Clojure compiler class cache and JVM soft references

A follow-up to the last post when I still thought it would be easy to clear JVM soft references. Spoiler alert, it's not.

A quick background on JVM references:

The reason the JVM has more than one reference type is garbage collection. They are all handled differently when the GC runs, so if you don't care about that, there is no reason to create any of the non-default types.

Strong references

These are the default references if you don't do anything special. As long as there is a strong reference somewhere, the referenced object will not be garbage collected.

Weak references

These exist so you can hold on to some object without owning it. This is useful for a publish/subscribe or caching scenario where you want the objects to be collected if no other code uses them anymore.

Soft references

These work like strong references until the JVM needs the used memory. This is the case when an OutOfMemory Exception would be thrown. Right before that happens the GC runs and soft reference gets collected to hopefully make enough space for the execution to continue.

Phantom references

These work like weak references, except that they don't allow you to actually get the object they reference. The use-case is a custom finalize workflow.

Clojure class cache

The Clojure DynamicClassLoader has a map of all the classes it has currently loaded. It holds those classes in soft references so that they do not get collected unless there is no more memory to hold on to them.

This makes total sense since recompiling those classes is expensive but possible.

This does pose a problem for though if you want to build a tool that determines which classes are actually reachable. As I mentioned above, the only way soft references get cleaned up is when the memory fills up. This means any anonymous function that's not in use anymore still stays around mostly forever.


Blow up your memory

If the only way to get rid of objects in soft references is to fill up the memory, why not just do that?

(defn oom []
(try (let [memKiller (java.util.ArrayList.)]
(loop [free 10000000]
(.add memKiller (object-array free))
(.get memKiller 0)
(recur (if (< (Math/abs (.. Runtime (getRuntime) (freeMemory))) Integer/MAX_VALUE)
(Math/abs (.. Runtime (getRuntime) (freeMemory)))

(catch OutOfMemoryError _
(println "freed"))

This function will produce empty arrays until there is no more space left. Sadly the JVM garbage collector is pretty smart and will sometimes optimize this away when the array is never accessed, so the .get is needed.

But even this does not always work since the GC is being smart again. It will not clean up the soft references if they are too small to help with the OOM exception. This can be fixed by creating even smaller arrays, but this will get pretty slow at some point.

The bigger problem is that your JVM instance will now expand its memory usage until your repl hit's the default 4gig. This is not a very good user experience.

JVM Tools Interface

The JVM TI allows you to get all strong references to an object. So this would be an easy solution, right?

Sadly not every JVM has support for the Tools Interface and even worse, it's hard to use since you have to write native code and connect that to the running JVM.


At this point, there is no good way to find out which functions in the running VM are still in use.

I'd love to be wrong about this, so if anyone has any more ideas I'm very open to suggestions.

Discuss this post here.

Published: 2022-01-07