原文
今日生产环境发现有些容器停止响应了，但是容器没有死，docker exec -it <container-name> /bin/bash 也能正常使用。
在容器内部使用 jstack <pid> 发现 log4j2 的 Console Appender 一直处于运行状态：
“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> 没有发现有新的日志输出，且访问该应用肯定会输出日志的接口也是没有任何日志输出，因此怀疑 log4j2 阻塞住了。
Google 到有人在 log4j 提出了类似了问题 LOG4J2-2239，官方给出的解释是问题出在 log4j2 之外。
于是查一下 logback 是否也有类似问题，找到 LOGBACK-1422，同样给出的解释是问题出在 logback 之外。
两个问题的共通点都是用 docker 运行，于是把应用直接进程方式运行，没有出现问题。
于是 Google 搜索 docker logging to stdout hangs，找到 SO 的这个回答，以及这个 issue，解决方案将 Docker 升级到 18.06。
查看生产环境的 docker 版本是 18.03，升级到 18.09 后问题解决。