PageHelper分页后，对list操作会导致分页无效的原因分析

矫情吗；* 2023-01-21 14:27 145阅读 0赞

## 你知道的越多，不知道的就越多，业余的像一棵小草！ ##

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 ##

![013bc836da55db9e07ed868b72bad369.png][]

#### 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

![23e2290b39f6337552ffdbfd4986039c.png][]

![e5c5dd7afadb282641b3a024ba4058df.png][]

#### 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。

![7c4ae37c9b24b2761968c7213e2d113a.png][]

![b26662e05a0bafe2b93078bbb346ac18.png][]

![8138aa6fdd61aadfa0f70abfb53cdc54.png][]

## 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);`

![e07132c1d5679385f6ed7322d485237b.png][]

/**
    * 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

![d0c603ed304c31f378ed47f2f97cdb64.png][]

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

![1c9f766748dcb7af731c4af04056e354.png][]

![15dd702aaf45ac75bc31edf464e0d93f.png][]

@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;
        }

![c935e344b5927ace8c6c8e1df0fc4820.png][] ![00d6eb13c9f5258dfd4aa7e66cdc0713.png][] ![bf3b642640701edbddfa789790f23d7a.png][]

#### 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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