From Programming Language Pragmatics, by Scott
To resume a thread that is suspended on a given object, some other
thread must execute the predefined method notify from within a
synchronized statement or method that refers to the same object. Like
wait, notify has no arguments. In response to a notify call, the
language run-time system picks an arbitrary thread suspended on the
object and makes it runnable. If there are no such threads, then the
notify is a no-op. As in Mesa, it may sometimes be appropriate to
awaken all threads waiting in a given object; Java provides a built-in
notifyAll method for this purpose.
If threads are waiting for more than one condition (i.e., if their waits are embedded in dissimilar loops), there is no guarantee that
the “right” thread will awaken. To ensure that an appropriate thread
does wake up, the programmer may choose to use notifyAll instead of
notify. To ensure that only one thread continues after wakeup, the
first thread to discover that its condition has been satisfied must
modify the state of the object in such a way that other awakened
threads, when they get to run, will simply go back to sleep.
Unfortunately, since all waiting threads will end up reevaluating
their conditions every time one of them can run, this “solution” to
the multiple-condition problem can be quite expensive.
When using notifyAll, all the awaken threads will contend to reacquire the lock, but only one can reacquire the lock, then return from wait() and then reevaluate the condition. So why does it say that "all waiting threads will end up reevaluating their conditions every time one of them can run"?
How does the thread, which reacquires the lock and rechecks that the condition become true, "modify the state of the object in such a way that other awakened threads, when they get to run, will simply go back to sleep"?
Thanks.
So why does it say that "all waiting threads will end up reevaluating their conditions every time one of them can run"?
After it will reacquire and release the lock a different thread will aquire it and run. This will continue until they all do that.
How does the thread, which reacquires the lock and rechecks that the condition become true, "modify the state of the object in such a way that other awakened threads, when they get to run, will simply go back to sleep"?
All the threads will have something like:
while (condition) {
wait();
}
The notifyAll() caller will set condition to false before calling it and then the awakened thread will exit the while loop and before it returns and releases it will do:
condition = true;
All the other threads will awaken, check the condition, stay in the while loop and call wait() (go back to sleep).
Additionally, you should use explicit locking mechanism because it allows you to have multiple conditions and condition queues for a single lock, which will enable you to use signal() instead of signalAll(). And that has better performance and less contention.
Condition API
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(); }
}
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
}
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 with sleep(), the OS won’t
even try to schedule your task unless someone calls notify() (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 a yield() 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.
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()
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()
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 { //... }
after the specified amount of time, 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
}
Difference between wait() and sleep()
The fundamental difference is that wait() is non static method of Object and sleep() is a static method of Thread.
The major difference is that 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.
To start a thread again from wait(), you have to call notify() or notifyAll() indefinitely. As for sleep(), the thread gets started definitely after a specified time interval.
Similarities
Both make the current thread go into the Not Runnable state.
Both are native methods.
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:
Call on:
wait(): Call on current thread that hold HandObject Object
sleep(): Call on Thread execute task get beer (is class method so affect on current running thread)
Synchronized:
wait(): when synchronized multi thread access same Object (HandObject) (When need communication between more than one thread (thread execute coding, thread execute get beer) access on same object HandObject )
sleep(): when waiting condition to continue execute (Waiting beer available)
Hold lock:
wait(): release the lock for other object have chance to execute (HandObject is free, you can do other job)
sleep(): keep lock for at least t times (or until interrupt) (My job still not finish, i'm continue hold lock and waiting some condition to continue)
Wake-up condition:
wait(): until call notify(), notifyAll() from object
sleep(): until at least time expire or call interrupt
And the last point is use when as estani indicate:
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 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.
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,
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.
Wait and sleep are two different things:
In sleep() the thread stops working for the specified duration.
In wait() the thread stops working until the object being waited-on is notified, generally by other threads.
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:
wait(): Call on an object; current thread must synchronize on the lock object.
sleep(): Call on a Thread; always currently executing thread.
Synchronized:
wait(): when synchronized multiple threads access same Object one by one.
sleep(): when synchronized multiple threads wait for sleep over of sleeping thread.
Hold lock:
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.
Wake-up condition:
wait(): until call notify(), notifyAll() from object
sleep(): until at least time expire or call interrupt().
Usage:
sleep(): for time-synchronization and;
wait(): for multi-thread-synchronization.
Ref:diff sleep and wait
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",
whereas 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/
wait() Method.
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.
sleep() Method
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.
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.
One potential big difference between sleep/interrupt and wait/notify is that
calling interrupt() during sleep() always throws an exception (e.g. InterruptedException), whereas
calling notify() 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.
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.
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...
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.
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 :
Main : Contains main method and two threads.
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
From oracle documentation page on wait() method of Object:
public final void wait()
Causes the current thread to wait until another thread invokes the 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).
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
interrupts and spurious wakeups are possible
This method should only be called by a thread that is the owner of this object's monitor
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)
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.
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).
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().
The method wait(1000) causes the current thread to sleep up to one second.
A thread could sleep less than 1 second if it receives the notify() or notifyAll() method call.
The call to sleep(1000) causes the current thread to sleep for exactly 1 second.
Also sleeping thread doesn't hold lock any resource. But waiting thread does.
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.
I am in doubt with that , in Java language, we need to acquire the lock, before we await some condition to be satisfied.
For example, int java monitor lock:
synchronized(lock){
System.out.println("before lock ...");
lock.wait();
System.out.println("after lock ...");
}
or the concurrency utils:
Lock lock = new ReentrantLock();
Condition cond = lock.newCondition();
lock.lock();
try{
System.out.println("before condition ...");
cond.await();
System.out.println("after condition ...");
}catch(Exception e){
e.printStackTrace();
}finally{
lock.unlock();
}
So, why we can't await, without hold the lock ?
Does other languages differ, or it's just in Java?
I hope you can explain the reason after the design, but not only for JAVA-SPEC definition.
Imagine you have something that a thread might need to wait for. Maybe you have a queue and a thread needs to wait until there's something on the queue so it can process it. The queue must be thread-safe, so it has to be protected by a lock. You might write the following code:
Acquire the lock.
Check if the queue is empty.
If the queue is empty, wait for the something to be placed on the queue.
Oops, that won't work. We hold the lock on the queue so how can another thread place something on it? Let's try again:
Acquire the lock.
Check if the queue is empty.
If the queue is empty, release the lock and wait for the something to be placed on the queue.
Oops, now we still have a problem. What if after we release the lock but before we wait for something to be placed on the queue, something is placed on the queue? In that case, we will be waiting for something that already happened.
Condition variables exist to solve this exact problem. They have an atomic "unlock and wait" operation that closes this window.
So await must hold the lock because otherwise there would be no way to ensure you weren't waiting for something that already happened. You must hold the lock to prevent another thread from racing with your wait.
Well, what are we waiting for? We are waiting for a condition to become true. Another thread will make the condition true, then notify the waiting threads.
Before entering wait, we must check that the condition is false; this check and the wait must be atomic, i.e. under the same lock. Otherwise, if we enter the wait while the condition is already true, we'll likely never wakeup.
Therefore it is necessary that the lock is already acquired before calling wait()
synchronized(lock)
{
if(!condition)
lock.wait();
If wait() automatically and silently acquires lock, a lot of bugs will go undetected.
Upon wakeup from wait(), we must check the condition again -- there's no guarantee that the condition must become true here (for lots of reasons - spurious wakeup; timeout, interruption, multiple waiters, multiple conditions)
synchronized(lock)
{
if(!condition)
lock.wait();
if(!condition) // check again
...
Typically, if the condition is still false, we'll wait again. Therefore the typical pattern is
while(!condition)
lock.wait();
But there are also cases where we don't want to wait again.
Could there ever be legit use cases where naked wait/notify make sense?
synchronized(lock){ lock.wait(); }
Sure; an application can be made up with naked wait/notify, with well defined behavior; argument can be made that this is the desired behavior; and this is the best implementation for that behavior.
However, that is not the typical usage pattern, and there is no reason to account for it in API design.
See the doc for Condition.
A Condition is like a wait pool or wait set of an object and it replaces the use of the Object monitor methods (wait, notify and notifyAll). Conditions enable one thread to suspend execution (to "wait") until notified by another thread that some state condition may now be true. A Condition instance is intrinsically bound to a lock just like the Object monitor methods require the lock of the shared object to wait or notify on. So before invoking await() on a condition, the thread must have locked the Lock object that is used to produce the condition. When the await() method is invoked, the lock associated with the condition is released.
