Does GC collects garbage from Metaspace?

Question

Always I thought that garbage collector clear only heap and now I think so.

In java 8 permGen was deleted and it was replaced by Metaspace.

And As I understood Metaspace is garbage collected(https://stackoverflow.com/a/24075360/2674303)

Who collects garbage from Metaspace?

Answer 1

I think your confusion stems from the colloquial term “garbage collection” which is widely used but doesn’t really describe what happens in a managed environment.

Memory Management is a complex process which is, simplified, about:

• Identifying the objects which are garbage, which is actually a process of determining which objects are reachable (read: not garbage) and consider everything not encountered to be garbage
• Enqueuing object references to reference queues and/or trigger finalization, if necessary
• Reclaiming memory formerly occupied by garbage, which might also be the other way around: sometimes the alive objects are moved to a different memory space instead

So for a memory space not consisting of Java objects, the first two points usually make not much sense which is what your question seems to be about. Algorithms addressing the first two points usually process the Java heap (defined as space containing ordinary Java object instances and similar structured data) only.

The statement you have linked, saying “Metaspace is GCed” seems to address mainly the third point. It’s about the fact that memory within the Metaspace might get reclaimed if not needed anymore. This does not imply that it requires a traversal of live references within the Metaspace or something similar. Obviously, class metadata are obsolete when their associated Class and ClassLoader have become unreachable, which are both ordinary (well, almost) objects living on the Java heap.

So when the Metaspace size reaches a limit, a garbage collection will be triggered, but regarding the first two bullet above, it will not process the Metaspace as it is not the Metaspace which can tell you whether a Class has become unused. It will be an ordinary garbage collection, but it will be a “Full GC” or whatever term the currently used GC algorithm has for the mode that includes collecting garbage within the memory segment (aka “generation”) which contains classes and class loaders.

Once Class and ClassLoader heap instances have been collected, their associated Metaspace data can be reclaimed as well during the cleanup.