Error log context with logback with cleanup

Log context limitations

In another article I created a Logback appender that collect log events and only writes those events to a log file when an error occurs. I did a fairly simple implementation that collects events in a ThreadLocal variable. The potential problem is that the list in the ThreadLocal can grow if it is never properly cleaned up.

Another issue is that in modern Java, you can easily use multiple threads with collections and parallelStream. My solution will collect a context for each stream because each stream runs in its own Thread and create a separate ThreadLocal.

This may lead, over time, to memory leaks.

Size limit and automatic cleanup.

To address these issues, I added a check in the code to make sure that the context does not grow uncontrollably. The code simply checks the size of the event list and removes events that are older than a set maximum age.

  if (events.size > maxContextSize) {
      val minTimestamp = System.currentTimeMillis() - maxEventAge;
      events =
              .filter {eventItem ->
                  if (eventItem is ILoggingEvent) {
                      eventItem.getTimeStamp() > minTimestamp
                  } else {

You may notice that this is not Java code, but Kotlin. I created a new implementation in Kotlin to get more experience with Kotlin. I like the language – it is similar to Java and at the same time different in many ways.

The above solution removes events that are older than a certain number of milliseconds. At first I thought this was a nice solution – you probably do not need events that happened 30 seconds ago, for example. On second thought, it could be that the application logs many events in a short time and in that case, no events will be removed at all and it will go through the loop every time – bad for performance.

A safer and simpler solution is to cut the list in half and effectively remove half of the events in the list.

  events = events.subList(events.size/2, events.size)

In the logback configuration, you can specify which appender to write the error log context to with errorLogger and errorAppender elements.

  <appender name="contextAppender" class="logback.LogContextAppender">

You will get a slight performance hit every time the limit is reached – I guess that is the price to pay for convenience, just like the JVM garbage collection.

You can look at the code on GitHub: