问题描述：

1：我们使用PageHelper插件的时候，PageHelper.startPage(pageNow,pageSize)要放在查询语句的前面

2：当startPage的后面有多次查询的话，它只对第一条查询语句有效果

3：假如要进行多次查询，然后对结果进行分页，关注点是：对最后的结果进行分页，而不是第一次查询出来的结果

最终解决办法有2种：第一种：依旧是用老套路，用原始自己写的PageUtil做【因为里面存放的数据就是List的，所以将最终查询的结果（不管你查询多少次）直接放进去即可】

　　　　　　　　　  第二种：重新写一个SQL语句，解决问题所需【这样就只需查询一次】

效果如下：

开始代码如下：

    @Override //重写父类的无条件分页查询
    public PageUtil queryAllByPage(int pageNow, int pageSize) {
        //先定义一个存放查询结果的地方
        List<SpecsParamVO> specsParam = new ArrayList<>();
        //先实例化一个Example对象
        TbSpuExample example = new TbSpuExample();
        //设置分页的数据
        PageHelper.startPage(pageNow,pageSize);
        //无条件查询tb_spu表中的所有数据
        List<TbSpu> tbSpus = spuMapper.selectByExample(example);
        log.info("共查询到："+tbSpus.size()+"条数据");
        //遍历每一条数据的id
        for (TbSpu spu:tbSpus){
            //根据id查询tb_sku表，每个id对应多少条数据
            TbSkuExample skuExample = new TbSkuExample();
            //利用skuExample进行条件查询
            TbSkuExample.Criteria criteria = skuExample.createCriteria();
            //in条件查询
            //涉及：long类型转integer类型：
            //criteria.andSpuIdEqualTo(Integer.valueOf(spu.getId()+""));
            criteria.andSpuIdEqualTo(Integer.valueOf(spu.getId().toString()));
            //开始查询
            List<TbSku> tbSkus = skuMapper.selectByExample(skuExample);
            //对应的条数就是规格数量
            long specsCount = tbSkus.size();
            log.info("id为："+spu.getId()+"查询到的规格数量一共是："+specsCount);
            //根据id查询tb_spu_detail表，统计参数数量
            TbSpuDetail detail = spuDetailMapper.selectByPrimaryKey(spu.getId());
            //获取detail的specifications，即所有的参数
            try {
                String[] split = detail.getSpecifications().split("group");
                //对应的数量就是参数数量
                long paramCount = split.length-1;
                log.info("id为："+spu.getId()+"查询到的参数数量一共是："+paramCount);
                //存放每条数据
                SpecsParamVO vo = new SpecsParamVO(spu.getId(), spu.getTitle(), paramCount, specsCount);
                specsParam.add(vo);
            }catch (Exception e){
                log.info("该条数据字符串切分异常："+spu.getId());
                //存放每条数据
                SpecsParamVO vo = new SpecsParamVO(spu.getId(), spu.getTitle(), (long)0, specsCount);
            }
        }

        //使用pageInfo对结果进行封装
        PageInfo<SpecsParamVO> pageInfo = new PageInfo<>(specsParam);
        log.info(pageInfo.getTotal());  //始终是15条数据
        log.info(pageInfo.getPages());  //总页数也是只有1条
        return pageInfo;
    }

解决方法一：

1.重新编写一个工具类PageUtil[粘贴即可用]

package com.blb.util;

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

public class PageUtil {
    /**
     * 总记录数
     */
    private int totalCount;
    /**
     * 每页的条数
     */
    private int pageSize;
    /**
     * 总页数
     */
    private int totalPage;
    /**
     * 当前是第几页
     */
    private int pageNow;
    /**
     * 当前页的数据
     */
    private List data;
    /**
     * 当前页的下一页是几
     */
    private int next;
    /**
     * 当前页的上一页是几
     */
    private int last;
    /**
     * 是否有下一页
     */
    private boolean hasNext;
    /**
     * 是否有上一页
     */
    private boolean hasLast;
    /**
     * 是否是第一页
     */
    private boolean isFirst;
    /**
     * 是否是最后一页
     */
    private boolean isLast;

    /**
     * 自定义的页码的格式
     */
    private int num = 7;
    /**
     * 页码数
     */
    private List pages = new ArrayList<>();

    // 用于循环的第一个
    private int forMin;

    // 用于循环的最后一个
    private int forMax;

    private String buidUrl;

