为什么webappclassloader不是垃圾收集？

Question

I have a web service developped by spring/hibernate, and when I do deployment and undeployment on Tomcat 7 many times, I get an outOfMemoryError PermGen Memory leak.

By using java VisualVM, I noticed that the previous webappClassLoader are still kept in JVM, so they are not garbage collected.

What can be the reasons of this problem ?

How can I detect which objects still have reference to the application class loader or any object loaded by it ?

Thanks,

Answer 1

The short answer is that you have a memory leak. From experience, this is most likely to be in your web application or a library it is using. It is also possible, but unlikely, that you have found a memory leak in Tomcat.

The short version for tracking down the memory leak is:

• Start Tomcat
• Undeploy and redeploy the problematic application once
• Use a profiler to examine the heap
• Look for an instances of the WebappClassLoader
• Find the one that is strongly held but has an attribute of started = false
• Trace the GC roots for that instance of the WebappClassLoader
• That will point to your memory leak

Finding the root cause of the memory leak might be a little harder.

For a fuller explanation of how to track down this sort of memory leak and what might cause them see this presentation: http://people.apache.org/~markt/presentations/2010-08-05-Memory-Leaks-JavaOne-60mins.pdf