对ibatis分页功能的改进
Posted on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); } } } ... }
