What is the difference between a wait() and sleep() in Threads?
Is my understanding that a wait()-ing Thread is still in running mode and uses CPU cycles but a sleep()-ing does not consume any CPU cycles correct?
Why do we have both wait() and sleep()?
How does their implementation vary at a lower level?
A wait can be "woken up" by another thread calling notify on the monitor which is being waited on whereas a sleep cannot. Also a wait (and notify) must happen in a block synchronized on the monitor object whereas sleep does not:
Object mon = ...;
synchronized (mon) {
mon.wait();
}
At this point the currently executing thread waits and releases the monitor. Another thread may do
synchronized (mon) { mon.notify(); }
(on the same mon object) and the first thread (assuming it is the only thread waiting on the monitor) will wake up.
You can also call notifyAll if more than one thread is waiting on the monitor – this will wake all of them up. However, only one of the threads will be able to grab the monitor (remember that the wait is in a synchronized block) and carry on – the others will then be blocked until they can acquire the monitor's lock.
Another point is that you call wait on Object itself (i.e. you wait on an object's monitor) whereas you call sleep on Thread.
Yet another point is that you can get spurious wakeups from wait (i.e. the thread which is waiting resumes for no apparent reason). You should always wait whilst spinning on some condition as follows:
synchronized {
while (!condition) { mon.wait(); }
}
wait/notify are typically used to wait for some other thread to accomplish a task, or to wait until a certain condition is satisfied. –
Statuette One key difference not yet mentioned is that:
sleep() does not release the lock it holds on the Thread,
synchronized(LOCK) {
Thread.sleep(1000); // LOCK is held
}
wait() releases the lock it holds on the object.
synchronized(LOCK) {
LOCK.wait(); // LOCK is not held
}
sleep doesn't have to be used with synchronized(LOCK). The code comparison you did above is a misleading one. –
Guff sleep holds java locks, but it doesn't. To have a fair comparison, we would compare synchronized(OUTER_LOCK){ Thread.sleep(1000); } with synchronized(OUTER_LOCK){ synchronized(LOCK){LOCK.wait();} } and we can see that both instructions don't release the OUTER_LOCK. If there's any difference, we can say that sleep doesn't explicitly use java locks, but the question is asking about quote "how does their implementation vary at a lower level?" unquote. –
Guff wait() is associated with the condition of the inner most lock it's called from, in your code example, wait() can only release LOCK and not OUTER_LOCK. That's how Java monitor is designed anyway. A fair comparison would be synchronized(OUTER_LOCK){ synchronized(LOCK) { Thread.sleep(1000); } } and synchronized(OUTER_LOCK){ synchronized(LOCK) { LOCK.wait(); } }. In this case sleep() will hold both locks while wait() will release LOCK but still hold OUTER_LOCK –
Tiffaneytiffani synchronized(LOCK) is part of wait just like how sle is part of sleep. You simply can't do wait without synchronized(LOCK), so in your example, we see 2 outer locks for thread but only 1 outer lock for wait. –
Guff I found this post helpful. It puts the difference between Thread.sleep(), Thread.yield(), and Object.wait() in human terms. To quote:
It all eventually makes its way down to the OS’s scheduler, which hands out timeslices to processes and threads.
sleep(n)says “I’m done with my timeslice, and please don’t give me another one for at least n milliseconds.” The OS doesn’t even try to schedule the sleeping thread until requested time has passed.
yield()says “I’m done with my timeslice, but I still have work to do.” The OS is free to immediately give the thread another timeslice, or to give some other thread or process the CPU the yielding thread just gave up.
wait()says “I’m done with my timeslice. Don’t give me another timeslice until someone calls notify().” As withsleep(), the OS won’t even try to schedule your task unless someone callsnotify()(or one of a few other wakeup scenarios occurs).Threads also lose the remainder of their timeslice when they perform blocking IO and under a few other circumstances. If a thread works through the entire timeslice, the OS forcibly takes control roughly as if
yield()had been called, so that other processes can run.You rarely need
yield(), but if you have a compute-heavy app with logical task boundaries, inserting ayield()might improve system responsiveness (at the expense of time — context switches, even just to the OS and back, aren’t free). Measure and test against goals you care about, as always.
sleep(n) is implicitly saying that the currently running thread relinquishes the lock's monitor voluntarily, which is not true. Quote from Thread's javadoc:"The thread does not lose ownership of any monitors." –
Stela sleep does not have any special behaviour regarding monitor than any other Java method call, that is, it does not interact or modify them in any way. If you would say something about monitors you should specify that wait will, in addition to the things said above, relinquish temporarily the lock on the object it is called on. –
Soul wait(n) to compare with sleep(n). There's no sense in comparing using the no-arg one. –
Guff 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.
