对ibatis分页功能的改进

 

今天无意间看到了一篇关于这方面的文章，觉得是网上改进ibatis分页方面比较好的文章，这里转摘一下，希望能让更多的人用的到，也希望别人能把更好的解决方案贡献出来！

使ibatis支持hibernate式的物理分页

一直以来ibatis的分页都是通过滚动ResultSet实现的，应该算是逻辑分页吧。逻辑分页虽然能很干净地独立于特定数据库，但效率在多数情 况下不及特定数据库支持的物理分页，而hibernate的分页则是直接组装sql，充分利用了特定数据库的分页机制，效率相对较高。本文讲述的就是如何 在不重新编译ibatis源码的前提下，为ibatis引入hibernate式的物理分页机制。

基本思路就是找到ibatis执行sql的地方，截获sql并重新组装sql。通过分析ibatis源码知道，最终负责执行sql的类是 com.ibatis.sqlmap.engine.execution.SqlExecutor，此类没有实现任何接口，这多少有点遗憾，因为接口是相 对稳定契约，非大的版本更新，接口一般是不会变的，而类就相对易变一些，所以这里的代码只能保证对当前版本（2.1.7）的ibatis有效。下面是 SqlExecutor执行查询的方法：

 

 

  /**
    * Long form of the method to execute a query
    *
    * @param request - the request scope
    * @param conn - the database connection
    * @param sql - the SQL statement to execute
    * @param parameters - the parameters for the statement
    * @param skipResults - the number of results to skip
    * @param maxResults - the maximum number of results to return
    * @param callback - the row handler for the query
    *
    * @throws SQLException - if the query fails
   */
  public void executeQuery(RequestScope request, Connection conn, String sql, Object[] parameters,
                           int skipResults, int maxResults, RowHandlerCallback callback)
      throws SQLException {
     ErrorContext errorContext = request.getErrorContext();
     errorContext.setActivity("executing query");
     errorContext.setObjectId(sql);

     PreparedStatement ps = null;
     ResultSet rs = null;

    try {
       errorContext.setMoreInfo("Check the SQL Statement (preparation failed).");

       Integer rsType = request.getStatement().getResultSetType();
      if (rsType != null) {
         ps = conn.prepareStatement(sql, rsType.intValue(), ResultSet.CONCUR_READ_ONLY);
       } else {
         ps = conn.prepareStatement(sql);
       }

       Integer fetchSize = request.getStatement().getFetchSize();
      if (fetchSize != null) {
         ps.setFetchSize(fetchSize.intValue());
       }

       errorContext.setMoreInfo("Check the parameters (set parameters failed).");
       request.getParameterMap().setParameters(request, ps, parameters);

       errorContext.setMoreInfo("Check the statement (query failed).");

       ps.execute();
       rs = getFirstResultSet(ps);

      if (rs != null) {
         errorContext.setMoreInfo("Check the results (failed to retrieve results).");
         handleResults(request, rs, skipResults, maxResults, callback);
       }

      // clear out remaining results
      while (ps.getMoreResults());

     } finally {
      try {
         closeResultSet(rs);
       } finally {
         closeStatement(ps);
       }
     }

   }

 

 其中handleResults(request, rs, skipResults, maxResults, callback)一句用于处理分页，其实此时查询已经执行完毕，可以不必关心handleResults方法，但为清楚起见，下面来看看 handleResults的实现：

 

private void handleResults(RequestScope request, ResultSet rs, int skipResults, int maxResults, RowHandlerCallback callback) throws SQLException {
    try {
       request.setResultSet(rs);
       ResultMap resultMap = request.getResultMap();
      if (resultMap != null) {
        // Skip Results
        if (rs.getType() != ResultSet.TYPE_FORWARD_ONLY) {
          if (skipResults > 0) {
             rs.absolute(skipResults);
           }
         } else {
          for (int i = 0; i < skipResults; i++) {
            if (!rs.next()) {
              break;
             }
           }
         }

        // Get Results
        int resultsFetched = 0;
        while ((maxResults == SqlExecutor.NO_MAXIMUM_RESULTS || resultsFetched < maxResults) && rs.next()) {
           Object[] columnValues = resultMap.resolveSubMap(request, rs).getResults(request, rs);
           callback.handleResultObject(request, columnValues, rs);
           resultsFetched++;
         }
       }
     } finally {
       request.setResultSet(null);
     }
   }

 此处优先使用的是ResultSet的absolute方法定位记录，是否支持absolute取决于具体数据库驱动，但一般当前版本的数据库都支 持该方法，如果不支持则逐条跳过前面的记录。由此可以看出如果数据库支持absolute，则ibatis内置的分页策略与特定数据库的物理分页效率差距 就在于物理分页查询与不分页查询在数据库中的执行效率的差距了。因为查询执行后读取数据前数据库并未把结果全部返回到内存，所以本身在存储占用上应该差距 不大，如果都使用索引，估计执行速度也差不太多。

