I doubt it. Needing to zero ultra large chunks of memory often enough that you care about performance to this extent is a very niche scenario. If I had to guess, Java probably just zeroes the memory upon garbage collecting it or before handing it back out; fresh pages from the OS are already zeroed.

But anything is possible. Maybe some big customer used this pattern and now Java detects it and does something fancy like what I suggested. But really, this is an unusual scenario for an application.

There's a brilliantly-named paper on this: Why Nothing Matters: The Impact of Zeroing.

Efficient zeroing is something JVMs put some thought into. Well, with a notable exception, in HotSpot itself: We were able to substantially improve its performance by using memset() to perform the zeroing.

[0] (PDF) http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.225...

There is no canonical answer to that question. Compilers wary wildly in implementation details.