今日生產環境發現有些容器停止響應了，但是容器沒有死，docker exec -it <container-name> /bin/bash也能正常使用。
在容器內部使用jstack <pid>發現log4j2的Console Appender一直處於運行狀態：
"AsyncAppender-asyncConsole" #21 daemon prio=5 os_prio=0 tid=0x00007fd968d07000 nid=0x1f runnable [0x00007fd91bffd000]
java.lang.Thread.State: RUNNABLE
at java.io.FileOutputStream.writeBytes(Native Method)
at java.io.FileOutputStream.write(FileOutputStream.java:326)
at java.io.BufferedOutputStream.write(BufferedOutputStream.java:122)
- locked <0x00000000f002b408> (a java.io.BufferedOutputStream)
at java.io.PrintStream.write(PrintStream.java:480)
- locked <0x00000000f002b3e8> (a java.io.PrintStream)
at org.apache.logging.log4j.core.util.CloseShieldOutputStream.write(CloseShieldOutputStream.java:53)
at org.apache.logging.log4j.core.appender.OutputStreamManager.writeToDestination(OutputStreamManager.java:262)
- locked <0x00000000f021d848> (a org.apache.logging.log4j.core.appender.OutputStreamManager)
at org.apache.logging.log4j.core.appender.OutputStreamManager.flushBuffer(OutputStreamManager.java:294)
- locked <0x00000000f021d848> (a org.apache.logging.log4j.core.appender.OutputStreamManager)
at org.apache.logging.log4j.core.appender.OutputStreamManager.drain(OutputStreamManager.java:351)
at org.apache.logging.log4j.core.layout.TextEncoderHelper.drainIfByteBufferFull(TextEncoderHelper.java:260)
- locked <0x00000000f021d848> (a org.apache.logging.log4j.core.appender.OutputStreamManager)
at org.apache.logging.log4j.core.layout.TextEncoderHelper.writeAndEncodeAsMuchAsPossible(TextEncoderHelper.java:199)
at org.apache.logging.log4j.core.layout.TextEncoderHelper.encodeChunkedText(TextEncoderHelper.java:159)
- locked <0x00000000f021d848> (a org.apache.logging.log4j.core.appender.OutputStreamManager)
at org.apache.logging.log4j.core.layout.TextEncoderHelper.encodeText(TextEncoderHelper.java:58)
at org.apache.logging.log4j.core.layout.StringBuilderEncoder.encode(StringBuilderEncoder.java:68)
at org.apache.logging.log4j.core.layout.StringBuilderEncoder.encode(StringBuilderEncoder.java:32)
at org.apache.logging.log4j.core.layout.PatternLayout.encode(PatternLayout.java:220)
at org.apache.logging.log4j.core.layout.PatternLayout.encode(PatternLayout.java:58)
at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.directEncodeEvent(AbstractOutputStreamAppender.java:177)
at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.tryAppend(AbstractOutputStreamAppender.java:170)
at org.apache.logging.log4j.core.appender.AbstractOutputStreamAppender.append(AbstractOutputStreamAppender.java:161)
at org.apache.logging.log4j.core.config.AppenderControl.tryCallAppender(AppenderControl.java:156)
at org.apache.logging.log4j.core.config.AppenderControl.callAppender0(AppenderControl.java:129)
at org.apache.logging.log4j.core.config.AppenderControl.callAppenderPreventRecursion(AppenderControl.java:120)
at org.apache.logging.log4j.core.config.AppenderControl.callAppender(AppenderControl.java:84)
at org.apache.logging.log4j.core.appender.AsyncAppender$AsyncThread.callAppenders(AsyncAppender.java:459)
at org.apache.logging.log4j.core.appender.AsyncAppender$AsyncThread.run(AsyncAppender.java:412)但用docker logs -f <container-name>沒有發現有新的日誌輸出，且訪問該應用肯定會輸出日誌的接口也是沒有任何日誌輸出，因此懷疑log4j2阻塞住了。
Google到有人在log4j提出了類似了問題LOG4J2-2239，官方給出的解釋是問題出在log4j2之外。
於是查一下logback是否也有類似問題，找到LOGBACK-1422，同樣給出的解釋是問題出在logback之外。
兩個問題的共通點都是用docker運行，於是把應用直接進程方式運行，沒有出現問題。
於是Google搜索docker logging to stdout hangs，找到SO的這個回答，以及這個issue，解決方案將Docker升級到18.06。
查看生產環境的docker版本是18.03，升級到18.09後問題解決。