 

继续我们的话题。其实只要在executeQuery执行前组装sql，然后将其传给executeQuery，并告诉handleResults 我们不需要逻辑分页即可。拦截executeQuery可以采用aop动态实现，也可直接继承SqlExecutor覆盖executeQuery来静态 地实现，相比之下后者要简单许多，而且由于SqlExecutor没有实现任何接口，比较易变，动态拦截反到增加了维护的工作量，所以我们下面来覆盖 executeQuery：

 

package com.aladdin.dao.ibatis.ext;

import java.sql.Connection;
import java.sql.SQLException;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import com.aladdin.dao.dialect.Dialect;
import com.ibatis.sqlmap.engine.execution.SqlExecutor;
import com.ibatis.sqlmap.engine.mapping.statement.RowHandlerCallback;
import com.ibatis.sqlmap.engine.scope.RequestScope;

public class LimitSqlExecutor extends SqlExecutor {

    private static final Log logger = LogFactory.getLog(LimitSqlExecutor.class);
    
    private Dialect dialect;

    private boolean enableLimit = true;

    public Dialect getDialect() {
        return dialect;
     }

    public void setDialect(Dialect dialect) {
        this.dialect = dialect;
     }

    public boolean isEnableLimit() {
        return enableLimit;
     }

    public void setEnableLimit(boolean enableLimit) {
        this.enableLimit = enableLimit;
     }

     @Override
    public void executeQuery(RequestScope request, Connection conn, String sql,
             Object[] parameters, int skipResults, int maxResults,
             RowHandlerCallback callback) throws SQLException {
        if ((skipResults != NO_SKIPPED_RESULTS || maxResults != NO_MAXIMUM_RESULTS)
                && supportsLimit()) {
             sql = dialect.getLimitString(sql, skipResults, maxResults);
            if(logger.isDebugEnabled()){
                 logger.debug(sql);
             }
             skipResults = NO_SKIPPED_RESULTS;
             maxResults = NO_MAXIMUM_RESULTS;            
         }
        super.executeQuery(request, conn, sql, parameters, skipResults,
                 maxResults, callback);
     }

    public boolean supportsLimit() {
        if (enableLimit && dialect != null) {
            return dialect.supportsLimit();
         }
        return false;
     }

}

 

 其中：

 

skipResults = NO_SKIPPED_RESULTS;
maxResults = NO_MAXIMUM_RESULTS;

 

 告诉handleResults不分页（我们组装的sql已经使查询结果是分页后的结果了），此处引入了类似hibenate中的数据库方言接口Dialect，其代码如下：

 

package com.aladdin.dao.dialect;

public interface Dialect {
    
    public boolean supportsLimit();

    public String getLimitString(String sql, boolean hasOffset);

    public String getLimitString(String sql, int offset, int limit);
}

 下面为Dialect接口的MySQL实现：package com.aladdin.dao.ibatis;

import java.io.Serializable;
import java.util.List;

import org.springframework.orm.ObjectRetrievalFailureException;
import org.springframework.orm.ibatis.support.SqlMapClientDaoSupport;

import com.aladdin.dao.ibatis.ext.LimitSqlExecutor;
import com.aladdin.domain.BaseObject;
import com.aladdin.util.ReflectUtil;
import com.ibatis.sqlmap.client.SqlMapClient;
import com.ibatis.sqlmap.engine.execution.SqlExecutor;
import com.ibatis.sqlmap.engine.impl.ExtendedSqlMapClient;

public abstract class BaseDaoiBatis extends SqlMapClientDaoSupport {

    private SqlExecutor sqlExecutor;

    public SqlExecutor getSqlExecutor() {
        return sqlExecutor;
     }

    public void setSqlExecutor(SqlExecutor sqlExecutor) {
        this.sqlExecutor = sqlExecutor;
     }

    public void setEnableLimit(boolean enableLimit) {
        if (sqlExecutor instanceof LimitSqlExecutor) {
             ((LimitSqlExecutor) sqlExecutor).setEnableLimit(enableLimit);
         }
     }

    public void initialize() throws Exception {
        if (sqlExecutor != null) {
             SqlMapClient sqlMapClient = getSqlMapClientTemplate()
                     .getSqlMapClient();
            if (sqlMapClient instanceof ExtendedSqlMapClient) {
                 ReflectUtil.setFieldValue(((ExtendedSqlMapClient) sqlMapClient)
                         .getDelegate(), "sqlExecutor", SqlExecutor.class,
                         sqlExecutor);
             }
         }
     }

     ...

}

 

 

 

评论

1 楼 vasuer 2010-08-29  

te