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>

 

（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));