PageHelper分页后，对list操作会导致分页无效

我就是我 2022-10-21 12:53 6阅读 0赞

![8a5bf6409bf5a18a611111004088f4cc.png][]

*来源：blog.csdn.net/qq\_35080796/article/details/105508744*

## 1.问题 ##

![c45ec95fe81d4a368418a3ba73eda903.png][] 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

![2a89af851bb51f6f2e3c98707e7603e6.png][] ![bc31c4f1512b3215a0aaf577eb39b78b.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。（搜索公众号Java知音，回复“2021”，送你一份Java面试题宝典）

![ed76b61a47ec5b279a20d3426e90f869.png][] ![47dfd0568f883c8103c2c2ec6ea67dc1.png][] ![371ebd545d381ea1dc217ef764bf19fa.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);`

![a6d06a47be55deb6398c9085f3cb095f.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

![17ff37834d8464247ea3ad732e979c7e.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

![66fba23512be9683350e2bc4064103f8.png][] ![f39e8a69be98133c7e1c0e95feec51ea.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;
        }

![78bb66e52585f670b234f4dc155fcb9a.png][] ![08a467c911826a987355d75004299154.png][] ![9e7439eca6286ac83720ee4710f5437d.png][]

#### 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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