Why PageHelper Pagination Breaks with List Operations and How to Fix It

Problem

When using PageHelper, pagination behaves incorrectly if the list is processed before or after the pagination is started.

1.1 Start pagination first, then manipulate the list

@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;
}

The total count equals the page size, indicating that pagination information is wrong.

1.2 Manipulate the list first, then start pagination

@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();
        // same field copying as above
        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;
}

Data is retrieved and the total count is correct, but the pagination does not take effect.

Root Cause

PageHelper's startPage only affects the SQL query that follows it.

When pagination is started before list processing, the subsequent conversion of the list to PageInfo does not influence the already‑executed query.

When pagination is started after the list has been built, there is no SQL statement for PageHelper to intercept, so pagination is ignored.

Solution

Operate on the list after PageHelper has produced the PageInfo object, then copy the pagination metadata back to a new PageInfo.

Extract the list from the source PageInfo.

Perform the required transformations on each element.

Copy pagination attributes to a new PageInfo and set the transformed list.

@Override
public PageInfo<HdQueryVo> getRecordsByView(int pageNo, int pageSize) {
    PageInfo<HdQueryVo> source = PageHelper.startPage(pageNo, pageSize).doSelectPageInfo(() -> {
        actionMapper.getActionByView();
    });
    // New PageInfo to hold transformed data
    PageInfo<HdQueryVo> target = new PageInfo<>();
    BeanUtils.copyProperties(source, target);
    List<HdQueryVo> transformed = source.getList().stream().map(hdQueryVo -> {
        HdQueryVo single = new HdQueryVo();
        single.setHdId(hdQueryVo.getHdId());
        single.setHdType(hdQueryVo.getHdType());
        single.setHdTitle(hdQueryVo.getHdTitle());
        single.setHdStartDate(hdQueryVo.getHdStartDate());
        single.setHdEndDate(hdQueryVo.getHdEndDate());
        single.setHdStatus(hdQueryVo.getHdStatus());
        single.setHdImage(hdQueryVo.getHdImage());
        single.setHdNumber(hdQueryVo.getHdNumber());
        single.setGmtCreate(hdQueryVo.getGmtCreate());
        single.setGmtModified(hdQueryVo.getGmtModified());
        single.setUserId(hdQueryVo.getUserId());
        if (hdQueryVo.getHdType().equals(0)) {
            single.setHdJoinUsers(onlineWorksMapper.getOnlineJoinUsers(hdQueryVo.getHdId()));
        } else {
            single.setHdJoinUsers(offlineUsersMapper.getOfflineJoinUsers(hdQueryVo.getHdId()));
        }
        return single;
    }).collect(Collectors.toList());
    target.setList(transformed);
    return target;
}

References: PageHelper official documentation and related articles on processing the result list after pagination.