Tuesday, November 14, 2006

More on... How to fix the dreaded "java.lang.OutOfMemoryError: PermGen space" exception (classloader leaks)

I got quite a few comments on my last blog (How to fix the dreaded "java.lang.OutOfMemoryError: PermGen space" exception (classloader leaks)). Apparently more people have been struggling with this problem.

Why bring this up? What's the news? Edward Chou continued to explore options to diagnose classloader leaks. First of all, he explored how to generate a list of orphaned classloaders with jhat. An orphaned classloader is a classloader that is not referenced by any object directly but cannot be garbage collected. The thinking behind this is that programs that create classloaders (e.g. application servers) do maintain references to them. So if there's a classloader that is no longer directly referenced, this classloader is probably a leak. Read about it on his blog (Find Orphaned Classloaders).

Still we were not satisfied: when examining some memory dumps from code that we were not familiar with, we explored yet some other options to diagnose classloader leaks: duplicate classes and duplicate classloaders. Let me explain.

Let's say that your application has a com.xyz.Controller class. If you find many instances of this class object, you likely have a classloader leak. Note the phrase "instances of this class object". What I mean by this: the class com.xyz.Controller is loaded multiple times, i.e. multiple instances of the com.xyz.Controller.class are present.  You can use jhat to run this query: simply list all instances of java.lang.Class.

Edward modified jhat to generate a list of all classloader instances that have an identical set of classes that it loaded. Typically there's no reason why someone would create two classloader instances and load exactly the same set of classes into them. If you find any in your memory dump, you should get suspicious and take a closer look. Monitor Edward's blog for more details on this.

One more thing: Edward found out that the method java.lang.String.intern() allocates memory in PermGen space. So if your application frequently uses this method with different strings, watch out. Fortunately these strings are subject to garbage collection. But if your application holds references to these strings, thereby making garbage collection impossible, your application may cause the dreaded "java.lang.OutOfMemoryError: PermGen space" exception. No classloaders involved this time.

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