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背景：

今天dba在进行DB的主从切换，导致应用一直报错，获取不到DB连接，druid的错误信息如下：

Could not open JDBC Connection for transaction; nested exception is com.alibaba.druid.pool.GetConnectionTimeoutException: wait millis 15000, active 20

可以看到活跃连接数active=20, 而应用配置中设置的maxActive=100，远远没有达到最大的连接数，为什么就创建不了连接呢？

查找真相

查看和db连接有关的错误日志，发现如下

ERROR com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread (DruidDataSource.java:1713) - create connection holder error
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
The last packet successfully received from the server was 0 milliseconds ago. 

错误信息展示CreateConnectionThread 创建连接线程报错，那问题就变成了猜测连接线程已经被中止了，导致没法创建连接，使用jstack命令导出应用线程，发现确实CreateConnectionThread 线程确实不存在了，所以问题已经明朗，是因为CreateConnectionThread 线程中止了，导致再也无法创建连接。

对CreateConnectionThread 线程的代码进行检查：

public void run() {initedLatch.countDown();int errorCount = 0;for (;;) {// addLasttry {lock.lockInterruptibly();} catch (InterruptedException e2) {break;}try {// 必须存在线程等待，才创建连接if (poolingCount >= notEmptyWaitThreadCount) {empty.await();}// 防止创建超过maxActive数量的连接if (activeCount + poolingCount >= maxActive) {empty.await();continue;}} catch (InterruptedException e) {lastCreateError = e;lastErrorTimeMillis = System.currentTimeMillis();break;} finally {lock.unlock();}Connection connection = null;try {connection = createPhysicalConnection();} catch (SQLException e) {LOG.error("create connection error", e);errorCount++;if (errorCount > connectionErrorRetryAttempts && timeBetweenConnectErrorMillis > 0) {if (breakAfterAcquireFailure) {break;}try {Thread.sleep(timeBetweenConnectErrorMillis);} catch (InterruptedException interruptEx) {break;}}} catch (RuntimeException e) {LOG.error("create connection error", e);continue;} catch (Error e) {LOG.error("create connection error", e);break;}if (connection == null) {continue;}DruidConnectionHolder holder = null;try {holder = new DruidConnectionHolder(DruidDataSource.this, connection);} catch (SQLException ex) {// 主从切换过程中，这里有报错，导致线程中止LOG.error("create connection holder error", ex);break;}lock.lock();try {connections[poolingCount++] = holder;if (poolingCount > poolingPeak) {poolingPeak = poolingCount;poolingPeakTime = System.currentTimeMillis();}errorCount = 0; // reset errorCountnotEmpty.signal();notEmptySignalCount++;} finally {lock.unlock();}}}
}

我使用的版本是druid 1.7，升级到最新的1.15版本后，这个bug已经没有解决了