But does Java allow easy management of memory, like .NET? From building a moderately high-performance packet-processing system in F#, apart from a few specific encoding parts where the machine code was subpar, the biggest hit to CPU seemed to be the GC. Removing a single allocation from a path provided a measurable improvement. Most of the gain was being able to allocate memory directly (1GB managed heap on top of 16GB+ unmanaged).

With Java, you don't have the unmanaged or struct support, so doesn't that really add up? If you go "native", isn't there significant overhead since you can't have pointers in Java (right - the bytecode doesn't support it?)?

People pull it off, but it seems that GC overhead would be a killer.