Difference between wait() and sleep()

Question

What is the difference between a wait() and sleep() in Threads?

Is my understanding that a wait()-ing Thread is still in runni

Answer 1

There are a lot of answers here but I couldn't find the semantic distinction mentioned on any.

It's not about the thread itself; both methods are required as they support very different use-cases.

sleep() sends the Thread to sleep as it was before, it just packs the context and stops executing for a predefined time. So in order to wake it up before the due time, you need to know the Thread reference. This is not a common situation in a multi-threaded environment. It's mostly used for time-synchronization (e.g. wake in exactly 3.5 seconds) and/or hard-coded fairness (just sleep for a while and let others threads work).

wait(), on the contrary, is a thread (or message) synchronization mechanism that allows you to notify a Thread of which you have no stored reference (nor care). You can think of it as a publish-subscribe pattern (wait == subscribe and notify() == publish). Basically using notify() you are sending a message (that might even not be received at all and normally you don't care).

To sum up, you normally use sleep() for time-syncronization and wait() for multi-thread-synchronization.

They could be implemented in the same manner in the underlying OS, or not at all (as previous versions of Java had no real multithreading; probably some small VMs doesn't do that either). Don't forget Java runs on a VM, so your code will be transformed in something different according to the VM/OS/HW it runs on.