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

44
votes

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 classpath, 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?

1
javagarbage-collectionclassloadermemory-leaks
Which JVM do you use (exact version)? Do you use any JVM options, that might affect classloading? Do you use any stuff from Sun's own implementations? Does the application manipulate byte code? ... What's the environment, that might affect class loading? - cafebabe
Not related to your main question, but did you consider something like OSGi instead of doing your own framework that supports hot deployment? - SteveD
@bfoo In our tests, we are using Java 6. No JVM options. We are not using Sun's impl of anything in the simplest case. No byte code manipulation. - onejigtwojig
@stevendick Yes we considered OSGi, but OSGI seemed to be overkill for our use case. The main hurdle to overcome was to avoid conflicts of third party libraries used by different sub-applications. A subapplication would use a completely equivalent libraries but in different classloaderes to avoid any potential conflicts and complexities. That does sound like it would use too much memory... Have you used OSGi? And if so, what are some of hurdles you faced? - onejigtwojig
To be clear, with usage of Sun implementations I mean to e.g. use socket communication with serialization, like RMI / HTTP/SOAP calls et cetera. Those implementations might use caching methodologies e.g. for serialization issues. This way, your classes (to be unloaded) are referenced by ClassLoaders above and/or aside the ClassLoader you like to have purged. Those implementations might use strong references to the classes in order to ensure serialization would not raise system/VM IO issues. - cafebabe

1 Answers

19
votes

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:

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:

I hope it will help you.