    /**
     * 分页时携带的查询条件
     */
    private Map param;

    public String getBuidUrl() {
        StringBuffer url = new StringBuffer();
        if (null != param && param.size() > 0) {
            Set<Map.Entry<String, String>> entrySet = param.entrySet();
            for (Map.Entry<String, String> e : entrySet) {
                url.append("&param." + e.getKey() + "=" + e.getValue());
            }
        }
        return url.toString();
    }

    public PageUtil(int totalCount, int pageNow, int pageSize, Map param) {
        this.totalCount = totalCount;
        this.pageSize = pageSize;
        this.pageNow = pageNow;
        this.param = param;
        // 先把总页数算出来
        this.totalPage = totalCount % pageSize == 0 ? totalCount / pageSize : (totalCount / pageSize) + 1;
        this.next = pageNow + 1;
        this.last = pageNow - 1;
        if (this.totalPage == 1) {
            this.isFirst = true;
            this.isLast = true;
            this.hasLast = false;
            this.hasNext = false;
        } else {
            if (this.pageNow <= 1) {
                this.pageNow = 1;
                this.last = 1;
                this.isFirst = true;
                this.hasLast = false;
                this.hasNext = true;
                this.isLast = false;
            } else if (this.pageNow >= this.totalPage) {
                this.pageNow = this.totalPage;
                this.next = this.totalPage;
                this.isFirst = false;
                this.isLast = true;
                this.hasLast = true;
                this.hasNext = false;
            } else {
                this.isFirst = false;
                this.isLast = false;
                this.hasLast = true;
                this.hasNext = true;
            }
        }
        // 设置最大值和最小值
        int min = (int) (this.pageNow - Math.floor(num / 2));
        int max = (int) (this.pageNow + Math.floor(num / 2));
        if (totalPage >= num) {
            if (min < 1) {
                forMin = 1;
                forMax = num;
                for (int i = forMin; i <= forMax; i++) {
                    pages.add(i);
                }
                return;
            }
            if (max > totalPage) {
                forMax = totalPage;
                forMin = totalPage - num + 1;
                for (int i = forMin; i <= forMax; i++) {
                    pages.add(i);
                }
                return;
            }
            for (int i = min; i <= max; i++) {
                pages.add(i);
            }
        } else {
            for (int i = 1; i <= totalPage; i++) {
                pages.add(i);
            }
        }
    }

    public int getTotalCount() {
        return totalCount;
    }

    public void setTotalCount(int totalCount) {
        this.totalCount = totalCount;
    }

    public int getPageSize() {
        return pageSize;
    }

    public void setPageSize(int pageSize) {
        this.pageSize = pageSize;
    }

    public int getTotalPage() {
        return totalPage;
    }

    public void setTotalPage(int totalPage) {
        this.totalPage = totalPage;
    }

