前言：
项目中数据分页是一个很常见的需求，目前大部分项目都会使用pagehelper进行分页，那么在使用的过程中是否考虑如下问题？
一、基本集成
引入jar包
<dependency>
<groupId>com.github.pagehelper</groupId>
<artifactId>pagehelper-spring-boot-starter</artifactId>
<version>${pagehelper.version}</version>
</dependency>Yml配置文件中添加相关配置
pagehelper:
helperDialect: mysql
reasonable: true
supportMethodsArguments: true
params: count=countSql封装相关分页方法
分页参数类
public class PageParam<T> implements Serializable
{
private static final long serialVersionUID = -7916211163897873899L;
private int pageNum=1;
private int pageSize=10;
//条件参数
private T param;
//排序字段
private String orderBy;
public int getPageSize()
{
return pageSize;
}
public void setPageSize(int pageSize)
{
this.pageSize = pageSize;
}
public int getPageNum()
{
return pageNum;
}
public void setPageNum(int pageNum)
{
this.pageNum = pageNum;
}
public T getParam()
{
return param;
}
public void setParam(T param)
{
this.param = param;
}
public String getOrderBy()
{
return orderBy;
}
public void setOrderBy(String orderBy)
{
//需要注意sql注入
this.orderBy = orderBy;
}
}
分页结果类
public class PagedList<T> implements Serializable
{
private static final long serialVersionUID = -1253790062865437768L;
private int pageNum = 1;
private List<T> data = null;
private int pageCount = 0;
private int recordCount = -1;
private int pagingType = 0;
private int pageSize;
private String orderBy;
public int getPageSize()
{
return pageSize;
}
public void setPageSize(int pageSize)
{
if (pageSize <= 0)
{
return;
}
this.pageSize = pageSize;
}
public int getPageCount()
{
return pageCount;
}
public void setPageCount(int pageCount)
{
if (pageCount <= 0)
{
return;
}
this.pageCount = pageCount;
}
public int getRecordCount()
{
return recordCount;
}
public void setRecordCount(int recordCount)
{
this.recordCount = recordCount;
calcPageCount();
}
private void calcPageCount()
{
if (this.recordCount < 0)
{
return;
}
int tmp = this.recordCount % getPageSize();
this.pageCount = (tmp == 0 ? (this.recordCount / getPageSize())
: (this.recordCount / getPageSize() + 1));
if (this.pageNum > this.pageCount && this.pageCount != 0)
{
this.pageNum = this.pageCount;
}
this.pageNum = this.pageCount;
}
public void setData(List<T> data)
{
this.data = data;
if (ObjectUtil.isNotEmpty(data) && this.recordCount == -1)
{
this.recordCount = data.size();
}
}
public List<T> getData()
{
return data;
}
public int getPagingType()
{
return pagingType;
}
public void setPagingType(int pagingType)
{
this.pagingType = pagingType;
}
public void setOrderBy(String orderBy)
{
this.orderBy = orderBy;
}
public int getPageNum()
{
return pageNum;
}
public void setPageNum(int pageNum)
{
this.pageNum = pageNum;
}
public String getOrderBy()
{
return orderBy;
}
}
分页工具类
public class PageUtils implements Serializable
{
private static final long serialVersionUID = 377943433889798799L;
public static <T> PagedList<T> exportPagedList(PageParam<T> pageParam)
{
PagedList<T> pl = new PagedList<T>();
// pagesize
int pageSize = pageParam.getPageSize();
if (pageSize <= 0)
{
pageSize = 10;
}
else
{
pl.setPageSize(pageSize);
}
int pageNum = pageParam.getPageNum();
pl.setPageNum(pageNum);
String orderBy= pageParam.getOrderBy();
if(StringUtil.isNotEmpty(orderBy))
{
//防止sql注入
String orderBySql=SQLFilter.sqlInject(orderBy);
pl.setOrderBy(orderBySql);
}
return pl;
}
public static <T>PagedList<T> toPageList(PageInfo<T> spage)
{
PagedList<T> pagedList = new PagedList<T>();
pagedList.setPageSize((int) spage.getPageSize());
pagedList.setPageNum((int) spage.getPageNum());
pagedList.setRecordCount((int) spage.getTotal());
pagedList.setData(spage.getList());
pagedList.setPageCount((int) spage.getPages());
return pagedList;
}
}示例代码
@PostMapping("getPageList")
public Result getPageList(@RequestBody PageParam<TUser> pageParm)
{
//接收参数
PagedList<TUser> pl =PageUtils.exportPagedList(pageParm);
return Result.success(userService.queryPageList(pl, pageParm.getParam()));
}
public PagedList<TUser> queryPageList(PagedList<TUser> page,TUser user)
{
PageInfo<TUser> pageInfo= PageHelper.startPage(page).doSelectPageInfo(()-> list(user));
//转换结果
return PageUtils.toPageList(pageInfo);
}前段传入参数
{
"pageSize":10,
"pageNum":"1",
//查询条件
"param":{
"name":"张三210001"
},
//排序字段
"orderBy":"age desc"
}执行结果
2022-04-15 22:26:39.914 [http-nio-9090-exec-9] DEBUG [613920d89eb54bfd8601c93ec8572dcf] c.s.f.m.UserMapper.queryPageList - ==> Preparing: SELECT * FROM t_user u LEFT JOIN t_user_role ur ON ur.userOid = u.oid WHERE name = ? order by age desc LIMIT ?