Here, I have listed few important differences between wait() and sleep() methods.
PS: Also click on the links to see library code (internal working, just play around a bit for better understanding).
wait() method releases the lock. wait() is the method of Object class.wait() is the non-static method - public final void wait() throws InterruptedException { //...}
wait() should be notified by notify() or notifyAll() methods.wait() method needs to be called from a loop in order to deal with false alarm.
wait() method must be called from synchronized context (i.e. synchronized method or block), otherwise it will throw IllegalMonitorStateException
sleep() method doesn't release the lock.sleep() is the method of java.lang.Thread class.sleep() is the static method - public static void sleep(long millis, int nanos) throws InterruptedException { //... }sleep() is completed.sleep() better not to call from loop(i.e. see code below).sleep() may be called from anywhere. there is no specific requirement. Ref: Difference between Wait and Sleep
Code snippet for calling wait and sleep method
synchronized(monitor){
while(condition == true){
monitor.wait() //releases monitor lock
}
Thread.sleep(100); //puts current thread on Sleep
}
Thread.sleep() is use for making processor time available to the other threads. the sleep period can be terminated by interrupts(i.e by JVM). Read this #4264855 –
Polito notify() or notifyAll() are Object class methods. hence they are available will obj of all class(i.e. here with Thread class too). see the code grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/… –
Polito wait() is non static method of Object and sleep() is a static method of Thread.wait() releases the lock while sleep() doesn’t release any lock while waiting.wait() is used for inter-thread communication while sleep() is used to introduce a pause on execution, generally.wait() should be called from inside synchronise or else we get an IllegalMonitorStateException, while sleep() can be called anywhere.wait(), you have to call notify() or notifyAll() indefinitely. As for sleep(), the thread gets started definitely after a specified time interval.There are some difference key notes i conclude after working on wait and sleep, first take a look on sample using wait() and sleep():
Example1: using wait() and sleep():
synchronized(HandObject) {
while(isHandFree() == false) {
/* Hand is still busy on happy coding or something else, please wait */
HandObject.wait();
}
}
/* Get lock ^^, It is my turn, take a cup beer now */
while (beerIsAvailable() == false) {
/* Beer is still coming, not available, Hand still hold glass to get beer,
don't release hand to perform other task */
Thread.sleep(5000);
}
/* Enjoy my beer now ^^ */
drinkBeers();
/* I have drink enough, now hand can continue with other task: continue coding */
setHandFreeState(true);
synchronized(HandObject) {
HandObject.notifyAll();
}
Let clarity some key notes:
you normally use sleep() for time-syncronization and wait() for multi-thread-synchronization.
Please correct me if i'm wrong.
This is a very simple question, because both these methods have a totally different use.
The major difference is to wait to release the lock or monitor while sleep doesn't release any lock or monitor while waiting. Wait is used for inter-thread communication while sleep is used to introduce pause on execution.
This was just a clear and basic explanation, if you want more than that then continue reading.
In case of wait() method thread goes in waiting state and it won't come back automatically until we call the notify() method (or notifyAll() if you have more then one thread in waiting state and you want to wake all of those thread). And you need synchronized or object lock or class lock to access the wait() or notify() or notifyAll() methods. And one more thing, the wait() method is used for inter-thread communication because if a thread goes in waiting state you'll need another thread to wake that thread.
But in case of sleep() this is a method which is used to hold the process for few seconds or the time you wanted. Because you don't need to provoke any notify() or notifyAll() method to get that thread back. Or you don't need any other thread to call back that thread. Like if you want something should happen after few seconds like in a game after user's turn you want the user to wait until the computer plays then you can mention the sleep() method.
And one more important difference which is asked often in interviews: sleep() belongs to Thread class and wait() belongs to Object class.
These are all the differences between sleep() and wait().
And there is a similarity between both methods: they both are checked statement so you need try catch or throws to access these methods.
I hope this will help you.
source : http://www.jguru.com/faq/view.jsp?EID=47127
Thread.sleep()sends the current thread into the "Not Runnable" state for some amount of time. The thread keeps the monitors it has aquired -- 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 callst.interrupt()it will wake up the sleeping thread.Note that 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.
t.suspend()is deprecated. Using it is possible to halt a thread other than the current thread. A suspended thread keeps all its monitors and since this state is not interruptable it is deadlock prone.
object.wait()sends the current thread into the "Not Runnable" state, likesleep(), but with a twist. Wait is called on an object, not a thread; we call this object the "lock object." Beforelock.wait()is called, the current thread must synchronize on the lock object;wait()then releases this lock, and adds the thread to the "wait list" associated with the lock. Later, another thread can synchronize on the same lock object and calllock.notify(). This wakes up the original, waiting thread. Basically,wait()/notify()is likesleep()/interrupt(), only the active thread does not need a direct pointer to the sleeping thread, but only to the shared lock object.
