Mysql jdbc的queryTimeout分析

Mysql的jdbc-driver

com.mysql.jdbc.Driver

设置queryTimeout方法

com.mysql.jdbc.StatementImpl.setQueryTimeout
StatementImpl实例有一个field:timeoutInMillis

public void setQueryTimeout(int seconds) throws SQLException { synchronized(this.checkClosed().getConnectionMutex()) { if(seconds < 0) { throw SQLError.createSQLException(Messages.getString("Statement.21"), "S1009", this.getExceptionInterceptor()); } else { this.timeoutInMillis = seconds * 1000; } }}

queryTimeout使用场景示例：

com.mysql.jdbc.StatementImpl.executeQuery

ResultSet executeQuery(String sql) throws SQLException;

executeQuery有一个较复杂的逻辑：

• 获取connection的互斥锁
• 校验、初始化一些配置，是否为ping请求
• sql转义，防sql注入
• 判断timeout是否有效，有效时创建一个CancelTask
• 将cancelTask放入Timer中延迟执行
  locallyScopedConn.getCancelTimer().schedule(timeoutTask, (long)this.timeoutInMillis);
  也就是在当前时间的timeoutInMillis后会执行这个Task
• 执行sql语句，获取结果

• 超时任务判断，如果有超时任务，分为两种情况：1 超时异常已经抛出，直接返回异常；1 超时任务未执行，cancel超时任务

   this.results = locallyScopedConn.execSQL(this, sql, this.maxRows, (Buffer)null, this.resultSetType, this.resultSetConcurrency, this.createStreamingResultSet(), this.currentCatalog, cachedFields); if(timeoutTask != null) { if(timeoutTask.caughtWhileCancelling != null) { throw timeoutTask.caughtWhileCancelling; } timeoutTask.cancel(); locallyScopedConn.getCancelTimer().purge(); timeoutTask = null; }

• 获取lastInsertId

• 返回results

StatementImpl.CancelTask

class CancelTask extends TimerTask { SQLException caughtWhileCancelling = null; StatementImpl toCancel; Properties origConnProps = null; String origConnURL = ""; long origConnId = 0L; CancelTask(StatementImpl cancellee) throws SQLException { this.toCancel = cancellee; this.origConnProps = new Properties(); Properties props = StatementImpl.this.connection.getProperties(); Enumeration keys = props.propertyNames(); while(keys.hasMoreElements()) { String key = keys.nextElement().toString(); this.origConnProps.setProperty(key, props.getProperty(key)); } this.origConnURL = StatementImpl.this.connection.getURL(); this.origConnId = StatementImpl.this.connection.getId(); } public void run() { Thread cancelThread = new Thread() { public void run() { Connection cancelConn = null; java.sql.Statement cancelStmt = null; try { MySQLConnection npe = (MySQLConnection)StatementImpl.this.physicalConnection.get(); if(npe != null) { if(npe.getQueryTimeoutKillsConnection()) { CancelTask.this.toCancel.wasCancelled = true; CancelTask.this.toCancel.wasCancelledByTimeout = true; npe.realClose(false, false, true, new MySQLStatementCancelledException(Messages.getString("Statement.ConnectionKilledDueToTimeout"))); } else { Object var4 = StatementImpl.this.cancelTimeoutMutex; synchronized(StatementImpl.this.cancelTimeoutMutex) { if(CancelTask.this.origConnURL.equals(npe.getURL())) { cancelConn = npe.duplicate(); cancelStmt = cancelConn.createStatement(); cancelStmt.execute("KILL QUERY " + npe.getId()); } else { try { cancelConn = (Connection)DriverManager.getConnection(CancelTask.this.origConnURL, CancelTask.this.origConnProps); cancelStmt = cancelConn.createStatement(); cancelStmt.execute("KILL QUERY " + CancelTask.this.origConnId); } catch (NullPointerException var25) { ; } } CancelTask.this.toCancel.wasCancelled = true; CancelTask.this.toCancel.wasCancelledByTimeout = true; } } } } catch (SQLException var27) { CancelTask.this.caughtWhileCancelling = var27; } catch (NullPointerException var28) { ; } finally { if(cancelStmt != null) { try { cancelStmt.close(); } catch (SQLException var24) { throw new RuntimeException(var24.toString()); } } if(cancelConn != null) { try { cancelConn.close(); } catch (SQLException var23) { throw new RuntimeException(var23.toString()); } } CancelTask.this.toCancel = null; CancelTask.this.origConnProps = null; CancelTask.this.origConnURL = null; } } }; cancelThread.start(); }}

timeout后执行的操作主要为：

• cancelConn = npe.duplicate(); //复制一个当前连接配置相同的连接
• cancelStmt = cancelConn.createStatement(); //创建一个Statement对象，用来发送sql语句到数据库
• cancelStmt.execute("KILL QUERY " + npe.getId()); //杀掉已经timeout的语句

可以看到，只要CancelTask执行，除了执行sql的连接压根没有成功生成外，都会执行KILL QUERY操作，里面不做任何请求是否已成功的判断。
原因也比较明显，凡是执行到CancelTask，说明确实超时了。