关于分页查询的基础案例

public class BroadScopeController {
    @Resource
    private BroadScopeService broadScopeService;

    @PostMapping ("/queryPage")
      /**
      * 分页查询
      */
    public Result queryPage(@RequestBody BroadScopePageDTO dto) {
        //如果穿了页数的话,就默认第一页,然后每页10条
        if (Objects.isNull(dto.getPage())||dto.getPage()<1) {
            dto.setPage(1);
        }
        if (Objects.isNull(dto.getPageSize())||dto.getPageSize()<1) {
            dto.setPageSize(10);
        }
        PageResult queryPage =broadScopeService.queryPage(dto);
        return Result.success(queryPage);
    }

@Service
public interface BroadScopeService {
    PageResult queryPage(BroadScopePageDTO dto);}

@Service
public class BroadScopeServiceImpl implements BroadScopeService {
    @Resource
    private BroadScopeMapper broadScopeMapper;
    @Resource
    private BroadScopePromptMapper broadScopePromptMapper;

    /**
     * 分页查询
     * @param dto
     * @return
     */
    @Override
    public PageResult queryPage(BroadScopePageDTO dto) {
        // 计算偏移量,防止sql注入
        int offset = (dto.getPage() - 1) * dto.getPageSize();

        List category = broadScopeMapper.queryPage(offset, dto.getPageSize());
        // 判断是否为空,没有配置指令,就返回null
        if (ObjectUtils.isEmpty(category)) {
            return null;
        }
        // 实体类转vo
        List categoryVO = new ArrayList<>();
        for (BroadScopePromptCategory broadScopePromptCategory : category) {
            BroadScopeVO broadScopeVO = new BroadScopeVO();
            // Bean拷贝失败,换个工具类,
            BeanUtils.copyProperties(broadScopePromptCategory, broadScopeVO, BroadScopeVO.class);
            categoryVO.add(broadScopeVO);
        }
        PageResult data = new PageResult<>();
        data.setPage(dto.getPage());
        data.setPageSize(dto.getPageSize());
        data.setRows(categoryVO);
        data.setTotal(category.size());
        return data;
    }

@Mapper
public interface BroadScopeMapper {
    
    /**
     * 分页查询
     * @param offset
     * @param pageSize
     * @return
     */

    List queryPage(@Param("offset") int offset,@Param("pageSize") Integer pageSize);

    select id,
    category_name,
    sort
    from broadscope_prompt_category
   order by sort
    LIMIT #{pageSize} OFFSET #{offset}

统一返回类

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Result {
    private Integer code;//响应码，1 代表成功; 0 代表失败
    private String msg;  //响应信息 描述字符串
    private Object data; //返回的数据

    //增删改 成功响应
    public static Result success(){
        return new Result(1,"success",null);
    }
    //查询 成功响应
    public static Result success(Object data){
        return new Result(1,"success",data);
    }
    //失败响应
    public static Result error(String msg){
        return new Result(0,msg,null);
    }
}

分页传入dto

@Data
public class BroadScopePageDTO {

    /**
     * 当前页
     */
    private Integer page;

    /**
     * 每页展示几条
     */
    private Integer pageSize;
    

}

返回的数据vo:

@Data
@AllArgsConstructor
@NoArgsConstructor
public class BroadScopeVO implements Serializable {

    private Long id;
    /**
     * 类目名称
     */

    private String categoryName;
    /**
     * 排序
     */

    private Long sort;
    
}

全局异常类:

/**
 * 自定义全局异常处理器
 */
public class BusinessException extends RuntimeException {

    public BusinessException(String message) {
        super(message);
    }

    public BusinessException(){
    }
}

信息常量类:

/**
 * 信息提示常量类
 */
public class MessageConstant {

    public static final String CATEGORY_EXISTS = "该分类已经存在";
    public static final String SAVE_FAILED = "新增失败";
    public static final String DELETE_FAILED = "删除失败";
    public static final String CATEGORY_DELETE = ",分类下有指令,删除失败";

    public static final String UPDATE_FAILED = "修改失败";
    public static final String SORT_NULL = "排序字段不能为空";
    public static final String SORT_FAILED = "排序失败";

    /**
     * 参数不能为空
     */
    private static final String PARAMETER_NULL = "参数不能为空";

    public static final String ID_NULL = "id不能为空";
}

测试: