Java中PageHelper分页后对list操作导致分页无效

目录1.问题1.1.PageHelper先开启分页，后对list数据操作1.2.先对list数据进行操作，后开启分页2.原因3.解决方案

1.问题

阿里巴巴Java开发手册

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

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①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②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

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