Mybatis使用篇之十二：实现物理分页

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>

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（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));