Wait and sleep are two different things:
sleep() the thread stops working for the specified duration.wait() the thread stops working until the object being waited-on is notified, generally by other threads.sleep, if you want it to stop until some input comes from the other you use wait/notify. interrupt is intended as a way to signal a thread that it should stop doing what it is doing and terminate. It is handled by sleep, wait but also blocking I/O functions (and you could implement functions with the same behaviour by calling the method Thread.interrupted()). As for the performance, functions are usually optimised for the goal they were designed. –
Soul sleep is a method of Thread, wait is a method of Object, so wait/notify is a technique of synchronizing shared data in Java (using monitor), but sleep is a simple method of thread to pause itself.
sleep() is a method which is used to hold the process for few seconds or the time you wanted 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 any 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. Note that 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.
object.wait() sends the current thread into the “Not Runnable” state, like sleep(), but with a twist. Wait is called on an object, not a thread; we call this object the “lock object.” Before lock.wait() is called, the current thread must synchronize on the lock object; wait() then releases this lock, and adds the thread to the “wait list” associated with the lock. Later, another thread can synchronize on the same lock object and call lock.notify(). This wakes up the original, waiting thread. Basically, wait()/notify() is like sleep()/interrupt(), only the active thread does not need a direct pointer to the sleeping thread, but only to the shared lock object.
synchronized(LOCK) {
Thread.sleep(1000); // LOCK is held
}
synchronized(LOCK) {
LOCK.wait(); // LOCK is not held
}
Let categorize all above points :
Call on:
Synchronized:
Hold lock:
Wake-up condition:
Usage:
In simple words, wait is wait Until some other thread invokes you whereas sleep is "dont execute next statement" for some specified period of time.
Moreover sleep is static method in Thread class and it operates on thread, whereas wait() is in Object class and called on an object.
Another point, when you call wait on some object, the thread involved synchronize the object and then waits. :)
wait and sleep methods are very different:
sleep has no way of "waking-up",wait has a way of "waking-up" during the wait period, by another thread calling notify or notifyAll.Come to think about it, the names are confusing in that respect; however sleep is a standard name and wait is like the WaitForSingleObject or WaitForMultipleObjects in the Win API.
From this post : http://javaconceptoftheday.com/difference-between-wait-and-sleep-methods-in-java/
1) The thread which calls wait() method releases the lock it holds.
2) The thread regains the lock after other threads call either notify() or notifyAll() methods on the same lock.
3) wait() method must be called within the synchronized block.
4) wait() method is always called on objects.
5) Waiting threads can be woken up by other threads by calling notify() or notifyAll() methods.
6) To call wait() method, thread must have object lock.
1) The thread which calls sleep() method doesn’t release the lock it holds.
2) sleep() method can be called within or outside the synchronized block.
3) sleep() method is always called on threads.
4) Sleeping threads can not be woken up by other threads. If done so, thread will throw InterruptedException.
5) To call sleep() method, thread need not to have object lock.
One potential big difference between sleep/interrupt and wait/notify is that
interrupt() during sleep() always throws an exception (e.g. InterruptedException), whereasnotify() during wait() does not.Generating an exception when not needed is inefficient. If you have threads communicating with each other at a high rate, then it would be generating a lot of exceptions if you were calling interrupt all the time, which is a total waste of CPU.
Here wait() will be in the waiting state till it notify by another Thread but where as sleep() will be having some time..after that it will automatically transfer to the Ready state...
wait() is a method of Object class.
sleep() is a method of Thread class.
sleep() allows the thread to go to sleep state for x milliseconds.
When a thread goes into sleep state it doesn’t release the lock.
wait() allows thread to release the lock and goes to suspended state.
This thread will be active when a notify() or notifAll() method is
called for the same object.
You are correct - Sleep() causes that thread to "sleep" and the CPU will go off and process other threads (otherwise known as context switching) wheras I believe Wait keeps the CPU processing the current thread.
We have both because although it may seem sensible to let other people use the CPU while you're not using it, actualy there is an overhead to context switching - depending on how long the sleep is for, it can be more expensive in CPU cycles to switch threads than it is to simply have your thread doing nothing for a few ms.
Also note that sleep forces a context switch.