If the thread were merely waiting for a signal to proceed there are other mechanisms for doing that. Presumably there is some state protected by the lock that the thread is waiting to be operated on and satisfy some condition. To properly protect that state the thread should have the lock before and after waiting for the condition, so it makes sense to require acquisition of the lock.
a sounds-reasonable answer
It is a JVM thing. An Object x has:
an Entry Set: a queue for threads attempting to synchronized(x)
a Waiting Set: a queue for threads called x.wait()
When you call x.wait(), JVM adds your current thread into Waiting Set; when you call x.notify()/x.notifyAll(), JVM removes one/all element from Waiting Set.
Multiple threads may call x.wait()/x.notify()/x.notifyAll() to modify the Waiting Set. In order to ensure the Waiting Set thread safety, JVM accepts only one operation from one thread at one time.
Simple answer is because otherwise you will get IllegalMonitorStateException which is specified in Object.wait javadoc. Internally, synchronization in Java uses underlying OS mechanizm. So it is not only Java.
I'm just trying to further my understanding of this concept.
We have a monitor, let's say a queue or a map of some sort. This monitor has methods to put objects on, and get objects off. In order to be thread safe, the monitor will lock on it's put methods and on it's get methods. When a thread is synchronized to this monitor, it's constantly trying to obtain this monitor's right's so it can proceed with what it needs to do. Does this sound right?
Another question, how does the flow of control work here. Which code is executed once the thread has gained access to the monitor? I'm finding it hard to debug multi-threaded programs with just print statements, it get's really messy and confusing.
public void run(){
try{
synchronized (monitor){
while (monitor is empty){
monitor.wait(); // Does this line pause the thread or the monitor?
}
System.out.println("Done Waiting");
}
System.out.println("Out of the synchronized block");
}
}
Here's the definition from the Java Language Specification:
The Java programming language provides multiple mechanisms for
communicating between threads. The most basic of these methods is
synchronization, which is implemented using monitors. Each object in
Java is associated with a monitor, which a thread can lock or unlock.
Only one thread at a time may hold a lock on a monitor. Any other
threads attempting to lock that monitor are blocked until they can
obtain a lock on that monitor. A thread t may lock a particular
monitor multiple times; each unlock reverses the effect of one lock
operation.
To answer
This monitor has methods to put objects on, and get objects off. In
order to be thread safe, the monitor will lock on it's put methods and
on it's get methods. When a thread is synchronized to this monitor,
it's constantly trying to obtain this monitor's right's so it can
proceed with what it needs to do. Does this sound right?
So you're not interacting with a monitor. A monitor doesn't have a concept of methods. Don't think of it like that. You interact with objects which have monitors. When a thread acquires an object's monitor, it doesn't need to constantly trying to obtain it, it already has it.
Another question, how does the flow of control work here. Which code
is executed once the thread has gained access to the monitor? I'm
finding it hard to debug multi-threaded programs with just print
statements, it get's really messy and confusing.
If execution enters the synchronized block on an object, the currently executing thread has acquired the monitor on the synchronized object, in this case the object referenced by the variable monitor.
I'll assume (thanks to Radiodeaf) that by monitor is empty, you mean your Map object doesn't have any entries.
When you call
monitor.wait();
the current thread releases the monitor on the object referenced by monitor and sleeps until it gets notified.
The javadoc of Object#wait() has more details.
So you will loop on the check for empty and wait if it returns true. We can assume that some other piece of code calls notify() when they put something into the Map.
When the object does get notified, the thread then has to compete to re-acquire the object's monitor. This is obviously necessary so that the thread can be executing inside a synchronized block on the object.
As we know We can call wait method only from synchronized context.
So By saying wait release the lock we mean that once lock is acquired on an object when it is in synchronized context , by calling wait method on same object , it release the lock and allow other thread to work on that object.
Someone at work just asked for the reasoning behind having to wrap a wait inside a synchronized.
Honestly I can't see the reasoning. I understand what the javadocs say--that the thread needs to be the owner of the object's monitor, but why? What problems does it prevent? (And if it's actually necessary, why can't the wait method get the monitor itself?)
I'm looking for a fairly in-depth why or maybe a reference to an article. I couldn't find one in a quick google.
