Difference between sleep() and wait() in Java

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.

2. Java sleep() and wait() – Example

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

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