How to diagnose a Java 8 metaspace leak?

Question

I have a J2EE application with some interesting behavior ... the heap seems to behave well, growing and shrinking with garbage collections as expected over time. There is no app

Answer 1

The main cause for the java.lang.OutOfMemoryError: Metaspace is:

• either too many classes or
• too big classes being loaded to the Metaspace.

If you want to recreate the problem use this code snippet:

public class Metaspace {
static javassist.ClassPool cp = javassist.ClassPool.getDefault();

public static void main(String[] args) throws Exception {
    for (int i = 0; ; i++) { 
        Class c = cp.makeClass("eu.plumbr.demo.Generated" + i).toClass();
    }
  }
}

All those generated class definitions end up consuming Metaspace.

Javaassist in Maven repo.

You can find a lot more about OOME here

Answer 2

Do a heap dump and analyze it with Eclipse MAT. Look at the classes you have loaded. Check if there's something unexpected, especially duplicate classes. It also has a classloader explorer.

Edit: In theory you could also be that you're constantly generating proxies.