This is a myth. Or more precisely, a mmaped region no longer in use will be unmapped whenever the JVM feels like it (which can, of course, be never). Try runnin

This is a myth. Or more precisely, a mmaped region no longer in use will be unmapped whenever the JVM feels like it (which can, of course, be never). Try running the loop: long size = 0; while (size >= 0) { size += Files.map(veryBigFile).capacity(); } You will see that VSIZE grows to be very big, but will decrease in size whenever the JVM runs the finalizers for the mmaped regions. Adding System.gc() to the loop makes the VSIZE stay constantly low on my machine, proving that at least my JVM (build 1.6.0_29-b11-402-11D50b on a mac) will unmap the underlying regions (JVM is allowed to ignore System.gc()).