2022-04-15 22:26:39.919 [http-nio-9090-exec-9] DEBUG [613920d89eb54bfd8601c93ec8572dcf] c.s.f.m.UserMapper.queryPageList - ==> Parameters: 张三210001(String), 10(Integer)
2022-04-15 22:26:40.267 [http-nio-9090-exec-9] DEBUG [613920d89eb54bfd8601c93ec8572dcf] c.s.f.m.UserMapper.queryPageList - <== Total: 1
基础的分页查询已经发完成了，下面解答上面的问题的方法
二、分页中的排序字段如何防止SQL注入问题
对于前段传入的排序字段，我们需要进行SQL过滤处理，关于这个问题其实在上述的分页封装类中已经进行了解决
示例代码：
public class SQLFilter
{
public static String sqlInject(String str)
{
if (StringUtil.isBlank(str))
{
return null;
}
// 去掉'|"|;|\字符
str = StringUtil.replace(str, "'", "");
str = StringUtil.replace(str, "\"", "");
str = StringUtil.replace(str, ";", "");
str = StringUtil.replace(str, "\\", "");
// 转换成小写
str = str.toLowerCase();
// 非法字符
String[] keywords = { "master", "truncate", "insert", "select",
"delete", "update", "declare", "alert", "drop" };
// 判断是否包含非法字符
for (String keyword : keywords)
{
if (str.indexOf(keyword) != -1)
{
throw new SysException("包含非法字符");
}
}
return str;
}
}
三、复杂的SQL分页语句
复杂的SQL分页语句，需要自定义SQL的count语句如何实现呢？
PageHelper实现分页，默认是查询自定义的count语句是否存在，如果存在就用自定义的语句，否则就在外层包装查询的语句，而自定义count语句只需要在在查询语句名称后面添加_COUNT即可。例如
查询集合的语句名称为queryPageList，那么查询count的语句为queryPageList_COUNT，返回Long类型即可。
<select id="queryPageList_COUNT" resultType="java.lang.Long">
select count(1) from t_user u
left join t_user_role ur on ur.userOid=u.oid
<where>
<if test="name != null">name=#{name}</if>
</where>
</select>四、分页失效的常见的场景有哪些？
1.pageHelper分页查询有个特殊的要求，查询下sql语句一定要紧跟在分页查询的后面，否则分页查询会失效。之前采用的如下写法容易失效，建议采用java8的写法
PageHelper.startPage(pagedList.getPageNum(),pagedList.getPageSize());
//紧跟分页查询后面
List<TUser> list = list(user);
PageInfo<TUser> pageInfo =new PageInfo<>(list);
return PageUtils.toPageList(pageInfo);2.注意pagehelper的reasonable 默认为false，遇到查询页数大于总页数时，出现分页失败
pagehelper的reasonable 默认为false，遇到查询页数大于总页数时，查询为空；当reasonable设置为true时，遇到查询页数大于总页数时，查询最后一页数据；
3.PageHelper先开启分页，后对list数据操作将会导致分页错误
示例代码：
public PageInfo<TUserVO> getUserPageList(int pageNum, int pageSize) {
PageHelper.startPage(pageNum,pageSize);
List<TUserVO> tUserVOsByView = userMapper.getUserList();
List<TUserVO> TUserVOs = new ArrayList<>();
for (TUserVO TUserVO : tUserVOsByView) {
TUserVO TUserVOSingle = new TUserVO();
TUserVOSingle.setHdId(TUserVO.getHdId());
TUserVOs.add(TUserVOSingle);
}
PageInfo<TUserVO> pageViewInfo = new PageInfo<>(TUserVOs);
return pageViewInfo;
}4.PageHelper先对list数据操作，后开启分页，将会导致分页失效
示例代码：
public PageInfo<TUserVO> getUserPageList(int pageNum, int pageSize) {
List<TUserVO> tUserVOsByView = userMapper.getUserList();
List<TUserVO> TUserVOs = new ArrayList<>();
for (TUserVO TUserVO : tUserVOsByView) {
TUserVO TUserVOSingle = new TUserVO();
TUserVOSingle.setHdId(TUserVO.getHdId());
}
PageHelper.startPage(pageNo,pageSize);
PageInfo<TUserVO> pageViewInfo = new PageInfo<>(TUserVOs);
return pageViewInfo;
}
大家需要注意下，抽时间可以去验证下结果。
五、大表数据PageHelper分页性能如何
PageHelper 对于大表查询数据量越大，性能越差，这是因为PageHelper分页是自动在sql语句后面拼接limit没有进行相关的优化，一旦数据大，性能就比较慢。
例如：
优化前SQL语句：
SELECT d.* FROM tag_detail d LIMIT 10000000,10 查询的时间大概需要10秒左右，执行速度比较慢。
优化后SQL语句：
SELECT d.* FROM tag_detail d
INNER JOIN
(SELECT oid FROM tag_detail LIMIT 10000000,10) t
ON d.oid= t.oid;子查询先通过分页查询主键字段，然后进行关联查询，经过优化后，查询时间大概为1秒左右。性能大幅度提升。
总结：
本文讲解了PageHelper的基本的使用和相关的问题，这些都是我从实际的项目中总结出来的问题以及相关的解决方案，大家在使用的时候要特别注意，不要放同样的错误。
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