Do you mean how is it implemented? If you go through the
Java® Language Specification I think you will find that is not specified. Nor does the
Virtual Machine Specification (try §2.11 and §3.14) give anything definite.
Obviously each object has a monitor which is probably implemented by changing a bit in it from 0 to 1 and
vice versa, but for more details you will have to download something like OpenJDK and go through its source code.
