Java notify vs notifyAll all over again

Question

If one Googles for  difference between notify   and notifyAll    then a lot of explanations will pop up  leaving apart the javadoc paragraphs   It all boils down to the number of waiting threads being waken up  one in notify   and all in notifyAll     However  if I do understand the difference between these methods right   only one thread is always selected for further monitor acquisition  in the first case the one selected by the VM  in the second case the one selected by the system thread scheduler  The exact selection procedures for both of them  in the general case  are not known to the programmer   What s the useful difference between notify   and notifyAll   then  Am I missing something

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I would like to mention what is explained in Java Concurrency in Practice:

First point, whether Notify or NotifyAll?

It will be NotifyAll, and reason is that it will save from signall hijacking.

If two threads A and B are waiting on different condition predicates of same condition queue and notify is called, then it is upto JVM to which thread JVM will notify.

Now if notify was meant for thread A and JVM notified thread B, then thread B will wake up and see that this notification is not useful so it will wait again. And Thread A will never come to know about this missed signal and someone hijacked it's notification.

So, calling notifyAll will resolve this issue, but again it will have performance impact as it will notify all threads and all threads will compete for same lock and it will involve context switch and hence load on CPU. But we should care about performance only if it is behaving correctly, if it's behavior itself is not correct then performance is of no use.

This problem can be solved with using Condition object of explicit locking Lock, provided in jdk 5, as it provides different wait for each condition predicate. Here it will behave correctly and there will not be performance issue as it will call signal and make sure that only one thread is waiting for that condition

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This answer is a graphical rewriting and simplification of the excellent answer by xagyg  including comments by eran   Why use notifyAll  even when each product is intended for a single consumer   Consider producers and consumers  simplified as follows   Producer   while   empty       wait      on full   put   notify     Consumer   while  empty       wait      on empty   take   notify     Assume 2 producers and 2 consumers  sharing a buffer of size 1  The following picture depicts a scenario leading to a deadlock  which would be avoided if all threads used notifyAll   Each notify is labeled with the thread being woken up

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When you call the wait   of the  object  expecting the object lock is acquired  intern this will release the lock on that object and help s the other threads to have lock on this  object   in this scenario there will be more than 1 thread waiting for the  resource object  considering the other threads also issued the wait on the same above object and down the way there will be a thread that fill the resource object and invokes notify notifyAll     Here when you issue the notify of the same object from the same other side of the process code  this will release a blocked and waiting single thread  not all the waiting threads  -- this released thread will be picked by JVM Thread Scheduler and all the lock obtaining process on the object is same as regular    If you have Only one thread that will be sharing working on this object   it is ok to use the notify   method alone in your wait-notify implementation   if you are in situation where more than one thread read s and writes on resources object  based on your business logic then you should go for notifyAll    now i am looking how exactly the jvm is identifying and breaking the waiting thread when we issue notify   on a object

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While there are some solid answers above  I am surprised by the number of confusions and misunderstandings I have read  This probably proves the idea that one should use java util concurrent as much as possible instead of trying to write their own broken concurrent code  Back to the question  to summarize  the best practice today is to AVOID notify   in ALL situations due to the lost wakeup problem  Anyone who doesn t understand this should not be allowed to write mission critical concurrency code  If you are worried about the herding problem  one safe way to achieve waking one thread up at a time is to   Build an explicit waiting queue for the waiting threads  Have each of the thread in the queue wait for its predecessor  Have each thread call notifyAll   when done   Or you can use Java util concurrent    which have already implemented this

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Here is an example  Run it  Then change one of the notifyAll   to notify   and see what happens   ProducerConsumerExample class  public class ProducerConsumerExample        private static boolean Even   true      private static boolean Odd   false       public static void main String   args            Dropbox dropbox   new Dropbox             new Thread new Consumer Even  dropbox    start             new Thread new Consumer Odd  dropbox    start             new Thread new Producer dropbox    start              Dropbox class  public class Dropbox        private int number      private boolean empty   true      private boolean evenNumber   false       public synchronized int take final boolean even            while  empty    evenNumber    even                try                   System out format   s is waiting      n   even    Even     Odd                    wait                  catch  InterruptedException e                        System out format   s took  d  n   even    Even     Odd   number           empty   true          notifyAll             return number             public synchronized void put int number            while   empty                try                   System out println  Producer is waiting                        wait                  catch  InterruptedException e                        this number   number          evenNumber   number   2    0          System out format  Producer put  d  n   number           empty   false          notifyAll              Consumer class  import java util Random   public class Consumer implements Runnable        private final Dropbox dropbox      private final boolean even       public Consumer boolean even  Dropbox dropbox            this even   even          this dropbox   dropbox             public void run             Random random   new Random            while  true                dropbox take even               try                   Thread sleep random nextInt 100                  catch  InterruptedException e                          Producer class  import java util Random   public class Producer implements Runnable        private Dropbox dropbox       public Producer Dropbox dropbox            this dropbox   dropbox             public void run             Random random   new Random            while  true                int number   random nextInt 10               try                   Thread sleep random nextInt 100                    dropbox put number                 catch  InterruptedException e

