What does JVM flag CMSClassUnloadingEnabled actually do?

Question

Hi

I cannot for the life of me find a definition of what the Java VM flag CMSClassUnloadingEnabled actually does, other than some very fuzzy high-level definitions such as "gets rid of your PermGen problems" (which it doesn't, btw).

I have looked on Sun's/Oracle's site, and even the options list doesn't actually say what it does.

Based upon the name of the flag, I'm guessing that the CMS Garbage Collector doesn't by default unload classes, and this flag turns it on - but I can't be sure.

Thanks in advance for any details about what it actually does.

Rich

Answer 1

The standard Oracle/Sun VM look on the world is: Classes are forever. So once loaded, they stay in memory even if no one cares anymore. This usually is no problem since you don't have that many purely "setup" classes (= used once for setup and then never again). So even if they take up 1MB, who cares.

But lately, we have languages like Groovy, that define classes at runtime. Every time you run a script, one (or more) new classes are created and they stay in PermGen forever. If you're running a server, that means you have a memory leak.

If you enable CMSClassUnloadingEnabled the GC will sweep PermGen, too, and remove classes which are no longer used.

Answer 2

According to this link, it determines if class unloading is enabled under the CMS garbage collector. The default is false. There is another option called ClassUnloading that is true by default which (presumably) affects the other garbage collectors.

The idea is that if the GC detects that a previously loaded class is no longer used anywhere in the JVM, it can reclaim the memory used to hold the classes bytecode and/or native code.

Setting CMSClassUnloadingEnabled might help with your permgen problem if you are currently using the CMS compiler. But the chances are that you are not using the CMS, or that you have a genuine classloader related memory leak. In the latter case, your class will never appear to the GC to be unused.