PageHelper分頁后，對list操作會導(dǎo)致分頁無效的原因分析

1.問題

1.1.PageHelper先開啟分頁，后對list數(shù)據(jù)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，但是數(shù)據(jù)量錯誤，total始終等于每頁數(shù)據(jù)量，即pageSize

1.2.先對list數(shù)據(jù)進(jìn)行操作，后開啟分頁

@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;
    }

數(shù)據(jù)可以查詢出來，總數(shù)total也正確，但是分頁功能失效。

2.原因

PageHelper中startPage開啟分頁方法只對后面的sql查詢起作用

1.1 錯誤原因是提前開啟分頁后，對list操作，即PageInfo pageViewInfo = new PageInfo<>(hdQueryVos);

/**
* 包裝Page對象
*
* @param list
*/
public PageInfo(List list) {
this(list, 8);
}

只是把list轉(zhuǎn)為PageInfo對象，不影響前面分頁數(shù)據(jù)的操作

1.2 錯誤原因是先對list操作后，開啟翻頁后沒有sql語句

即sql語句沒有參與分頁查詢

3.解決方案

直接對分頁后的PageInfo對象中的數(shù)據(jù)進(jìn)行操作

• 對list集合操作，先取出PageInfo里的list集合數(shù)據(jù)，再對數(shù)據(jù)進(jìn)行相關(guān)操作
• 將操作完后的list集合再次存到PageInfo里，進(jìn)行return

@Override
    public PageInfo<HdQueryVo> getRecordsByView(int pageNo, int pageSize) {
        PageInfo<HdQueryVo> source = PageHelper.startPage(pageNo, pageSize).doSelectPageInfo(()->{
            actionMapper.getActionByView();
        });
        // 需要轉(zhuǎn)換的對象
        PageInfo<HdQueryVo> target = new PageInfo<>();
        // 復(fù)制分頁屬性
        BeanUtils.copyProperties(source, target);
        // 對查詢的list進(jìn)行下一步操作，比如類型轉(zhuǎn)換后
        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);
        }
        // 加工后的數(shù)據(jù)放入新的pageinfo
        target.setList(hdQueryVos);

        return target;
    }

問題看起來不復(fù)雜，但初級和中級程序員遇到后基本上都素手無策。不會看源碼，不知道鉆研，永遠(yuǎn)都只停留在CRUD上，最多也就算是入了個門。

參考資料：

• PageHelper官方文檔：apidoc.gitee.com/free/Mybatis_PageHelper
• PageHelper分頁查詢結(jié)果后再對數(shù)據(jù)List操作的方法