Also - in general it's not possible to control context switching - during the Wait the OS may (and will for longer waits) choose to process other threads.
interrupt. The ending time is n in wait(n). ¶¶ It's been 8 years now and still no one has the answer ! –
Guff The methods are used for different things.
Thread.sleep(5000); // Wait until the time has passed.
Object.wait(); // Wait until some other thread tells me to wake up.
Thread.sleep(n) can be interrupted, but Object.wait() must be notified.
It's possible to specify the maximum time to wait: Object.wait(5000) so it would be possible to use wait to, er, sleep but then you have to bother with locks.
Neither of the methods uses the cpu while sleeping/waiting.
The methods are implemented using native code, using similar constructs but not in the same way.
Look for yourself: Is the source code of native methods available? The file /src/share/vm/prims/jvm.cpp is the starting point...
Thread.sleep(big_num) must be interrupted. Object.wait(small_num) can be notified. –
Guff Wait() and sleep() Differences?
Thread.sleep() Once its work completed then only its release the lock to everyone. until its never release the lock to anyone.
Sleep() take the key, its never release the key to anyone, when its work completed then only its release then only take the key waiting stage threads.
Object.wait() When its going to waiting stage, its will be release the key and its waiting for some of the seconds based on the parameter.
For Example:
you are take the coffee in yours right hand, you can take another anyone of the same hand, when will your put down then only take another object same type here. also. this is sleep() you sleep time you didn't any work, you are doing only sleeping.. same here also.
wait(). when you are put down and take another one mean while you are waiting , that's wait
you are play movie or anything in yours system same as player you can't play more than one at a time right, thats its here, when you close and choose another anyone movie or song mean while is called wait
wait releases the lock and sleep doesn't. A thread in waiting state is eligible for waking up as soon as notify or notifyAll is called. But in case of sleep the thread keeps the lock and it'll only be eligible once the sleep time is over.
InterruptedException is thrown, just as it says in the Javadoc. –
Attested sleep() method causes the current thread to move from running state to block state for a specified time. If the current thread has the lock of any object then it keeps holding it, which means that other threads cannot execute any synchronized method in that class object.
wait() method causes the current thread to go into block state either for a specified time or until notify, but in this case the thread releases the lock of the object (which means that other threads can execute any synchronized methods of the calling object.
In my opinion, the main difference between both mechanisms is that sleep/interrupt is the most basic way of handling threads, whereas wait/notify is an abstraction aimed to do thread inter-communication easier. This means that sleep/interrupt can do anything, but that this specific task is harder to do.
Why is wait/notify more suitable? Here are some personal considerations:
It enforces centralization. It allows to coordinate the communication between a group of threads with a single shared object. This simplifies the work a lot.
It enforces synchronization. Because it makes the programmer wrap the call to wait/notify in a synchronized block.
It's independent of the thread origin and number. With this approach you can add more threads arbitrarily without editing the other threads or keeping a track of the existing ones. If you used sleep/interrupt, first you would need to keep the references to the sleeping threads, and then interrupt them one by one, by hand.
An example from the real life that is good to explain this is a classic restaurant and the method that the personnel use to communicate among them: The waiters leave the customer requests in a central place (a cork board, a table, etc.), ring a bell, and the workers from the kitchen come to take such requests. Once that there is any course ready, the kitchen personnel ring the bell again so that the waiters are aware and take them to the customers.
Example about sleep doesn’t release lock and wait does
Here there are two classes :
Singleton : This is singleton class with two static methods getInstance() and getInstance(boolean isWait).
public class Main {
private static Singleton singletonA = null;
private static Singleton singletonB = null;
public static void main(String[] args) throws InterruptedException {
Thread threadA = new Thread() {
@Override
public void run() {
singletonA = Singleton.getInstance(true);
}
};
Thread threadB = new Thread() {
@Override
public void run() {
singletonB = Singleton.getInstance();
while (singletonA == null) {
System.out.println("SingletonA still null");
}
if (singletonA == singletonB) {
System.out.println("Both singleton are same");
} else {
System.out.println("Both singleton are not same");
}
}
};
threadA.start();
threadB.start();
}
}
and
public class Singleton {
private static Singleton _instance;
public static Singleton getInstance() {
if (_instance == null) {
synchronized (Singleton.class) {
if (_instance == null)
_instance = new Singleton();
}
}
return _instance;
}
public static Singleton getInstance(boolean isWait) {
if (_instance == null) {
synchronized (Singleton.class) {
if (_instance == null) {
if (isWait) {
try {
// Singleton.class.wait(500);//Using wait
Thread.sleep(500);// Using Sleep
System.out.println("_instance :"
+ String.valueOf(_instance));
} catch (InterruptedException e) {
e.printStackTrace();
}
}
_instance = new Singleton();
}
}
}
return _instance;
}
}
Now run this example you will get below output :
_instance :null
Both singleton are same
Here Singleton instances created by threadA and threadB are same. It means threadB is waiting outside until threadA release it’s lock.
