PageHelper 分页无效的坑。。。

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。（这份阿里 Java 开发手册可以关注公众号Java核心技术，回复：手册）

2.原因

PageHelper中startPage开启分页方法只对后面的sql查询起作用。另外，关注公众号Java技术栈，在后台回复：面试，可以获取我整理的 Java 系列面试题和答案，非常齐全。

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。Spring Boot 学习笔记，分享给你。

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;
}

原文链接：https://blog.csdn.net/qq_35080796/article/details/105508744

参考资料：

版权声明：本文为CSDN博主「He Ain't Hero」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。

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