padding: 10px 0">
      <el-pagination
          @size-change="handleSizeChange"
          @current-change="handleCurrentChange"
          :current-page="pageNum"
          :page-sizes="[ 5, 10, 15]"
          :page-size="pageSize"
          layout="total, sizes, prev, pager, next, jumper"
          :total="total">
      </el-pagination>
    </div>

3.数据渲染

我们默认查询处理的数据是0，设置页数为一页，一页为5条数据

data() {
    return {
      tableData: [],
      total: 0,
      pageNum: 1,
      pageSize: 5,
      username: "",
      form: {},
      dialogFormVisible: false,
      multipleSelection: [],
      headerBg: "headerBg",
      roles: []
    }
  }

 最后进行数据请求，请求后台写好的page接口

methods: {
    load: function () {
      this.request.get("/user/page", {
        params: {
          pageNum: this.pageNum,
          pageSize: this.pageSize,
          username: this.username,
        }
      }).then(res => {
        this.tableData = res.data.records
        this.total = res.data.total
      })
      this.request.get("/role").then(res => {
        this.roles = res.data
      })
    }
}

八、功能展示 

最后附上前端完整代码

<template>
  <div>
    <div style="margin: 10px 0">
      <el-input style="width: 200px; margin-left: 10px" placeholder="请输入用户名" clearable suffix-icon="el-icon-user" v-model="username" ></el-input>
      <el-button class="ml-5" type="primary" @click="load"><i class="el-icon-search" />搜索</el-button>
      <el-button type="warning" @click="reset"><i class="el-icon-refresh" />刷新</el-button>
    </div>
    <div style="margin: 10px 0">
      <el-button type="primary" @click="handleAdd" class="ml-10"><i class="el-icon-circle-plus-outline" />新增</el-button>
      <el-popconfirm
          class="ml-5"
          confirm-button-text='确认'
          cancel-button-text='取消'
          icon="el-icon-info"
          icon-color="red"
          title="确定批量删除这些信息吗？"
          @confirm="delBatch">
        <el-button type="danger" slot="reference" ><i class="el-icon-remove-outline" />删除</el-button>
      </el-popconfirm>
      <el-upload action="http://localhost:9090/user/import" :show-file-list="false" accept=".xlsx"
                 :on-success="handleExcelImportSuccess" style="display: inline-block">
        <el-button type="primary" class="ml-5"><i class="el-icon-upload"></i>导入</el-button>
      </el-upload>
      <el-button type="primary" class="ml-5" @click="exp"><i class="el-icon-download" />导出</el-button>
    </div>
    <el-table :data="tableData" border stripe :header-cell-class-name="headerBg"
              @selection-change="handleSelectionChange">
      <el-table-column type="selection" width="55"></el-table-column>
      <el-table-column prop="username" label="用户名" ></el-table-column>
      <el-table-column prop="nickname" label="昵称" ></el-table-column>
      <el-table-column prop="email" label="邮箱" ></el-table-column>
      <el-table-column prop="phone" label="联系方式" ></el-table-column>
      <el-table-column prop="address" label="地址"></el-table-column>
<!--      <el-table-column prop="role" label="身份"></el-table-column>-->
      <el-table-column label="操作">
        <template slot-scope="scope">
          <el-button type="success" @click="handleEdit(scope.row)"><i class="el-icon-edit-outline" />编辑</el-button>
        </template>
      </el-table-column>
    </el-table>
    <div style="padding: 10px 0">
      <el-pagination
          @size-change="handleSizeChange"
          @current-change="handleCurrentChange"
          :current-page="pageNum"
          :page-sizes="[ 5, 10, 15]"
          :page-size="pageSize"
          layout="total, sizes, prev, pager, next, jumper"
          :total="total">
      </el-pagination>
    </div>
    <el-dialog title="用户信息" :visible.sync="dialogFormVisible" width="30%">
      <el-form :model="form" label-width="100px" size="small">
        <el-form-item label="用户名" >
          <el-input v-model="form.username" autocomplete="off"></el-input>
        </el-form-item>
        <el-form-item label="昵称" >
          <el-input v-model="form.nickname" autocomplete="off"></el-input>
        </el-form-item>
        <el-form-item label="邮箱" >
          <el-input v-model="form.email" autocomplete="off"></el-input>
        </el-form-item>
        <el-form-item label="联系方式" >
          <el-input v-model="form.phone" autocomplete="off"></el-input>
        </el-form-item>
        <el-form-item label="地址" >
          <el-input v-model="form.address" autocomplete="off"></el-input>
        </el-form-item>
      </el-form>
      <div slot="footer" class="dialog-footer">
        <el-button @click="dialogFormVisible = false">取 消</el-button>
        <el-button type="primary" @click="save">确 定</el-button>
      </div>
    </el-dialog>
  </div>
</template>

<script>
export default {
  name: "User",
  data() {
    return {
      tableData: [],
      total: 0,
      pageNum: 1,
      pageSize: 5,
      username: "",
      form: {},
      dialogFormVisible: false,
      multipleSelection: [],
      headerBg: "headerBg",
      roles: []
    }
  },
  created() {
    this.load()
  },
  methods: {
    load: function () {
      this.request.get("/user/page", {
        params: {
          pageNum: this.pageNum,
          pageSize: this.pageSize,
          username: this.username,
        }
      }).then(res => {
        this.tableData = res.data.records
        this.total = res.data.total
      })
      this.request.get("/role").then(res => {
        this.roles = res.data
      })
    },
    home() {
      this.$router.push("/")
    },
    save() {
      this.request.post("/user", this.form).then(res => {
        if (res.code === '200') {
          this.$message.success("保存成功")
          this.dialogFormVisible = false
          this.load()
        } else {
          this.$message.error("保存失败")
        }
      })
    },
    handleAdd() {
      this.dialogFormVisible = true
      this.form = {}
    },
    handleEdit(row) {
      this.form = row
      this.dialogFormVisible = true
    },
    handleSelectionChange(val) {
      console.log(val)
      this.multipleSelection = val;
    },
    exp() {
      window.open("http://localhost:9090/user/export")
    },
    handleExcelImportSuccess() {
      this.$message.success("文件导入成功")
      this.load()
    },
    delBatch() {
      let ids = this.multipleSelection.map(v => v.id)  //[{}, {}, {}] => [1,2,3]
      this.request.post("/user/del/batch", ids).then(res => {
        if (res.code === '200') {
          this.$message.success("删除用户成功")
          this.load()
        } else {
          this.$message.error("删除用户失败")
        }
      })
    },
    reset() {
      this.username = ""
      this.load()
    },
    handleSizeChange(pageSize) {
      console.log(pageSize)
      this.pageSize = pageSize
      this.load()
    },
    handleCurrentChange(pageNum) {
      console.log(pageNum)
      this.pageNum = pageNum
      this.load()
    },
  }
}
</script>

<style>
.headerBg {
  background: #eee!important;
}
</style>

⛵小结

        以上就是对SpringBoot整合mybatis-plus实现分页查询简单的概述，现在我们的项目就更加的趋于完美了，也提升了我们对于编程的能力和思维！

        如果这篇文章有帮助到你，希望可以给作者点个赞👍，创作不易，如果有对后端技术、前端领域感兴趣的，也欢迎关注 ，我将会给你带来巨大的收获与惊喜💝💝💝！