Is the garbage collector guaranteed to run before Out of Memory Error?

Question

In case the heap is full, the JVM throws an OutOfMemoryError. But is it assured that a (full) garbage collection always takes place before such an exception is thrown?

This would mean that the memory is full only with strong referenced objects (or reachable by GC Roots) when the exception is thrown.

Edit: Assume the Sun JVM - HotSpot is in discussion.

Answer 1

The garbage collector will usually be run before an OutOfMemoryError is thrown. However you might get an OOME without a GC if you

• try to create a very large object (e.g. larger than the heap)
• start a thread and there is not enough virtual memory or resources to start the thread.
• older versions of Java would throw this error if you reached your maximum direct memory.

Answer 2

The Java Machine Specification states in section 6.3 (emphasis mine):

OutOfMemoryError: The Java virtual machine implementation has run out of either virtual or physical memory, and the automatic storage manager was unable to reclaim enough memory to satisfy an object creation request.

So the JVM does give a guarantee that it will try what it can to free up memory through garbage collection before it throws an OOME.