Mysql JDBC-mysql-Driver queryTimeout分析

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

         if (locallyScopedConn.getEnableQueryTimeouts() && this.timeoutInMillis != 0 && locallyScopedConn.versionMeetsMinimum(5, 0, 0)) {
                        timeoutTask = new CancelTask(this);
                         //每个连接会有一个CancelTimer，一个deamon线程
                        locallyScopedConn.getCancelTimer().schedule(timeoutTask, 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。

connectTimeout=5000&socketTimeout=10000

其实，设置了queryTimeout也不一定生效，上述代码中无论是成功执行，还是CancelTask，都会涉及到对socket的操作，socket操作是底层的，它也有timeout选项，错误的配置或不配置，会采用操作系统的默认配置，这个时间可能是长达30分钟的。一旦网络出现问题，调用socket.read()时阻塞了，都到导致应用程序假死。

解决办法

在jdbc.url中配置参数connectTimeout和socketTimeout参数，当然他们的值应该大于计划内的程序执行sql的最长耗时时间，否则可能中断正常的sql执行。