Oh, also, how does thread.sleep compare?
edit: Great set of answers--I really wish I could select more than one because they all helped me understand what was going on.
Lots of good answers here already. But just want to mention here that the other MUST DO when using wait() is to do it in a loop dependent on the condition you are waiting for in case you are seeing spurious wakeups, which in my experience do happen.
To wait for some other thread to change a condition to true and notify:
synchronized(o) {
while(! checkCondition()) {
o.wait();
}
}
Of course, these days, I'd recommend just using the new Condition object as it is clearer and has more features (like allowing multiple conditions per lock, being able to check wait queue length, more flexible schedule/interrupt, etc).
Lock lock = new ReentrantLock();
Condition condition = lock.newCondition();
lock.lock();
try {
while (! checkCondition()) {
condition.await();
}
} finally {
lock.unlock();
}
}
If the object does not own the object monitor when it calls Object.wait(), it will not be able to access the object to setup a notify listener until the the monitor is released. Instead, it will be treated as a thread attempting to access a method on a synchronized object.
Or to put it another way, there is no difference between:
public void doStuffOnThisObject()
and the following method:
public void wait()
Both methods will be blocked until the object monitor is released. This is a feature in Java to prevent the state of an object from being updated by more than one thread. It simply has unintended consequences on the wait() method.
Presumably, the wait() method is not synchronized because that could create situations where the Thread has multiple locks on the object. (See Java Language Specifications/Locking for more info on this.) Multiple locks are a problem because the wait() method will only undo one lock. If the method were synchronized, it would guarantee that only the method's lock would be undone while still leaving a potential outer lock undone. This would create a deadlock condition in the code.
To answer your question on Thread.sleep(), Thread.sleep() does not guarantee that whatever condition you are waiting on has been met. Using Object.wait() and Object.notify() allows a programmer to manually implement blocking. The threads will unblock once a notify is sent that a condition has been met. e.g. A read from disk has finished and data can be processed by the thread. Thread.sleep() would require the programmer to poll if the condition has been met, then fall back to sleep if it has not.
It needs to own the monitor, since the purpose of the wait() is to release the monitor and let other threads obtain the monitor to do processing of their own. The purpose of these methods (wait/notify) is to coordinate access to synchronized code blocks between two threads that require each other to perform some functionality. It is not simply a matter of making sure access to a data structure is threadsafe, but to coordinate events between multiple threads.
A classic example would be a producer/consumer case where one thread pushes data to a queue, and another thread consumes the data. The consuming thread would always require the monitor to access the queue, but would release the monitor once the queue is empty. The producer thread would then only get access to write to the thread when the consumer is no longer processing. It would notify the consumer thread once it has pushed more data into the queue, so it can regain the monitor and access the queue again.
Wait gives up the monitor, so you must have it to give it up. Notify must have the monitor as well.
The main reason why you want to do this is to ensure that you have the monitor when you come back from wait() -- typically, you are using the wait/notify protocol to protect some shared resource and you want it to be safe to touch it when wait returns. The same with notify -- usually you are changing something and then calling notify() -- you want to have the monitor, make changes, and call notify().
If you made a function like this:
public void synchWait() {
syncronized { wait(); }
}
You would not have the monitor when wait returned -- you could get it, but you might not get it next.
Here's my understanding on why the restriction is actually a requirement. I'm basing this on a C++ monitor implementation I made a while back by combining a mutex and a condition variable.
In a mutex+condition_variable=monitor system, the wait call sets the condition variable into a wait state and releases the mutex. The condition variable is shared state, so it needs to be locked to avoid race conditions between threads that want to wait and threads that want to notify. Instead of introducing yet another mutex to lock its state, the existing mutex is used. In Java, the mutex is correctly locked when the about-to-wait thread owns the monitor.
Mostly wait is done if there is a condition say a queue is empty.
If(queue is empty)
queue.wait();
Let us assume the queue is empty.
In case if the current thread pre-empts after checking the queue, then if another
thread adds few elements to queue, the current thread will not know and will go for wait
state. Thats wrong.
So we should have something like
Synchornized(queue)
{
if(queue is empty)
queue.wait();
}
Now let us consider what if they made wait itself as synchronized. As already mentioned in one of the comments, it releases only one lock. That means if wait() was synchronized in the above code only one lock would have been released. Implies that current thread will go for wait with the lock for the queue.