    public int getPageNow() {
        return pageNow;
    }

    public void setPageNow(int pageNow) {
        this.pageNow = pageNow;
    }

    public List getData() {
        return data;
    }

    public void setData(List data) {
        this.data = data;
    }

    public int getNext() {
        return next;
    }

    public void setNext(int next) {
        this.next = next;
    }

    public int getLast() {
        return last;
    }

    public void setLast(int last) {
        this.last = last;
    }

    public boolean getHasNext() {
        return hasNext;
    }

    public void setHasNext(boolean hasNext) {
        this.hasNext = hasNext;
    }

    public boolean getHasLast() {
        return hasLast;
    }

    public void setHasLast(boolean hasLast) {
        this.hasLast = hasLast;
    }

    public boolean getIsFirst() {
        return isFirst;
    }

    public void setFirst(boolean isFirst) {
        this.isFirst = isFirst;
    }

    public boolean getIsLast() {
        return isLast;
    }

    public void setLast(boolean isLast) {
        this.isLast = isLast;
    }

    public int getNum() {
        return num;
    }

    public void setNum(int num) {
        this.num = num;
    }

    public List getPages() {
        return pages;
    }

    public void setPages(List pages) {
        this.pages = pages;
    }

    public Map getParam() {
        return param;
    }

    public void setParam(Map param) {
        this.param = param;
    }

}

2.service的代码修改

    @Override //重写父类的无条件分页查询
    public PageUtil queryAllByPage(int pageNow, int pageSize) {
        //先定义一个存放查询结果的地方
        List<SpecsParamVO> specsParam = new ArrayList<>();
        //先实例化一个Example对象
        TbSpuExample example = new TbSpuExample();
        //设置分页的数据
        PageHelper.startPage(pageNow,pageSize);
        //无条件查询tb_spu表中的所有数据
        List<TbSpu> tbSpus = spuMapper.selectByExample(example);
        log.info("共查询到："+tbSpus.size()+"条数据");
        //遍历每一条数据的id
        for (TbSpu spu:tbSpus){
            //根据id查询tb_sku表，每个id对应多少条数据
            TbSkuExample skuExample = new TbSkuExample();
            //利用skuExample进行条件查询
            TbSkuExample.Criteria criteria = skuExample.createCriteria();
            //in条件查询
            //涉及：long类型转integer类型：
            //criteria.andSpuIdEqualTo(Integer.valueOf(spu.getId()+""));
            criteria.andSpuIdEqualTo(Integer.valueOf(spu.getId().toString()));
            //开始查询
            List<TbSku> tbSkus = skuMapper.selectByExample(skuExample);
            //对应的条数就是规格数量
            long specsCount = tbSkus.size();
            log.info("id为："+spu.getId()+"查询到的规格数量一共是："+specsCount);
            //根据id查询tb_spu_detail表，统计参数数量
            TbSpuDetail detail = spuDetailMapper.selectByPrimaryKey(spu.getId());
            //获取detail的specifications，即所有的参数
            try {
                String[] split = detail.getSpecifications().split("group");
                //对应的数量就是参数数量
                long paramCount = split.length-1;
                log.info("id为："+spu.getId()+"查询到的参数数量一共是："+paramCount);
                //存放每条数据
                SpecsParamVO vo = new SpecsParamVO(spu.getId(), spu.getTitle(), paramCount, specsCount);
                specsParam.add(vo);
            }catch (Exception e){
                log.info("该条数据字符串切分异常："+spu.getId());
                //存放每条数据
                SpecsParamVO vo = new SpecsParamVO(spu.getId(), spu.getTitle(), (long)0, specsCount);
            }
        }　　　　　　
        PageUtil pageUtil = new PageUtil((int) spuMapper.countByExample(example), pageNow, pageSize, null);
        pageUtil.setData(specsParam);
        return pageUtil;
    }

3.jsp页面分页代码

<!--分页-->
<div id="u1950-${pageUtil.data.size()+1}" class="preeval" style="width: 1100px; height: 49px;">
  <div id="u1961_div" class="ax_default _表格 u1951" style="visibility: inherit">
  <div class="pages">
      <ul>
        <li>
          <span>共</span><span style="color:#FF3C2C;">${pageUtil.totalPage}</span><span>页/</span><span style="color:#FF0000;">${pageUtil.totalCount}</span><span>条数据</span>
        </li>
        <c:if test="${!pageUtil.isFirst}">
          <li>
            <a href="${pageContext.request.contextPath}/specsParam/1/15">首页</a>
          </li>
        </c:if>
        <c:if test="${pageUtil.hasLast}">
          <li>
            <a href="${pageContext.request.contextPath}/specsParam/${pageUtil.last}/15">上一页</a>
          </li>
        </c:if>
        <c:forEach items="${pageUtil.pages}" var="page">
          <c:if test="${pageUtil.pageNow eq page}">
            <li>
              <a id="curr" href="${pageContext.request.contextPath}/specsParam/${page}/15">${page}</a>
            </li>
          </c:if>
          <c:if test="${pageUtil.pageNow ne page}">
            <li>
              <a href="${pageContext.request.contextPath}/specsParam/${page}/15">${page}</a>
            </li>
          </c:if>
        </c:forEach>
        <c:if test="${pageUtil.hasNext}">
          <li>
            <a href="${pageContext.request.contextPath}/specsParam/${pageUtil.next}/15">下一页</a>
          </li>
        </c:if>
        <c:if test="${!pageUtil.isLast}">
          <li>
            <a href="${pageContext.request.contextPath}/specsParam/${pageUtil.totalPage}/15">尾页</a>
          </li>
        </c:if>
      </ul>
  </div>
  </div>
</div>
</div>

解决办法二：

第一种办法显得太low了，所以我们通过写一个SQL语句解决问题，只用查询一次，也就不用自己写工具类了

1.写一个数据类用于存放最后查询出来的结果

package com.blb.vo;

import lombok.Data;

@Data
public class SpecsParamVO {
    private Long id;
    private String title;
    private Long specsCount;
    private String paramCount;

    public SpecsParamVO(Long id, String title, Long specsCount, String paramCount) {
        this.id = id;
        this.title = title;
        this.specsCount = specsCount;
        this.paramCount = paramCount;
    }

    public SpecsParamVO() {

    }
}

2.在mapper.xml中添加SQL语句

还有在对应的mapper接口添加该方法：

List<SpecsParamVO> getAll_SpecsParam();

