Mybatis技术（三） 实现物理分页

Mybatis技术（三） 实现物理分页

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• Mybatis技术内幕

Mybatis 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));