When and how is a java classloader marked for garbage collection?

We are creating multiple child classloaders to load in multiple subapplications into a Java application "container", prototyping hot deployment. When the classpath of a particular classloader has changed (i.e. jars have been added, deleted, updated), the old classloader is thrown away (unreferenced) and a new classloader is created for the new classpath of jars.

After updating the claspath, triggering the hot deployment, we took a heap dump. The heap dump (using Memory Analyzer) indicates that the old classloaders were not being garbage collected. Certain classes in the parent classloader were caching the old classloaders. The following things were invoked to clear these caches:

java.lang.ResourceBundle.clearCache(classLoader);
org.apache.commons.logging.LogFactory.release(classLoader);
java.beans.Introspector.flushCaches();

Even after clearing the above caches, the old classloader were still not being garbage collected. The remaining references to the classloader included the following:

• the classes loaded by the classloader
• java.lang.Package's created by the classloader itself
• java.lang.ProtectionDomain created by the classloader itself

All the above are circular references within the classloader, which should trigger a garbage collection. I'm not sure why it is not. Does anybody know why the old classloaders are still not being garbage collected even with the circular references?

From stackoverflow

• I always heard that Classloader unloading was problematic. They are theoretically garbage collected when there is not reference to the object instances and class unloading is not necessary, but in practice it seems like to be more problematic. Subtle references may leak and prevent the Classloader from being reclaimed. In application servers, after numerous redeploy cycle, I sometimes got a OutOfMemoryError: PermGen space.

  All that to say that I guess there is a nasty reference somewhere that prevent it from being collected -- maybe the memory analyzer didn't followed the link correctly. It seems like all this can happen, as described in these articles:

  • Classloader leaks: the dreaded PermGen space exception
  • How to fix the dreaded PermGen space exception

  Also, I don't know exactly what you are doing, but if you can wait for JDK 7, you could have a look at AnonymousClassLoader. They will be introduced to better support dynamic language, as explained in this post:

  • A first taste of InvokeDynamic

  I hope it will help you.

  jigjig : One sub-application is very simple. The only third party library it uses is log4j. Nothing else. The rest is the basic JDK API. It also uses the ResourceBundle class which keeps cache. After calling clearCache(), those references did disappear, but it still does not get garbage collected. But Thanks for the answer :)

  jigjig : oh and by the way, great links!

  ewernli : I can only suggest to try with the simplest sub-app ever (say with one class) and then add more and more classes and dependencies to see at which point it stops the GC of the classloader. But I know it can be extremely time-consuming. BTW, Frank Kievet wrote also interesting posts on XA transactions, if you're interested.

  jigjig : @ewernli, you are correct. A nasty reference is preventing it from being collected. However, the Eclipse Memory Analyzer was after all displaying all the references correctly. The actual architecture of the system is bit more complex than I described above, written for the brevity and simplicity of the question. I will provide further details in an answer.

  ewernli : Glad to hear that there is not bug in the GC and it's indeed a leak :)