Reproduce a race-condition in WebappClassLoader

Please see ReproduceTest

ReproduceTest.shouldProvokeRaceConditionIn_findResourceInternalFromJars concurrently calls WebappClassLoader.addJar(), WebappClassLoader.findResource() and WebappClassLoader.closeJARs().

The exceptions thrown by findResource() (written to the console / System.out) show that findResource() and closeJARs() are not properly synchronized with each other.

Please note: The occurrence of this problem strongly depends on the threads timings and thus on the executing machine. Basically, this works on my machine; you might have to change the timings by playing around with the Thread.sleep() intervals to provoke the issue.

Proposed Solution

closeJARs() (and some other methods) synchronizes on the field jarFilesLock. findResource()'s helper method findClassInternal() synchronizes on jarFiles and is the only method that does that.

I think it's very likely that it was meant to synchronize on jarFilesLock, too.

Additionally IntelliJ warns:

Synchronization on a non-final field 'jarFiles' Reports synchronized statements where the lock expression is a reference to a non-final field. Such statements are unlikely to have useful semantics, as different threads may be locking on different objects even when operating on the same object.

As e.g. closeJARs() mutates the jarFiles array, generally all accesses to it should synchronize with jarFilesLock.