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  1. Log4j 2
  2. LOG4J2-485

toString methods that perform logging can deadlock AsyncAppender



    • Bug
    • Status: Closed
    • Major
    • Resolution: Fixed
    • 2.0-beta9
    • 2.0-rc1, 2.0
    • Appenders
    • None


      During analysis of LOG4J2-471, I found the same issue can happen with AsyncAppender. Summary of the problem:

      Application thread A calls Logger.log(), which puts LogEvents in a queue. Appender thread B takes LogEvents off the queue, converts them to a byte[] array and saves this byte array to disk. The conversion from a LogEvent to a byte[] array may itself result in calls to Logger.log() (particularly if logging is done from the toString() method of an object that is being logged).
      Here is where the problem occurs: if the Appender thread calls Logger.log() when processing LogEvents from the queue, and it happens that the queue is full, then in blocking mode, the Appender thread will block until space is available in the queue. Unfortunately space will only become available if the Appender consumes LogEvents, but the Appender thread is blocked... resulting in deadlock.

      It is possible to configure AsyncAppender with blocking="false", which will log to an errorAppender when the queue is full, but a better solution would be to prevent deadlock in the above scenario.

      The problem can be reproduced with this program:

      package demo.deadlock;
      import org.apache.logging.log4j.LogManager;
      import org.apache.logging.log4j.Logger;
      public class DeadlockDemo {
          static Logger sLog = LogManager.getLogger(DeadlockDemo.class);
          public static void main(String[] args) throws Exception {
              try {
                  System.out.println("(Sysout) Before throwing ex...");
                  sLog.info("(Log) Before throwing ex...");
                  throw new LoggingEx();
              } catch (Exception ex) {
                  sLog.warn("Caught a {} problem", ex, ex);
                  sLog.error(ex, ex);
              System.out.println("(Sysout) After catching ex");
              sLog.info("(Log) After catching ex");
              System.out.println("(Sysout) After 1 sec..."); // I still see this
              sLog.info("(Log) After 1 sec..."); // causes deadlock on my laptop
              System.out.println("(Sysout) After 2 sec..."); // I don't see this output
              sLog.info("(Log) After 2 sec...");
              sLog.info("Done. Completed without deadlock.");
          public static class LoggingEx extends Exception {
              static final Logger exLog = LogManager.getLogger(LoggingEx.class);
              public String toString() {
                  System.out.println("(Sysout) Before LoggingEx calling logger.debug 300 times...");
                  // fill up AsyncAppender queue (128 slots)
                  for(int i = 0; i < 300; i++) {
                      exLog.debug("(Log) in LoggingEx.toString() {}...", i);
                  System.out.println("(Sysout) After LoggingEx called logger.debug 300 times...");
                  try {
                  } catch (InterruptedException e) {
                  return "oops";

      ... and this log4j2.xml config:

      <?xml version="1.0" encoding="UTF-8"?>
      <configuration status="TRACE" name="deadlock-demo" packages="">
            <Console name="STDOUT" target="SYSTEM_OUT">
              <PatternLayout pattern="%m%n"/>
          <Async name="Async" bufferSize="128" errorRef="STDOUT">
            <appenderRef ref="STDOUT" />
          <root level="DEBUG">
            <appenderRef ref="Async" />


      (Sysout) Before throwing ex...
      (Log) Before throwing ex...
      (Sysout) Before LoggingEx calling logger.debug 300 times...
      (Log) in LoggingEx.toString() 0...
      (Log) in LoggingEx.toString() 1...
      (Log) in LoggingEx.toString() 2...
      (Log) in LoggingEx.toString() 169...
      (Log) in LoggingEx.toString() 170...
      (Sysout) After LoggingEx called logger.debug 300 times...
      (Log) in LoggingEx.toString() 171...
      (Log) in LoggingEx.toString() 172...
      (Log) in LoggingEx.toString() 298...
      (Log) in LoggingEx.toString() 299...
      Caught a oops problem
      	at demo.deadlock.DeadlockDemo.main(DeadlockDemo.java:32) [bin/:?]
      (Sysout) After catching ex
      (Sysout) Before LoggingEx calling logger.debug 300 times...
      (Sysout) After 1 sec...

      ... and the application hangs.


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              rpopma Remko Popma
              rpopma Remko Popma
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