pagehelper分页原理如何实现的?

1.pagehelper原理

PageHelper做的是什么呢?它封装了分页的后台部分,说得更简单点,就是你不需要每个POJO类的增删改查里都包括那两个方法了,它帮你做了。你只需要有一个selectAll的方法,它会根据你使用的数据库来将你selectAll的sql改装成一个分页查询的sql,并顺带生成一个查询总数的sql。

1.1如何进行拦截的?

通常我们使用这个插件的 ,配置一个拦截器“com.github.pagehelper.PageInterceptor”,让我们看看这个拦截器里面做了什么操作?

/*
* The MIT License (MIT)
*
* Copyright (c) 2014-2017 abel533@gmail.com
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the “Software”), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
*/

package com.github.pagehelper;

import com.github.pagehelper.cache.Cache;
import com.github.pagehelper.cache.CacheFactory;
import com.github.pagehelper.util.MSUtils;
import com.github.pagehelper.util.StringUtil;
import org.apache.ibatis.cache.CacheKey;
import org.apache.ibatis.executor.Executor;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.plugin.*;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;

import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Properties;

/**
* Mybatis – 通用分页拦截器<br/>
* 项目地址 : http://git.oschina.net/free/Mybatis_PageHelper
*
* @author liuzh/abel533/isea533
* @version 5.0.0
*/
@SuppressWarnings({“rawtypes”, “unchecked”})
@Intercepts(
{
@Signature(type = Executor.class, method = “query”, args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),
@Signature(type = Executor.class, method = “query”, args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class}),
}
)
public class PageInterceptor implements Interceptor {
//缓存count查询的ms
protected Cache<CacheKey, MappedStatement> msCountMap = null;
private Dialect dialect;
private String default_dialect_class = “com.github.pagehelper.PageHelper”;
private Field additionalParametersField;

@Override
public Object intercept(Invocation invocation) throws Throwable {
try {
Object[] args = invocation.getArgs();
MappedStatement ms = (MappedStatement) args[0];
Object parameter = args[1];
RowBounds rowBounds = (RowBounds) args[2];
ResultHandler resultHandler = (ResultHandler) args[3];
Executor executor = (Executor) invocation.getTarget();
CacheKey cacheKey;
BoundSql boundSql;
//由于逻辑关系,只会进入一次
if(args.length == 4){
//4 个参数时
boundSql = ms.getBoundSql(parameter);
cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);
} else {
//6 个参数时
cacheKey = (CacheKey) args[4];
boundSql = (BoundSql) args[5];
}
List resultList;
//调用方法判断是否需要进行分页,如果不需要,直接返回结果
if (!dialect.skip(ms, parameter, rowBounds)) {
//反射获取动态参数
Map<String, Object> additionalParameters = (Map<String, Object>) additionalParametersField.get(boundSql);
//判断是否需要进行 count 查询
if (dialect.beforeCount(ms, parameter, rowBounds)) {
//创建 count 查询的缓存 key
CacheKey countKey = executor.createCacheKey(ms, parameter, RowBounds.DEFAULT, boundSql);
countKey.update(“_Count”);
MappedStatement countMs = msCountMap.get(countKey);
if (countMs == null) {
//根据当前的 ms 创建一个返回值为 Long 类型的 ms
countMs = MSUtils.newCountMappedStatement(ms);
msCountMap.put(countKey, countMs);
}
//调用方言获取 count sql
String countSql = dialect.getCountSql(ms, boundSql, parameter, rowBounds, countKey);
BoundSql countBoundSql = new BoundSql(ms.getConfiguration(), countSql, boundSql.getParameterMappings(), parameter);
//当使用动态 SQL 时,可能会产生临时的参数,这些参数需要手动设置到新的 BoundSql 中
for (String key : additionalParameters.keySet()) {
countBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
}
//执行 count 查询
Object countResultList = executor.query(countMs, parameter, RowBounds.DEFAULT, resultHandler, countKey, countBoundSql);
Long count = (Long) ((List) countResultList).get(0);
//处理查询总数
//返回 true 时继续分页查询,false 时直接返回
if (!dialect.afterCount(count, parameter, rowBounds)) {
//当查询总数为 0 时,直接返回空的结果
return dialect.afterPage(new ArrayList(), parameter, rowBounds);
}
}
//判断是否需要进行分页查询
if (dialect.beforePage(ms, parameter, rowBounds)) {
//生成分页的缓存 key
CacheKey pageKey = cacheKey;
//处理参数对象
parameter = dialect.processParameterObject(ms, parameter, boundSql, pageKey);
//调用方言获取分页 sql
String pageSql = dialect.getPageSql(ms, boundSql, parameter, rowBounds, pageKey);
BoundSql pageBoundSql = new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);
//设置动态参数
for (String key : additionalParameters.keySet()) {
pageBoundSql.setAdditionalParameter(key, additionalParameters.get(key));
}
//执行分页查询
resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageKey, pageBoundSql);
} else {
//不执行分页的情况下,也不执行内存分页
resultList = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, cacheKey, boundSql);
}
} else {
//rowBounds用参数值,不使用分页插件处理时,仍然支持默认的内存分页
resultList = executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);
}
return dialect.afterPage(resultList, parameter, rowBounds);
} finally {
dialect.afterAll();
}
}

