PageHelper分页后，对list操作会导致分页无效 |
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PageHelper分页后，对list操作会导致分页无效
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来源：blog.csdn.net/qq_35080796/article/details/105508744 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 getRecordsByView(int pageNo, int pageSize) {
PageHelper.startPage(pageNo,pageSize);
List hdQueryVosByView = actionMapper.getActionByView();
List 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 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 getRecordsByView(int pageNo, int pageSize) {
List hdQueryVosByView = actionMapper.getActionByView();
List 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 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 getRecordsByView(int pageNo, int pageSize) {
PageInfo source = PageHelper.startPage(pageNo, pageSize).doSelectPageInfo(()->{
actionMapper.getActionByView();
});
// 需要转换的对象
PageInfo 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 collect = source.getList().stream().collect(Collectors.toList());
List 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 推荐好文 强大，10k+点赞的 SpringBoot 后台管理系统竟然出了详细教程！ 分享一套基于SpringBoot和Vue的企业级中后台开源项目，代码很规范！ 能挣钱的，开源 SpringBoot 商城系统，功能超全，超漂亮 |
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