对ibatis分页功能的改进

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对ibatis分页功能的改进

对ibatis**分页功能的改进**

2008-05-26 14:45 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.dialect; public class MySQLDialect implements Dialect { protected static final String SQL_END_DELIMITER = ";"; public String getLimitString(String sql, boolean hasOffset) { return new StringBuffer(sql.length() + 20).append(trim(sql)).append( hasOffset ? " limit ?,?" : " limit ?") .append(SQL_END_DELIMITER).toString(); } public String getLimitString(String sql, int offset, int limit) { sql = trim(sql); StringBuffer sb = new StringBuffer(sql.length() + 20); sb.append(sql); if (offset > 0) { sb.append(" limit ").append(offset).append(',').append(limit) .append(SQL_END_DELIMITER); } else { sb.append(" limit ").append(limit).append(SQL_END_DELIMITER); } return sb.toString(); } public boolean supportsLimit() { return true; } private String trim(String sql) { sql = sql.trim(); if (sql.endsWith(SQL_END_DELIMITER)) { sql = sql.substring(0, sql.length() - 1

                - SQL_END_DELIMITER.length());
     }
    return sql;
 }

}

接下来的工作就是把LimitSqlExecutor注入ibatis中。我们是通过spring来使用ibatis的,所以在我们的dao基类中执行注入,代码如下:

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); } } } ... }


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