mybatis 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：

Java代码 
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：



java代码：

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：



java代码：
查看复制到剪贴板打印01.Java代码    
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");    

      String originalSql = (String)metaStatementHandler.getValue("delegate.boundSql.sql");     

        Map parameterMap = (Map)defaultParameterHandler.getParameterObject();     
  if(parameterMap!=null)
  {
   Object sidx = parameterMap.get("_sidx");
   Object sord = parameterMap.get("_sord");
 
   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));