I am observing logback 1.2.3 getting stuck with stacktrace like below . In case the email gets truncated I am also placing the above logs in the below gist <https://gist.github.com/debraj-manna/d3b8de557ae3514368ab66b947d0ed14> . https://gist.github.com/debraj-manna/d3b8de557ae3514368ab66b947d0ed14 I am using java 8 Schedule_CISCON9K_10.22.8.7_config_OpMgr-0" #874742 prio=5 os_prio=0 tid=0x00007fff54789000 nid=0x31f0 waiting for monitor entry [0x00007fff0e323000] java.lang.Thread.State: BLOCKED (on object monitor) at ch.qos.logback.core.util.CachingDateFormatter.format(CachingDateFormatter.java:45) - waiting to lock <0x00000006488ca358> (a ch.qos.logback.core.util.CachingDateFormatter) at ch.qos.logback.classic.pattern.DateConverter.convert(DateConverter.java:61) at ch.qos.logback.classic.pattern.DateConverter.convert(DateConverter.java:23) at ch.qos.logback.core.pattern.FormattingConverter.write(FormattingConverter.java:36) at ch.qos.logback.core.pattern.PatternLayoutBase.writeLoopOnConverters(PatternLayoutBase.java:115) at ch.qos.logback.classic.PatternLayout.doLayout(PatternLayout.java:141) at ch.qos.logback.classic.PatternLayout.doLayout(PatternLayout.java:39) at ch.qos.logback.core.encoder.LayoutWrappingEncoder.encode(LayoutWrappingEncoder.java:115) at ch.qos.logback.core.OutputStreamAppender.subAppend(OutputStreamAppender.java:230) at ch.qos.logback.core.OutputStreamAppender.append(OutputStreamAppender.java:102) at ch.qos.logback.core.UnsynchronizedAppenderBase.doAppend(UnsynchronizedAppenderBase.java:84) at ch.qos.logback.core.spi.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:51) at ch.qos.logback.classic.Logger.appendLoopOnAppenders(Logger.java:270) at ch.qos.logback.classic.Logger.callAppenders(Logger.java:257) at ch.qos.logback.classic.Logger.buildLoggingEventAndAppend(Logger.java:421) at ch.qos.logback.classic.Logger.filterAndLog_0_Or3Plus(Logger.java:383) at ch.qos.logback.classic.Logger.info(Logger.java:579) at com.van.dataproviders.core.impl.cas.n5k.tasks.N5KSynchronousTask.doRun(N5KSynchronousTask.java:63) at com.van.dataproviders.core.common.impl.dataprovider.tasks.AbstractTask.run(AbstractTask.java:104) at com.van.dataproviders.tasker.Tasker$ScheduledTaskWrapper.run(Tasker.java:359) at com.google.common.util.concurrent.MoreExecutors$ScheduledListeningDecorator$NeverSuccessfulListenableFutureTask.run(MoreExecutors.java:678) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308) at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180) at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) Most thread trying to write to log are blocked on a `synchronize(this)` lock in `CachingDateFormatter` of logback. But another thread, which actually has the above lock (thus blocking the other threads) is itself blocked as follows: "Schedule_CISCON9K_10.22.8.149_metric_OpMgr-0" #871618 prio=5 os_prio=0 tid=0x00007fff54740800 nid=0x7e56 waiting for monitor entry [0x00007fff15f1a000] java.lang.Thread.State: BLOCKED (on object monitor) at java.util.ResourceBundle.getBundleImpl(ResourceBundle.java:1333) at java.util.ResourceBundle.getBundle(ResourceBundle.java:899) at sun.util.resources.LocaleData$1.run(LocaleData.java:167) at sun.util.resources.LocaleData$1.run(LocaleData.java:163) at java.security.AccessController.doPrivileged(Native Method) at sun.util.resources.LocaleData.getBundle(LocaleData.java:163) at sun.util.resources.LocaleData.getDateFormatData(LocaleData.java:127) at java.text.DateFormatSymbols.initializeData(DateFormatSymbols.java:710) at java.text.DateFormatSymbols.<init>(DateFormatSymbols.java:145) at sun.util.locale.provider.DateFormatSymbolsProviderImpl.getInstance(DateFormatSymbolsProviderImpl.java:85) at java.text.DateFormatSymbols.getProviderInstance(DateFormatSymbols.java:364) at java.text.DateFormatSymbols.getInstance(DateFormatSymbols.java:340) at java.util.Calendar.getDisplayName(Calendar.java:2110) at java.text.SimpleDateFormat.subFormat(SimpleDateFormat.java:1125) at java.text.SimpleDateFormat.format(SimpleDateFormat.java:966) at java.text.SimpleDateFormat.format(SimpleDateFormat.java:936) at java.text.DateFormat.format(DateFormat.java:345) at ch.qos.logback.core.util.CachingDateFormatter.format(CachingDateFormatter.java:48) - locked <0x00000006488ca358> (a ch.qos.logback.core.util.CachingDateFormatter) at ch.qos.logback.classic.pattern.DateConverter.convert(DateConverter.java:61) Loback.xml <configuration scan="true" debug="true"> <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender"> <Target>System.out</Target> <encoder> <pattern>%p [%d{yyyy-MM-dd HH:mm:ss,SSS}] %c{3}:[%C{1}:%M:%L] - [%t] - %replace(%.-20480m){'[^A-Za-z0-9=<>:@ /.,"\'\-\[\]\{\}\(\)\n\s!|]','_'}%n</pattern> </encoder> </appender> <root level="INFO"> <appender-ref ref="CONSOLE"/> </root> </configuration>