PageHelper 分頁無效的坑。。。

1.1.PageHelper先開啟分頁，後對list資料操作

@Override
public PageInfo<HdQueryVo> getRecordsByView(int pageNo, int pageSize) {
    PageHelper.startPage(pageNo,pageSize);
    List<HdQueryVo> hdQueryVosByView = actionMapper.getActionByView();

    List<HdQueryVo> hdQueryVos = new ArrayList<>();

    for (HdQueryVo hdQueryVo : hdQueryVosByView) {
        HdQueryVo hdQueryVoSingle = new HdQueryVo();
        hdQueryVoSingle.setHdId(hdQueryVo.getHdId());
        hdQueryVoSingle.setHdType(hdQueryVo.getHdType());
        hdQueryVoSingle.setHdTitle(hdQueryVo.getHdTitle());
        hdQueryVoSingle.setHdStartDate(hdQueryVo.getHdStartDate());
        hdQueryVoSingle.setHdEndDate(hdQueryVo.getHdEndDate());
        hdQueryVoSingle.setHdStatus(hdQueryVo.getHdStatus());
        hdQueryVoSingle.setHdImage(hdQueryVo.getHdImage());
        hdQueryVoSingle.setHdNumber(hdQueryVo.getHdNumber());
        hdQueryVoSingle.setGmtCreate(hdQueryVo.getGmtCreate());
        hdQueryVoSingle.setGmtModified(hdQueryVo.getGmtModified());
        hdQueryVoSingle.setUserId(hdQueryVo.getUserId());
        if (hdQueryVo.getHdType().equals(0)) {
            hdQueryVoSingle.setHdJoinUsers(onlineWorksMapper.getOnlineJoinUsers(hdQueryVo.getHdId()));
        } else {
            hdQueryVoSingle.setHdJoinUsers(offlineUsersMapper.getOfflineJoinUsers(hdQueryVo.getHdId()));
        }
        hdQueryVos.add(hdQueryVoSingle);
    }
    PageInfo<HdQueryVo> pageViewInfo = new PageInfo<>(hdQueryVos);

    return pageViewInfo;
}      

可以分頁，但是資料量錯誤，total始終等于每頁資料量，即pageSize

@Override
public PageInfo<HdQueryVo> getRecordsByView(int pageNo, int pageSize) {

    List<HdQueryVo> hdQueryVosByView = actionMapper.getActionByView();

    List<HdQueryVo> hdQueryVos = new ArrayList<>();

    for (HdQueryVo hdQueryVo : hdQueryVosByView) {
        HdQueryVo hdQueryVoSingle = new HdQueryVo();
        hdQueryVoSingle.setHdId(hdQueryVo.getHdId());
        hdQueryVoSingle.setHdType(hdQueryVo.getHdType());
        hdQueryVoSingle.setHdTitle(hdQueryVo.getHdTitle());
        hdQueryVoSingle.setHdStartDate(hdQueryVo.getHdStartDate());
        hdQueryVoSingle.setHdEndDate(hdQueryVo.getHdEndDate());
        hdQueryVoSingle.setHdStatus(hdQueryVo.getHdStatus());
        hdQueryVoSingle.setHdImage(hdQueryVo.getHdImage());
        hdQueryVoSingle.setHdNumber(hdQueryVo.getHdNumber());
        hdQueryVoSingle.setGmtCreate(hdQueryVo.getGmtCreate());
        hdQueryVoSingle.setGmtModified(hdQueryVo.getGmtModified());
        hdQueryVoSingle.setUserId(hdQueryVo.getUserId());
        if (hdQueryVo.getHdType().equals(0)) {
            hdQueryVoSingle.setHdJoinUsers(onlineWorksMapper.getOnlineJoinUsers(hdQueryVo.getHdId()));
        } else {
            hdQueryVoSingle.setHdJoinUsers(offlineUsersMapper.getOfflineJoinUsers(hdQueryVo.getHdId()));
        }
        hdQueryVos.add(hdQueryVoSingle);
    }
    PageHelper.startPage(pageNo,pageSize);
    PageInfo<HdQueryVo> pageViewInfo = new PageInfo<>(hdQueryVos);

    return pageViewInfo;
}      

資料可以查詢出來，總數total也正确，但是分頁功能失效。（這份阿裡 Java 開發手冊可以關注公衆号Java核心技術，回複：手冊）

2.原因

PageHelper中startPage開啟分頁方法隻對後面的sql查詢起作用

1.1 錯誤原因是提前開啟分頁後，對list操作，即

PageInfo pageViewInfo = new PageInfo<>(hdQueryVos);

即sql語句沒有參與分頁查詢

3.解決方案

直接對分頁後的PageInfo對象中的資料進行操作

• 對list集合操作，先取出PageInfo裡的list集合資料，再對資料進行相關操作
• 将操作完後的list集合再次存到PageInfo裡，進行return

@Override
public PageInfo<HdQueryVo> getRecordsByView(int pageNo, int pageSize) {
    PageInfo<HdQueryVo> source = PageHelper.startPage(pageNo, pageSize).doSelectPageInfo(()->{
        actionMapper.getActionByView();
    });
    // 需要轉換的對象
    PageInfo<HdQueryVo> target = new PageInfo<>();
    // 複制分頁屬性
    BeanUtils.copyProperties(source, target);
    // 對查詢的list進行下一步操作，比如類型轉換後
    List<HdQueryVo> collect = source.getList().stream().collect(Collectors.toList());
    List<HdQueryVo> hdQueryVos = new ArrayList<>();

    for (HdQueryVo hdQueryVo : collect) {
        HdQueryVo hdQueryVoSingle = new HdQueryVo();
        hdQueryVoSingle.setHdId(hdQueryVo.getHdId());
        hdQueryVoSingle.setHdType(hdQueryVo.getHdType());
        hdQueryVoSingle.setHdTitle(hdQueryVo.getHdTitle());
        hdQueryVoSingle.setHdStartDate(hdQueryVo.getHdStartDate());
        hdQueryVoSingle.setHdEndDate(hdQueryVo.getHdEndDate());
        hdQueryVoSingle.setHdStatus(hdQueryVo.getHdStatus());
        hdQueryVoSingle.setHdImage(hdQueryVo.getHdImage());
        hdQueryVoSingle.setHdNumber(hdQueryVo.getHdNumber());
        hdQueryVoSingle.setGmtCreate(hdQueryVo.getGmtCreate());
        hdQueryVoSingle.setGmtModified(hdQueryVo.getGmtModified());
        hdQueryVoSingle.setUserId(hdQueryVo.getUserId());
        if (hdQueryVo.getHdType().equals(0)) {
            hdQueryVoSingle.setHdJoinUsers(onlineWorksMapper.getOnlineJoinUsers(hdQueryVo.getHdId()));
        } else {
            hdQueryVoSingle.setHdJoinUsers(offlineUsersMapper.getOfflineJoinUsers(hdQueryVo.getHdId()));
        }
        hdQueryVos.add(hdQueryVoSingle);
    }
    // 加工後的資料放入新的pageinfo
    target.setList(hdQueryVos);

    return target;
}