Log4j and javax.mail are deadlocking

Issue

• JBoss became unresponsive. Thread dumps show it was due to threads deadlocking between log4j SMTP appenders and direct usage of javax.mail from the application:

  "WorkManager(2)-1" J9VMThread:0x000000011877C800, j9thread_t:0x0000000118754320, java/lang/Thread:0x070000003BD50160, state:B, prio=5
  (native thread ID:0x36C0D9, native priority:0x5, native policy:UNKNOWN)
  Java callstack:
  at org/apache/log4j/Category.callAppenders(Category.java:201(Compiled Code))
  at org/apache/log4j/Category.forcedLog(Category.java:388(Compiled Code))
  at org/apache/log4j/Category.log(Category.java:849(Compiled Code))
  at org/jboss/logbridge/LogBridgeHandler.publish(LogBridgeHandler.java:72(Compiled Code))
  at java/util/logging/Logger.log(Logger.java:1121(Compiled Code))
  at java/util/logging/Logger.log(Logger.java:1021(Compiled Code))
  at com/sun/activation/registries/LogSupport.log(LogSupport.java:73)
  at javax/activation/MailcapCommandMap.createDataContentHandler(MailcapCommandMap.java:548)
  at javax/activation/DataHandler.getDataContentHandler(DataHandler.java:625)
  at javax/activation/DataHandler.writeTo(DataHandler.java:329)
  at javax/mail/internet/MimeUtility.getEncoding(MimeUtility.java:264)
  at javax/mail/internet/MimeBodyPart.updateHeaders(MimeBodyPart.java:1299)
  at javax/mail/internet/MimeMessage.updateHeaders(MimeMessage.java:2071)
  at javax/mail/internet/MimeMessage.saveChanges(MimeMessage.java:2039)
  at javax/mail/Transport.send(Transport.java:119)
  at com/package/Helper.sendDeadlockingMail(Helper.java:166)
  
  "WorkerThread#2[10.2.52.204:58412]" J9VMThread:0x0000000119B48400, j9thread_t:0x0000000119914A80, java/lang/Thread:0x070000003C6E2AE0, state:B, prio=5
  (native thread ID:0x2C508B, native priority:0x5, native policy:UNKNOWN)
  Java callstack:
  at javax/activation/DataHandler.getDataContentHandler(DataHandler.java:625)
  at javax/activation/DataHandler.writeTo(DataHandler.java:329)
  at javax/mail/internet/MimeUtility.getEncoding(MimeUtility.java:264)
  at javax/mail/internet/MimeBodyPart.updateHeaders(MimeBodyPart.java:1299)
  at javax/mail/internet/MimeBodyPart.updateHeaders(MimeBodyPart.java:1008)
  at javax/mail/internet/MimeMultipart.updateHeaders(MimeMultipart.java:414)
  at javax/mail/internet/MimeBodyPart.updateHeaders(MimeBodyPart.java:1285)
  at javax/mail/internet/MimeMessage.updateHeaders(MimeMessage.java:2071)
  at javax/mail/internet/MimeMessage.saveChanges(MimeMessage.java:2039)
  at javax/mail/Transport.send(Transport.java:119)
  at org/apache/log4j/net/SMTPAppender.sendBuffer(Bytecode PC:216)
  at org/apache/log4j/net/SMTPAppender.append(Bytecode PC:56)
  at org/apache/log4j/AppenderSkeleton.doAppend(AppenderSkeleton.java:230(Compiled Code))
  at org/apache/log4j/helpers/AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:65(Compiled Code))
  at org/apache/log4j/Category.callAppenders(Category.java:203(Compiled Code))
  at org/apache/log4j/Category.forcedLog(Category.java:388(Compiled Code))
  at org/apache/log4j/Category.error(Category.java:302)