PageHelper分页后，对list操作会导致分页无效

1.问题

1.1.PageHelper先开启分页，后对list数据操作

@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;
    }

可以分页，但是数据量错误，total始终等于每页数据量，即pageSize

1.2.先对list数据进行操作，后开启分页

@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;
    }

数据可以查询出来，总数total也正确，但是分页功能失效。（搜索公众号Java知音，回复“2021”，送你一份Java面试题宝典）

2.原因

PageHelper中startPage开启分页方法只对后面的sql查询起作用

1.1 错误原因是提前开启分页后，对list操作，即PageInfo pageViewInfo = new PageInfo<>(hdQueryVos);

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

只是把list转为PageInfo对象，不影响前面分页数据的操作

1.2 错误原因是先对list操作后，开启翻页后没有sql语句

即sql语句没有参与分页查询

3.解决方案

直接对分页后的PageInfo对象中的数据进行操作

• 对list集合操作，先取出PageInfo里的list集合数据，再对数据进行相关操作
• 将操作完后的list集合再次存到PageInfo里，进行return

@Override
    public PageInfo<HdQueryVo> getRecordsByView(int pageNo, int pageSize) {
        PageInfo<HdQueryVo> source = PageHelper.startPage(pageNo, pageSize).doSelectPageInfo(()->{
            actionMapper.getActionByView();
        });
        // 需要转换的对象
        PageInfo<HdQueryVo> target = new PageInfo<>();
        // 复制分页属性
        BeanUtils.copyProperties(source, target);
        // 对查询的list进行下一步操作，比如类型转换后
        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);
        }
        // 加工后的数据放入新的pageinfo
        target.setList(hdQueryVos);

        return target;
    }

参考资料：

• PageHelper官方文档
• PageHelper分页查询结果后再对数据List操作的方法