Mybatis使用篇之十二：实现物理分页

hyadijxp

　　Mybatis的自带分页方法只是逻辑分页，如果数据量很大，内存会溢出，不知道为什么开源组织不在里面实现类似Hibernate的物理分页处理方法。在不改动Mybatis源代码的情况下，怎么使Mybatis支持物理分页呢？下面我们来看看。
　　（1）新建一个Java类Dialect.java，该类的内容如下：

package org.mybatis.extend.interceptor;
public abstract class Dialect {
public static enum Type{
MYSQL,
ORACLE
}
public abstract String getLimitString(String sql, int skipResults, int maxResults);
}
　　（2）新建一个Java类OracleDialect.java，该类继承Dialect 类，具体的内容如下：

package org.mybatis.extend.interceptor;
public class OracleDialect extends Dialect{
/* (non-Javadoc)
* @see org.mybatis.extend.interceptor.IDialect#getLimitString(java.lang.String, int, int)
*/
@Override
public String getLimitString(String sql, int offset, int limit) {
sql = sql.trim();
StringBuffer pagingSelect = new StringBuffer(sql.length() + 100);
pagingSelect.append("select * from ( select row_.*, rownum rownum_ from ( ");
pagingSelect.append(sql);
pagingSelect.append(" ) row_ ) where rownum_ > ").append(offset).append(" and rownum_ <= ").append(offset + limit);
return pagingSelect.toString();
}
}
　　（3）新建一个Mybaits的拦截器PaginationInterceptor.java，实现Interceptor接口，该类的内容如下：

package org.mybatis.extend.interceptor;
import java.sql.Connection;
import java.util.Map;
import java.util.Properties;
import org.apache.ibatis.executor.parameter.DefaultParameterHandler;
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.session.Configuration;
import org.apache.ibatis.session.RowBounds;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@Intercepts({@Signature(type=StatementHandler.class,method="prepare",args={Connection.class})})
public class PaginationInterceptor implements Interceptor {
//日志对象
protected static Logger log = LoggerFactory.getLogger(PaginationInterceptor.class);
/* (non-Javadoc)
* @see org.apache.ibatis.plugin.Interceptor#intercept(org.apache.ibatis.plugin.Invocation)
*/
@Override
public Object intercept(Invocation invocation) throws Throwable {
StatementHandler statementHandler = (StatementHandler)invocation.getTarget();
MetaObject metaStatementHandler = MetaObject.forObject(statementHandler);
RowBounds rowBounds = (RowBounds)metaStatementHandler.getValue("delegate.rowBounds");
if(rowBounds == null || rowBounds == RowBounds.DEFAULT){
return invocation.proceed();
}
DefaultParameterHandler defaultParameterHandler = (DefaultParameterHandler)metaStatementHandler.getValue("delegate.parameterHandler");
Map parameterMap = (Map)defaultParameterHandler.getParameterObject();
Object sidx = parameterMap.get("_sidx");
Object sord = parameterMap.get("_sord");
String originalSql = (String)metaStatementHandler.getValue("delegate.boundSql.sql");
if(sidx != null && sord != null){
originalSql = originalSql + " order by " + sidx + " " + sord;
}
Configuration configuration = (Configuration)metaStatementHandler.getValue("delegate.configuration");
Dialect.Type databaseType  = null;
try{
databaseType = Dialect.Type.valueOf(configuration.getVariables().getProperty("dialect").toUpperCase());
} catch(Exception e){
//ignore
}
if(databaseType == null){
throw new RuntimeException("the value of the dialect property in configuration.xml is not defined : " + configuration.getVariables().getProperty("dialect"));
}
Dialect dialect = null;
switch(databaseType){
case ORACLE:
dialect = new OracleDialect();
break;
case MYSQL://需要实现MySQL的分页逻辑
break;
}

metaStatementHandler.setValue("delegate.boundSql.sql", dialect.getLimitString(originalSql, rowBounds.getOffset(), rowBounds.getLimit()) );
metaStatementHandler.setValue("delegate.rowBounds.offset", RowBounds.NO_ROW_OFFSET );
metaStatementHandler.setValue("delegate.rowBounds.limit", RowBounds.NO_ROW_LIMIT );
if(log.isDebugEnabled()){
BoundSql boundSql = statementHandler.getBoundSql();
log.debug("生成分页SQL : " + boundSql.getSql());
}
return invocation.proceed();
}
/* (non-Javadoc)
* @see org.apache.ibatis.plugin.Interceptor#plugin(java.lang.Object)
*/
@Override
public Object plugin(Object target) {
return Plugin.wrap(target, this);
}
/* (non-Javadoc)
* @see org.apache.ibatis.plugin.Interceptor#setProperties(java.util.Properties)
*/
@Override
public void setProperties(Properties arg0) {
// TODO Auto-generated method stub
}
}

　　（4）将Mybatis的拦截器配置到Mybatis的全局配置文件(mybatis.cfg.xml)中，具体如下：

<?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="dialect" value="oracle"/>
</properties>
<plugins>
<plugin interceptor="org.mybatis.extend.interceptor.PaginationInterceptor"/>
</plugins>
</configuration>
　　（5）使用方法同Mybatis逻辑分页一样，拦截器会自动拦截执行SQL的地方，加上分页代码：

getSqlSession().selectList(sqlId, paramMap,new RowBounds(pageId, pageSize));