Biased Locking January 9, 2010 Recently I re-did HotSpot's internal locking mechanism for Azul's JVM. The old locking mechanism is approaching 15 years old and features a number of design decisions that are now out-dated: Recursion counts are kept as a NULL word on the stack for every recursion depth (i.e., counting in Base 1 math) in order to save a few instructions and a few bits of memory.