PageHelper 分頁無效的坑。。。

1.問題

阿里巴巴Java開發手冊：

1.1.PageHelper先開啟分頁，后對list數據操作

@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，但是數據量錯誤，total始終等於每頁數據量，即pageSize

1.2.先對list數據進行操作，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;
}

數據可以查詢出來，總數total也正確，但是分頁功能失效。（這份阿里 Java 開發手冊可以關注公眾號Java核心技術，回復：手冊）

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轉為PageInfo對象，不影響前面分頁數據的操作

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對象中的數據進行操作

• 對list集合操作，先取出PageInfo里的list集合數據，再對數據進行相關操作
• 將操作完后的list集合再次存到PageInfo里，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<>();
    // 復制分頁屬性
    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);
    }
    // 加工后的數據放入新的pageinfo
    target.setList(hdQueryVos);

    return target;
}

參考資料：

https://apidoc.gitee.com/free/Mybatis_PageHelper/

https://blog.csdn.net/shanglezeichaun/article/details/105039460

原文鏈接：https://blog.csdn.net/qq_35080796/article/details/105508744

版權聲明：本文為CSDN博主「He Ain't Hero」的原創文章，遵循CC 4.0 BY-SA版權協議，轉載請附上原文出處鏈接及本聲明。

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