iBatis3基于方言(Dialect)

(注:以下代码是基于ibatis3 beta4的扩展，ibatis3正式版如果实现改变，将会继续跟进修改)

iBatis3默认使用的分页是基于游标的分页，而这种分页在不同的数据库上性能差异不一致，最好的办法当然是使用类似hibernate的基于方言(Dialect)的物理分页功能。

iBatis3现在提供插件功能，通过插件我们可以编写自己的拦截器来拦截iBatis3的主要执行方法来完成相关功能的扩展。

能够拦截的的类如下:

Java代码 收藏代码
Executor
(update,query,flushStatements,commit,rollback,getTransaction,close,isClosed)
ParameterHandler
(getParameterObject,setParameters)
ResultSetHandler
(handleResultSets,handleOutputParameters)
StatementHandler
(prepare,parameterize,batch,update,query)

但此次我们感兴趣的是Executor.query()方法，查询方法最终都是执行该方法。

该方法完整是:

Java代码 收藏代码
Executor.query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws sqlException;

分页方言的基本实现:

以Mysql数据库示例,即有一个方法 query(sql,offset,limit);

Java代码 收藏代码
query("select * from user",5,10);

经过我们的拦截器拦截处理,变成

Java代码 收藏代码
query("select * from user limit 5,10",0,0);

1.拦截类实现:

Java代码 收藏代码
@Intercepts({@Signature(
type= Executor.class,
method = "query",
args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class})})
public class OffsetLimitInterceptor implements Interceptor{
static int MAPPED_STATEMENT_INDEX = 0;
static int PARAMETER_INDEX = 1;
static int ROWBOUNDS_INDEX = 2;
static int RESULT_HANDLER_INDEX = 3;

Dialect dialect;  

public Object intercept(Invocation invocation) throws Throwable {  
    processIntercept(invocation.getArgs());  
    return invocation.proceed();  
}  

void processIntercept(final Object[] queryArgs) {  
    //queryArgs = query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler)  
    MappedStatement ms = (MappedStatement)queryArgs[MAPPED_STATEMENT_INDEX];  
    Object parameter = queryArgs[PARAMETER_INDEX];  
    final RowBounds rowBounds = (RowBounds)queryArgs[ROWBOUNDS_INDEX];  
    int offset = rowBounds.getOffset();  
    int limit = rowBounds.getLimit();  

    if(dialect.supportsLimit() && (offset != RowBounds.NO_ROW_OFFSET || limit != RowBounds.NO_ROW_LIMIT)) {  
        BoundSql boundSql = ms.getBoundSql(parameter);  
        String sql = boundSql.getSql().trim();  
        if (dialect.supportsLimitOffset()) {  
            sql = dialect.getLimitString(sql, offset, limit);  
            offset = RowBounds.NO_ROW_OFFSET;  
        } else {  
            sql = dialect.getLimitString(sql, 0, limit);  
        }  
        limit = RowBounds.NO_ROW_LIMIT;  

        queryArgs[ROWBOUNDS_INDEX] = new RowBounds(offset,limit);  
        BoundSql newBoundSql = new BoundSql(sql, boundSql.getParameterMappings(), boundSql.getParameterObject());  
        MappedStatement newMs = copyFromMappedStatement(ms, new BoundSqlSqlSource(newBoundSql));  
        queryArgs[MAPPED_STATEMENT_INDEX] = newMs;  
    }  
}  

private MappedStatement copyFromMappedStatement(MappedStatement ms,SqlSource newSqlSource) {  
    Builder builder = new MappedStatement.Builder(ms.getConfiguration(),ms.getId(),newSqlSource,ms.getSqlCommandType());  
    builder.resource(ms.getResource());  
    builder.fetchSize(ms.getFetchSize());  
    builder.statementType(ms.getStatementType());  
    builder.keyGenerator(ms.geTKEyGenerator());  
    builder.keyProperty(ms.getKeyProperty());  
    builder.timeout(ms.getTimeout());  
    builder.parameterMap(ms.getParameterMap());  
    builder.resultMaps(ms.getResultMaps());  
    builder.cache(ms.getCache());  
    MappedStatement newMs = builder.build();  
    return newMs;  
}  

public Object plugin(Object target) {  
    return Plugin.wrap(target, this);  
}  

public void setProperties(Properties properties) {  
    String dialectClass = new PropertiesHelper(properties).getRequiredString("dialectClass");  
    try {  
        dialect = (Dialect)Class.forName(dialectClass).newInstance();  
    } catch (Exception e) {  
        throw new RuntimeException("cannot create dialect instance by dialectClass:"+dialectClass,e);  
    }   
    System.out.println(OffsetLimitInterceptor.class.getSimpleName()+".dialect="+dialectClass);  
}  

public static class BoundSqlSqlSource implements SqlSource {  
    BoundSql boundSql;  
    public BoundSqlSqlSource(BoundSql boundSql) {  
        this.boundSql = boundSql;  
    }  
    public BoundSql getBoundSql(Object parameterObject) {  
        return boundSql;  
    }  
}  

}

2.ibatis3配置文件内容:

Xml代码 收藏代码
<configuration>
<plugins>
<!-- 指定数据库分页方言Dialect, 其它方言:oracleDialect,SQLServerDialect,SybaseDialect,DB2Dialect,postgresqlDialect,mysqlDialect,DerbyDialect-->
<plugin interceptor="cn.org.rapid_framework.ibatis3.plugin.OffsetLimitInterceptor">
<property name="dialectClass" value="cn.org.rapid_framework.jdbc.dialect.MySQLDialect"/>
</plugin>
</plugins>

<environments default="development">  
    <environment id="development">  
        <transactionManager type="JDBC" />  
        <dataSource type="POOLED">  
            <property name="driver" value="com.mysql.jdbc.Driver" />  
            <property name="url" value="jdbc:mysql://localhost:3306/test" />  
            <property name="username" value="root" />  
            <property name="passWord" value="123456" />  
        </dataSource>  
    </environment>  
</environments>  

<mappers>  
    <mapper resource="com/company/project/model/mapper/BlogMapper.xml" />  
</mappers>  

</configuration>

3.MysqlDialect实现

Java代码 收藏代码
public class MySQLDialect extends Dialect{

public boolean supportsLimitOffset(){  
    return true;  
}  

public boolean supportsLimit() {     
    return true;     
}    

public String getLimitString(String sql, int offset, int limit) {  
        return getLimitString(sql,offset,String.valueOf(offset),limit,String.valueOf(limit));  
}  

public String getLimitString(String sql, int offset,String offsetPlaceholder, int limit, String limitPlaceholder) {  
    if (offset > 0) {     
        return sql + " limit "+offsetPlaceholder+","+limitPlaceholder;   
    } else {     
        return sql + " limit "+limitPlaceholder;  
    }    
}     

}

其它数据库的Dialect实现请查看:

Http://rapid-framework.Googlecode.com/svn/trunk/rapid-framework/src/rapid_framework_common/cn/org/rapid_framework/jdbc/dialect/

4.分页使用

直接调用SqlSession.selectList(statement, parameter, new RowBounds(offset,limit))即可使用物理分页

5.存在的问题

现不是使用占位符的方式(即不是“limit ?,?”而是使用“limit 8,20”)使用分页

完整代码可以查看即将发布的rapid-framework 3.0的ibatis3插件