I ran into a problem recently where, if I created and used a lot of Parser objects in a single thread, my application would OOM. Looking at the heap dump, I saw that that thread had thousands of inheritable thread locals, each of which had a DynamicVariable that held a reference to a Parser$Failure object, keeping my parser alive.

This DynamicVariable was introduced by the fix in #4929, specifically scala/scala@dce6b34

I'm not familiar with DynamicVariable so I don't know if the behavior I'm seeing is a bug in DynamicVariable or a bug in how the parser libraries use DynamicVariable - all I know is that, in this instance, they're combining to keep objects alive in a way that the garbage collector apparently can't see through.

I'm attaching a file to reproduce it; compile the file, then do

   JAVA_OPTS="-verbose:gc -Xmx256m -Xms64m -XX:+UseCompressedOops -XX:+HeapDumpOnOutOfMemoryError" scala ParserLoop

and you'll have a heap dump in short order.