[Java][MyBatis]物理分页实现
Mybatis3.0出来已有段时间了，其实自己挺喜欢这样的一个持久化框架的，因为它简单实用，学习成本低。Mybatis3.0在整体结构上和ibatis2.X差不多，改进特性如下：
1.解析xml引进了Xpath，不像ibatis2.x那样业余
2.动态sql用OGNL解析
3.加入注解配置sql，感觉没什么特别大的用途，我更喜欢xml方式，代码和配置分离，这也是ibatis的初衷
4.加强了缓存这块的功能。Mybatis3.0把缓存模块分得更细，分为“持久实现（prepetual）”和“资源回收策略实现（eviction）”，更好的对缓存功能进行自己组合和扩展
5.终于加入的plugin功能，就像struts一样，这样就可以很好的扩展内部的Executor,，StatementHandler….等内部对象功能。
MyBatis的分页功能还是基于内存分页（查找出所有记录再取出偏移量的记录，如果jdbc驱支持absolute定位或者rs.next()到指定偏移位置），其实这样的分页实现基本没用，特别是大量数据情况下。不过我们可以通过plugin功能来扩展MyBatis的分页功能、实现物理分页。具体做法如下：
1、编写分页插件类：
/**
* 版权所有：华信软件
* 项目名称:ACWS框架类
* 创建者: Wangdf
* 创建日期: 2014-4-2
* 文件说明: ACWS框架分页接口类
*/
package framework.core.interceptor;
import java.sql.Connection;
import java.util.Map;
import java.util.Properties;
import org.apache.commons.collections.MapUtils;
import org.apache.ibatis.executor.statement.StatementHandler;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.plugin.Interceptor;
import org.apache.ibatis.plugin.Intercepts;
import org.apache.ibatis.plugin.Invocation;
import org.apache.ibatis.plugin.Plugin;
import org.apache.ibatis.plugin.Signature;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.reflection.factory.DefaultObjectFactory;
import org.apache.ibatis.reflection.factory.ObjectFactory;
import org.apache.ibatis.reflection.wrapper.DefaultObjectWrapperFactory;
import org.apache.ibatis.reflection.wrapper.ObjectWrapperFactory;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.RowBounds;
import framework.core.util.DBUtil;
/**
* ACWS框架分页接口类
* @author Wangdf
*
*/
@Intercepts({ @Signature(type = StatementHandler.class, method = "prepare", args = { Connection.class }) })
public class PaginationInterceptor implements Interceptor {
private static final ObjectFactory DEFAULT_OBJECT_FACTORY = new DefaultObjectFactory();
private static final ObjectWrapperFactory DEFAULT_OBJECT_WRAPPER_FACTORY = new DefaultObjectWrapperFactory();
@Override
public Object intercept(Invocation invocation) throws Throwable {
StatementHandler statementHandler = (StatementHandler) invocation.getTarget();
MetaObject metaStatementHandler = MetaObject.forObject(statementHandler, DEFAULT_OBJECT_FACTORY,
DEFAULT_OBJECT_WRAPPER_FACTORY);
RowBounds rowBounds = (RowBounds) metaStatementHandler.getValue("delegate.rowBounds");
if (rowBounds == null || rowBounds == RowBounds.DEFAULT) {
return invocation.proceed();
}
// 分离代理对象链(由于目标类可能被多个拦截器拦截，从而形成多次代理，通过下面的两次循环可以分离出最原始的的目标类)
while (metaStatementHandler.hasGetter("h")) {
Object object = metaStatementHandler.getValue("h");
metaStatementHandler = MetaObject.forObject(object, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);
}
// 分离最后一个代理对象的目标类
while (metaStatementHandler.hasGetter("target")) {
Object object = metaStatementHandler.getValue("target");
metaStatementHandler = MetaObject.forObject(object, DEFAULT_OBJECT_FACTORY, DEFAULT_OBJECT_WRAPPER_FACTORY);
}
BoundSql boundSql = (BoundSql) metaStatementHandler.getValue("delegate.boundSql");
Object params = boundSql.getParameterObject();
if(params instanceof Map){
@SuppressWarnings("unchecked")
Map<String, Object> paramsMap = (Map<String, Object>)params;
MapUtils.safeAddToMap(paramsMap, "STARTROW", String.valueOf(rowBounds.getOffset()));
MapUtils.safeAddToMap(paramsMap, "ENDROW", String.valueOf(rowBounds.getLimit()));
}
String sql = boundSql.getSql();
StringBuffer sbSql = new StringBuffer();
// 重写sql
Configuration configuration = (Configuration)metaStatementHandler.getValue("delegate.configuration");
DBUtil dbUtil = new DBUtil(configuration);
if(dbUtil.isMySQL()){
sbSql.append(sql).append(" LIMIT ").append(rowBounds.getOffset()-1).append(", ").append(rowBounds.getLimit()-1);
metaStatementHandler.setValue("delegate.boundSql.sql", sbSql.toString());
// 采用物理分页后，就不需要mybatis的内存分页了，所以重置下面的两个参数
metaStatementHandler.setValue("delegate.rowBounds.offset", RowBounds.NO_ROW_OFFSET);
metaStatementHandler.setValue("delegate.rowBounds.limit", RowBounds.NO_ROW_LIMIT);
} else if(dbUtil.isOracle()){
sbSql.append("SELECT * ");
sbSql.append(" FROM (SELECT ROWNUM RN, NOPAGESQL.* ");
sbSql.append(" FROM (").append(sql).append(") NOPAGESQL ");
sbSql.append(" WHERE ROWNUM <= ").append(rowBounds.getLimit()).append(")");
sbSql.append(" WHERE RN >= ").append(rowBounds.getOffset());
metaStatementHandler.setValue("delegate.boundSql.sql", sbSql.toString());
// 采用物理分页后，就不需要mybatis的内存分页了，所以重置下面的两个参数
metaStatementHandler.setValue("delegate.rowBounds.offset", RowBounds.NO_ROW_OFFSET);
metaStatementHandler.setValue("delegate.rowBounds.limit", RowBounds.NO_ROW_LIMIT);
} else {
}
// 将执行权交给下一个拦截器
return invocation.proceed();
}
@Override
public Object plugin(Object target) {
// 当目标类是StatementHandler类型时，才包装目标类，否者直接返回目标本身,减少目标被代理的次数
if (target instanceof StatementHandler) {
return Plugin.wrap(target, this);
} else {
return target;
}
}
@Override
public void setProperties(Properties properties) {
// TODO Auto-generated method stub
}
}
2、配置读取类
/**
* 版权所有：华信软件
* 项目名称:ACWS框架类
* 创建者: Wangdf
* 创建日期: 2014-4-2
* 文件说明: ACWS框架数据库相关工具类
*/
package framework.core.util;
import org.apache.commons.lang.StringUtils;
import org.apache.ibatis.session.Configuration;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* ACWS框架数据库相关工具类
* @author Wangdf
*/
public class DBUtil {
private static final String DBTYPE_MYSQL = "MySQL";//支持的类型：MySQL,Oracle
private static final String DBTYPE_ORACLE = "Oracle";//支持的类型：MySQL,Oracle
private static Logger logger = LoggerFactory.getLogger(DBUtil.class);
private Configuration configuration = null;
private String dbType = "";
private String defaultDateFormat = "";
public DBUtil(Configuration configuration){
if(configuration == null){
logger.error("系统启动失败：MyBatis Configuration 对象为空！");
throw new IllegalArgumentException("系统启动失败：MyBatis Configuration 对象为空！");
}
this.configuration = configuration;
this.dbType = this.configuration.getVariables().getProperty("dbtype");
if(StringUtils.isBlank(dbType)){
logger.error("数据库类型没有配置！");
} else {
logger.info("数据库类型为："+dbType);
}
this.defaultDateFormat = this.configuration.getVariables().getProperty("defaultDateFormat");
if(StringUtils.isBlank(this.defaultDateFormat)){
this.defaultDateFormat="yyyy-MM-dd";
logger.info("数据库日期默认格式字符串没有指定！系统默认为：yyyy-MM-dd");
} else {
logger.info("数据库日期默认格式字符串："+this.defaultDateFormat);
}
}
/**
* 判断是否是Oracle数据库
* @return
* @author wangdf
*/
public boolean isOracle(){
return DBTYPE_ORACLE.equals(this.dbType);
}
/**
* 判断是否是MySQL数据库
* @return
* @author wangdf
*/
public boolean isMySQL(){
return DBTYPE_MYSQL.equals(this.dbType);
}
/**
* 取��数据库类型
* @return
* @author wangdf
*/
public String getDbType(){
return this.dbType;
}
/**
* 取得默认日期格式
* @return
* @author wangdf
*/
public String getDefaultDateFormat(){
return this.defaultDateFormat;
}
}
3、在mybatis全局配置文件设置
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<properties>
<property name="dbtype" value="MySQL"/><!-- 数据库类型：MySQL、Oracle -->
<property name="defaultDateFormat" value="yyyy-MM-dd"/>
</properties>
<settings>
<setting name="cacheEnabled" value="true" />
<setting name="lazyLoadingEnabled" value="true" />
<setting name="multipleResultSetsEnabled" value="true" />
<setting name="useColumnLabel" value="true" />
<setting name="defaultExecutorType" value="REUSE" />
<setting name="defaultStatementTimeout" value="25000" />
</settings>
<plugins>
<plugin interceptor="framework.core.interceptor.PaginationInterceptor"></plugin>
</plugins>
</configuration>