Now change the Singleton.java by commenting Thread.sleep(500); method and uncommenting Singleton.class.wait(500); . Here because of Singleton.class.wait(500); method threadA will release all acquire locks and moves into the “Non Runnable” state, threadB will get change to enter in synchronized block.
Now run again :
SingletonA still null
SingletonA still null
SingletonA still null
_instance :com.omt.sleepwait.Singleton@10c042ab
SingletonA still null
SingletonA still null
SingletonA still null
Both singleton are not same
Here Singleton instances created by threadA and threadB are NOT same because of threadB got change to enter in synchronised block and after 500 milliseconds threadA started from it’s last position and created one more Singleton object.
Should be called from synchronized block : wait() method is always called from synchronized block i.e. wait() method needs to lock object monitor before object on which it is called. But sleep() method can be called from outside synchronized block i.e. sleep() method doesn’t need any object monitor.
IllegalMonitorStateException : if wait() method is called without acquiring object lock than IllegalMonitorStateException is thrown at runtime, but sleep() method never throws such exception.
Belongs to which class : wait() method belongs to java.lang.Object class but sleep() method belongs to java.lang.Thread class.
Called on object or thread : wait() method is called on objects but sleep() method is called on Threads not objects.
Thread state : when wait() method is called on object, thread that holded object’s monitor goes from running to waiting state and can return to runnable state only when notify() or notifyAll() method is called on that object. And later thread scheduler schedules that thread to go from from runnable to running state.
when sleep() is called on thread it goes from running to waiting state and can return to runnable state when sleep time is up.
When called from synchronized block : when wait() method is called thread leaves the object lock. But sleep() method when called from synchronized block or method thread doesn’t leaves object lock.
For More Reference
wait() is given inside a synchronized method
whereas sleep() is given inside a non-synchronized method because wait() method release the lock on the object but sleep() or yield() does release the lock().
sleep() can be inside a synchronized block or method. Answer explains nothing. –
Attested wait(1000) causes the current thread to sleep up to one second.
notify() or notifyAll() method call.sleep(1000) causes the current thread to sleep for exactly 1 second.
sleep(1000) doesn't guaranty to sleep for exactly 1 second. It may may be interrupted before. –
Daleth Actually, all this is clearly described in Java docs (but I realized this only after reading the answers).
http://docs.oracle.com/javase/8/docs/api/index.html :
wait() - The current thread must own this object's monitor. The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object's monitor to wake up either through a call to the notify method or the notifyAll method. The thread then waits until it can re-obtain ownership of the monitor and resumes execution.
sleep() - Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds, subject to the precision and accuracy of system timers and schedulers. The thread does not lose ownership of any monitors.
wait() with a timeout value can wakeup upon timeout value elapsed or notify whichever is earlier (or interrupt as well),
whereas, a sleep() wakes up on timeout value elapsed or interrupt whichever is earlier. wait() with no timeout value will wait for ever until notified or interrupted.
From oracle documentation page on wait() method of Object:
public final void wait()
notify() method or the notifyAll() method for this object. In other words, this method behaves exactly as if it simply performs the call wait(0).This method throws
IllegalMonitorStateException - if the current thread is not the owner of the object's monitor.
InterruptedException - if any thread interrupted the current thread before or while the current thread was waiting for a notification. The interrupted status of the current thread is cleared when this exception is thrown.
From oracle documentation page on sleep() method of Thread class:
public static void sleep(long millis)
This method throws:
IllegalArgumentException - if the value of millis is negative
InterruptedException - if any thread has interrupted the current thread. The interrupted status of the current thread is cleared when this exception is thrown.
Other key difference:
wait() is a non-static method (instance method) unlike static method sleep() (class method).
Lets assume you are hearing songs.
As long as the current song is running, the next song wont play, i.e Sleep() called by next song
If you finish the song it will stop and until you select play button(notify()) it wont play, i.e wait() called by current song.
In this both cases songs going to Wait states.
wait() must surround with synchronized，when the thread entry the synchronized block or method, it means get the lock，so it not block，invoke wait mean release the lock，so it exit the synchronized then block, Thread.sleep() causes the current thread to suspend execution for a specified period.The thread does not lose ownership of any monitors
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