How does Thread.sleep() work when called from multiple threads

Question

sleep() is a static method of class Thread. How does it work when called from multiple threads. and how does it figure out the current thread of execution. ?

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Answer 1

When the virtual machine encounters a sleep(long)-statement, it will interrupt the Thread currently running. "The current Thread" on that moment is always the thread that called Thread.sleep(). Then it says:

Hey! Nothing to do in this thread (Because I have to wait). I'm going to continue an other Thread.

Changing thread is called "to yield". (Note: you can yield by yourself by calling Thread.yield();)

So, it doesn't have to figure out what the current Thread is. It is always the Thread that called sleep(). Note: You can get the current thread by calling Thread.currentThread();

A short example:

// here it is 0 millis
blahblah(); // do some stuff
// here it is 2 millis
new Thread(new MyRunnable()).start(); // We start an other thread
// here it is 2 millis
Thread.sleep(1000);
// here it is 1002 millis

MyRunnable its run() method:

// here it is 2 millis; because we got started at 2 millis
blahblah2(); // Do some other stuff
// here it is 25 millis;
Thread.sleep(300); // after calling this line the two threads are sleeping...
// here it is 325 millis;
... // some stuff
// here it is 328 millis;
return; // we are done;