When does a sleeping thread continue its execution in Java?

Question

Let's say I have 2 threads a and b running inside the same process. The processor runs a few instructions from a, a few from b and so on until it reaches a line of code like this: Thread.sleep(1000).

The problem is that I don't really understand what the processor will do next. I suspect these 2 scenerios:

1)

• Thread a starts to sleep for 1000 miliseconds
• meanwhile b is running
• the 1000 milisecond interval is over so:
  • if the proccessor still runs code from Thread b
    • then wait until it finishes
    • and run more code from Thread a
  • else
    • run more code from Thread a

2)

• Thread a starts to sleep for 1000 miliseconds
• meanwhile b is running

• the 1000 milisecond interval is over so:

  • if the proccessor still runs code from Thread b

    • STOP THREAD B. since Thread a has higher priority and it's code must be run IMMEDIATELY after the 1000 miliseconds interval it's finished
    • and run more code from Thread a
    • THEN run code from Thread B from where we left.
  • else
    • run more code from Thread a

Which one resembles what's actually going on behind the scenes? If both are wrong then please indicate me the correct answer.

Answer 1

A lot of factors come into play, so it's tough to provide an absolute answer for your question.

But based on the available options you have given, #2 will be closest.

A different thread is invoked to run in case one of the following events occur:

....
A thread with a higher priority than the thread currently running enters the Runnable state. The lower priority thread is preempted and the higher priority thread is scheduled to run.

Here's the source.