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Difference between sleep() and wait() in Java

By | Filed Under: Multi Threading

Learn the differences between sleep() and wait() methods in Java. Learn when to use which method and what effect they bring in Java concurrency.

1. Java sleep() and wait() – Discussion

sleep() is a method which is used to pause the process for few seconds or the time we want to. But in case of wait() method, thread goes in waiting state and it won’t come back automatically until we call the notify() or notifyAll().

The major difference is that wait() releases the lock or monitor while sleep() doesn’t releases the lock or monitor while waiting. wait() is used for inter-thread communication while sleep() is used to introduce pause on execution, generally.

Thread.sleep() sends the current thread into the “Not Runnable” state for some amount of time. The thread keeps the monitors it has acquired — i.e. if the thread is currently in a synchronized block or method no other thread can enter this block or method. If another thread calls t.interrupt(). it will wake up the sleeping thread.

While sleep() is a static method which means that it always affects the current thread (the one that is executing the sleep method). A common mistake is to call t.sleep() where t is a different thread; even then, it is the current thread that will sleep, not the t thread.

Read more : Working with wait() and notify()

2. Java sleep() and wait() – Example

synchronized(LOCK) {   
    Thread.sleep(1000); // LOCK is held
}

synchronized(LOCK) 
{   
    LOCK.wait(); // LOCK is not held
}

Read more : Difference between yield() and join()

3. Java sleep() vs wait() – Summary

Let categorize all above points in short to remember.

3.1. Method called on

  • wait() – Call on an object; current thread must synchronize on the lock object.
  • sleep() – Call on a Thread; always currently executing thread.

3.2. Synchronized

  • wait() – when synchronized multiple threads access same Object one by one.
  • sleep() – when synchronized multiple threads wait for sleep over of sleeping thread.

3.3. Lock duration

  • wait() – release the lock for other objects to have chance to execute.
  • sleep() – keep lock for at least t times if timeout specified or somebody interrupt.

3.4. wake up condition

  • wait() – until call notify(), notifyAll() from object
  • sleep() – until at least time expire or call interrupt().

3.5. Usage

  • sleep() – for time-synchronization
  • wait() – for multi-thread-synchronization.

Hope above information will add some value in your knowledge-base.

Happy Learning !!

Thread sleep method Java doc
Object wait() method Java doc

Feedback, Discussion and Comments

  1. which one hits a performance ? sleep or wait ?

    Reply

  2. Awesome explanation. Thanks keep up the good work and knowledge sharing.

    Reply

  3. Great tutorial, thanks!

    Reply

  4. Very informative article. Thanks

    Reply

  5. very informative one…thank you

    Reply

  6. Good Article..Really useful

    Reply

  7. Hi Lokesh,

    many thanks for your valuable posts .

    The only site I refer for any java queries is this !!!!!!!!!!

    Reply

  8. Hi Lokesh,
    Nice article. Please explain/add yield() and join() method to this article as sometime this 4 api looks similar yet so diffrent.

    Reply

  9. I have found the article very clear and explanatory.

    Reply

  10. you write in easy way…really you are doing good job ..james gosling invented java and lokesh is making easyjava :)

    Reply

  11. Hi.. I have a question!!!
    Why wait(), notift(),notifyAll() always works in Synchronized Block?

    Reply

    • Puneet, because it was designed in this way only. These methods were introduced as means of making safe interactions between threads. If they were allowed without synchronization, there were occur lots of other problems such as race condition, data corruption and even program crash.

      Please note that a program does not get any kind of surety from low level OS thread scheduling, so it depends on language runtime to manage its thread.

      Reply

      • Thanks for the answer Lokesh..
        I have 1 more question!!!
        why wait(), notift(),notifyAll() are the methods of Object class, insteed of Thread Class?

        Reply

  12. Lokesh, What happens if someone dosent call notify( earlier thread called wait) ?

    will this keep the program in hanged state? will it not allow program to exit/terminate?

    Reply

    • Tricky question.. In my view, no major impact on application just because one of several hundreds thread is blocked somewhere. Rest of application will run fine. This is very much clear and easy.

      No, program will not hang. According to theory, it should not allow to terminate and you might need to do it forcefully.

      Reply

  13. Good points you are covering here, it will be more helpful if you take up the relatively new topics like concurrent package, annotations, generics, nio etc

    Reply

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