Thread.sleep() implementation

Question

Today I had an interview on which I asked candidate quite usual and basic question about the difference between Thread.sleep() and Object.wait(). I exp

Answer 1

One could easily say Occam's Razor cuts the other way. The normal/expected implementation of the JVM underlying JDK is assumed to bind java 'threads' onto native threads most of the time, and putting a thread to sleep is a fundamental function of the underlying platform. Why reimplement it in java if thread code is going to be native anyway? The simplest solution is use the function that's already there.

Some other considerations: Uncontested synchronization is negligible in modern JVMs, but this wasn't always so. It used to be a fairly "expensive" operation to acquire that object monitor.

If you implement thread sleeping inside java code, and the way you implement it does not also bind to a native thread wait, the operating system has to keep scheduling that thread in order to run the code that checks if it's time to wake up. As hashed out in the comments, this would obviously not be true for your example on a modern JVM, but it's tough to say 1) what may have been in place and expected at the time the Thread class was first specified that way. and 2) If that assertion works for every platform one may have ever wanted to implement a JVM on.