<select id="getAll_SpecsParam" resultType="com.blb.vo.SpecsParamVO">
  select s.id, s.title,specifications as paramCount,count(s.id) as specsCount from
  (select * from tb_spu spu left join tb_spu_detail detail on spu.id = detail.spu_id) s
  left join tb_sku sku on s.id = sku.spu_id
  group by s.id
</select>

3.写service层的代码

特别注意：要注意对结果进行封装的位置

　　【你是先查询，然后对结果进行字符串切分等操作，做完之后再进行封装】 --> 该做法会出问题，导致PageHelper分页不了

　　【你是先查询，然后将结果进行封装，封装完之后将需要做字符串切分等操作取出来，做完操作之后重新设置回去(setList)】--> 完美解决问题

    @Override //通过SQL语句解决多次查询的问题
    public PageInfo queryAllByPage2(int pageNow, int pageSize) {
        //定义一个List集合，存放查询出来的数据
        ArrayList<SpecsParamVO> info = new ArrayList<>();
        //设置分页的数据
        PageHelper.startPage(pageNow,pageSize);
        //只需查询一次即可
        List<SpecsParamVO> list = spuMapper.getAll_SpecsParam();　　　　　//需要特别注意对查询结果封装的位置
        PageInfo<SpecsParamVO> infos = new PageInfo<>(list); //特别注意......
        log.info(infos.getTotal());
        log.info(infos.getPages());
        //遍历查询出来的数据--为了获取所有参数值
        for (SpecsParamVO sp : infos.getList()){
            String specifications = sp.getParamCount();
            //对字符串做切分操作
            try {
                String[] split = specifications.split("group");
                //对应的数量就是参数数量
                long paramCount = split.length-1;
                log.info("id为："+sp.getId()+"查询到的参数数量一共是："+paramCount);
                //存放每条数据
                SpecsParamVO vo = new SpecsParamVO(sp.getId(), sp.getTitle(), sp.getSpecsCount(), paramCount+"");
                info.add(vo);
            }catch (Exception e){
                log.info("该条数据字符串切分异常："+sp.getId());
                //存放每条数据
                SpecsParamVO vo = new SpecsParamVO(sp.getId(), sp.getTitle(), sp.getSpecsCount(), "0");
                info.add(vo);
            }
        }
        infos.setList(info);　　　　　错误做法：　
//        //对结果进行封装
//        PageInfo<SpecsParamVO> pageInfo = new PageInfo<>(info);
//        log.info(pageInfo.getTotal());
//        log.info(pageInfo.getPages());
        return infos;
    }

4.jsp页面分页代码

<!--分页-->
<div id="u1950-${pageInfo.list.size()+1}" class="preeval" style="width: 1100px; height: 49px;">
  <div id="u1961_div" class="ax_default _表格 u1951" style="visibility: inherit">
    <div class="pages">
      <ul>
        <li>
          <span>共</span><span style="color:#FF3C2C;">${pageInfo.pages}</span><span>页/</span><span style="color:#FF0000;">${pageInfo.total}</span><span>条数据</span>
        </li>
        <c:if test="${!pageInfo.isFirstPage}">
          <li>
            <a href="${pageContext.request.contextPath}/specsParam/1/15">首页</a>
          </li>
        </c:if>
        <c:if test="${pageInfo.hasPreviousPage}">
          <li>
            <a href="${pageContext.request.contextPath}/specsParam/${pageInfo.lastPage}/15">上一页</a>
          </li>
        </c:if>
        <c:forEach items="${pageInfo.navigatepageNums}" var="page">
          <c:if test="${pageInfo.pageNum eq page}">
            <li>
              <a id="curr" href="${pageContext.request.contextPath}/specsParam/${page}/15">${page}</a>
            </li>
          </c:if>
          <c:if test="${pageInfo.pageNum ne page}">
            <li>
              <a href="${pageContext.request.contextPath}/specsParam/${page}/15">${page}</a>
            </li>
          </c:if>
        </c:forEach>
        <c:if test="${pageInfo.hasNextPage}">
          <li>
            <a href="${pageContext.request.contextPath}/specsParam/${pageInfo.nextPage}/15">下一页</a>
          </li>
        </c:if>
        <c:if test="${!pageInfo.isLastPage}">
          <li>
            <a href="${pageContext.request.contextPath}/specsParam/${pageInfo.pages}/15">尾页</a>
          </li>
        </c:if>
      </ul>
    </div>
  </div>
</div>

PageHelper遇到问题可以参考：https://www.cnblogs.com/my12/p/10096076.html

PageHelper分页失效原因：https://blog.csdn.net/qq_40995335/article/details/80845914

原文地址：https://www.cnblogs.com/IT_CH/p/12357660.html