@Override
public Object plugin(Object target) {
return Plugin.wrap(target, this);
}

@Override
public void setProperties(Properties properties) {
//缓存 count ms
msCountMap = CacheFactory.createCache(properties.getProperty(“msCountCache”), “ms”, properties);
String dialectClass = properties.getProperty(“dialect”);
if (StringUtil.isEmpty(dialectClass)) {
dialectClass = default_dialect_class;
}
try {
Class<?> aClass = Class.forName(dialectClass);
dialect = (Dialect) aClass.newInstance();
} catch (Exception e) {
throw new PageException(e);
}
dialect.setProperties(properties);
try {
//反射获取 BoundSql 中的 additionalParameters 属性
additionalParametersField = BoundSql.class.getDeclaredField(“additionalParameters”);
additionalParametersField.setAccessible(true);
} catch (NoSuchFieldException e) {
throw new PageException(e);
}
}

}

通过源码发现:拦截器会根据参数来判断使用需要进行物理分页,不使用分页插件处理时,仍然支持默认的内存分页。需要分页时候,根据条件判断是否需要count  和做limit处理

1.2如何处理分页的逻辑?

对应不同的数据库有不同的方言,如常见的Mysql,分页关键字是limit,它就是这样组装你的sql。

/*
* The MIT License (MIT)
*
* Copyright (c) 2014-2017 abel533@gmail.com
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the “Software”), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
*/

package com.github.pagehelper.dialect.rowbounds;

import com.github.pagehelper.dialect.AbstractRowBoundsDialect;
import org.apache.ibatis.cache.CacheKey;
import org.apache.ibatis.session.RowBounds;

/**
* mysql 基于 RowBounds 的分页
*
* @author liuzh
*/
public class MySqlRowBoundsDialect extends AbstractRowBoundsDialect {

@Override
public String getPageSql(String sql, RowBounds rowBounds, CacheKey pageKey) {
StringBuilder sqlBuilder = new StringBuilder(sql.length() + 14);
sqlBuilder.append(sql);
if (rowBounds.getOffset() == 0) {
sqlBuilder.append(” LIMIT “);
sqlBuilder.append(rowBounds.getLimit());
} else {
sqlBuilder.append(” LIMIT “);
sqlBuilder.append(rowBounds.getOffset());
sqlBuilder.append(“,”);
sqlBuilder.append(rowBounds.getLimit());
pageKey.update(rowBounds.getOffset());
}
pageKey.update(rowBounds.getLimit());
return sqlBuilder.toString();
}

}

至此,总结PageHelper:

封装分页sql,使我们不需要每个地方都去写分页的查询语句;同时,使我们select的sql语句向下兼容,换了数据库也不需要更改sql代码;

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