PageHelper分页后，对list操作会导致分页无效的原因分析
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成功路上并不拥挤，因为坚持的人不多。
编辑：业余草
blog.csdn.net/qq_35080796
推荐：https://www.xttblog.com/?p=5184
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。
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;
}
问题看起来不复杂，但初级和中级程序员遇到后基本上都素手无策。不会看源码，不知道钻研，永远都只停留在CRUD上，最多也就算是入了个门。
参考资料：
PageHelper官方文档：apidoc.gitee.com/free/Mybatis_PageHelper PageHelper分页查询结果后再对数据List操作的方法
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