背景

在业务中我们在前端总是需要展示数据，将后端得到的数据进行分页处理，通过pagehelper实现动态的分页查询，将查询页数和分页数通过前端发送到后端，后端使用pagehelper，底层是封装threadlocal得到页数和分页数并动态的拼接到sql中完成分页的查询

代码

菜品的分页查询展示
前端传递对象

@Data
public class DishPageQueryDTO implements Serializable {

    private int page;

    private int pageSize;

    private String name;

    //分类id
    private Integer categoryId;

    //状态 0表示禁用 1表示启用
    private Integer status;

}

controller层

/**
     * 菜品分页查询
     *
     * @param dishPageQueryDTO
     * @return
     */
    @GetMapping("/page")
    @ApiOperation("菜品分页查询")
    public Result<PageResult> page(DishPageQueryDTO dishPageQueryDTO) {
        PageResult pageResult = dishService.pageQuery(dishPageQueryDTO);
        return Result.success(pageResult);
    }

service层

PageResult pageQuery(DishPageQueryDTO dishPageQueryDTO);

public PageResult pageQuery(DishPageQueryDTO dishPageQueryDTO) {
        PageHelper.startPage(dishPageQueryDTO.getPage(), dishPageQueryDTO.getPageSize());
        Page<DishVO> page = dishMapper.pageQuery(dishPageQueryDTO);
        return new PageResult(page.getTotal(), page.getResult());
    }

将封装好的需要查询的页面和页面大小传给PageHelper，由它为我们动态的拼接到SQL语句中实现分页查询的效果
Mapper层

Page<DishVO> pageQuery(DishPageQueryDTO dishPageQueryDTO);

 <select id="pageQuery" resultType="com.sky.vo.DishVO">
        select d.* , c.name as categoryName from dish d left outer join category c on d.category_id = c.id
        <where>
            <if test="name != null">
                and d.name like concat('%',#{name},'%')
            </if>
            <if test="categoryId != null">
                and d.category_id = #{categoryId}
            </if>
            <if test="status != null">
                and d.status = #{status}
            </if>
        </where>
        order by d.create_time desc
    </select>