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Short summary   Always prefer notifyAll   over notify   unless you have a massively parallel application where a large number of threads all do the same thing   Explanation      notify         wakes up a single   thread  Because notify   doesn t allow you to specify the thread that is   woken up  it is useful only in massively parallel applications     that   is  programs with a large number of threads  all doing similar chores    In such an application  you don t care which thread gets woken up    source  https   docs oracle com javase tutorial essential concurrency guardmeth html  Compare notify   with notifyAll   in the above described situation  a massively parallel application where threads are doing the same thing  If you call notifyAll   in that case  notifyAll   will induce the waking up  i e  scheduling  of a huge number of threads  many of them unnecessarily  since only one thread can actually proceed  namely the thread which will be granted the monitor for the object wait    notify    or notifyAll   was called on   therefore wasting computing resources   Thus  if you don t have an application where a huge number of threads do the same thing concurrently  prefer notifyAll   over notify    Why  Because  as other users have already answered in this forum  notify        wakes up a single thread that is waiting on this object s monitor        The   choice is arbitrary and occurs at the discretion of the   implementation    source  Java SE8 API  https   docs oracle com javase 8 docs api java lang Object html notify--   Imagine you have a producer consumer application where consumers are ready  i e  wait   ing  to consume  producers are ready  i e  wait   ing  to produce and the queue of items  to be produced   consumed  is empty  In that case  notify   might wake up only consumers and never producers because the choice who is waken up is arbitrary  The producer consumer cycle wouldn t make any progress although producers and consumers are ready to produce and consume  respectively  Instead  a consumer is woken up  i e  leaving the wait   status   doesn t take an item out of the queue because it s empty  and notify   s another consumer to proceed   In contrast  notifyAll   awakens both producers and consumers  The choice who is scheduled depends on the scheduler  Of course  depending on the scheduler s implementation  the scheduler might also only schedule consumers  e g  if you assign consumer threads a very high priority   However  the assumption here is that the danger of the scheduler scheduling only consumers is lower than the danger of the JVM only waking up consumers because any reasonably implemented scheduler doesn t make just arbitrary decisions  Rather  most scheduler implementations make at least some effort to prevent starvation

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Useful differences    Use notify   if all your waiting threads are interchangeable  the order they wake up doesn t matter   or if you only ever have one waiting thread  A common example is a thread pool used to execute jobs from a queue--when a job is added  one of threads is notified to wake up  execute the next job and go back to sleep  Use notifyAll   for other cases where the waiting threads may have different purposes and should be able to run concurrently  An example is a maintenance operation on a shared resource  where multiple threads are waiting for the operation to complete before accessing the resource

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All the above answers are correct  as far as I can tell  so I m going to tell you something else  For production code you really should use the classes in java util concurrent  There is very little they cannot do for you  in the area of concurrency in java

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notify   - Selects a random thread from the wait set of the object and puts it in the BLOCKED state  The rest of the threads in the wait set of the object are still in the WAITING state   notifyAll   - Moves all the threads from the wait set of the object to BLOCKED state  After you use notifyAll    there are no threads remaining in the wait set of the shared object because all of them are now in BLOCKED state and not in WAITING state    BLOCKED - blocked for lock acquisition  WAITING - waiting for notify   or blocked for join completion

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I am very surprised that no one mentioned the infamous  lost wakeup  problem  google it    Basically    if you have multiple threads waiting on a same condition and   multiple threads that can make you transition from state A to state B and  multiple threads that can make you transition from state B to state A  usually the same threads as in 1   and  transitioning from state A to B should notify threads in 1      THEN you should use notifyAll unless you have provable guarantees that lost wakeups are impossible   A common example is a concurrent FIFO queue where  multiple enqueuers  1  and 3  above  can transition your queue from empty to non-empty  multiple dequeuers  2  above  can wait for the condition  the queue is not empty  empty -  non-empty should notify dequeuers  You can easily write an interleaving of operations in which  starting from an empty queue  2 enqueuers and 2 dequeuers interact and 1 enqueuer will remain sleeping   This is a problem arguably comparable with the deadlock problem

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Clearly  notify wakes  any  one thread in the wait set  notifyAll wakes all threads in the waiting set  The following discussion should clear up any doubts  notifyAll should be used most of the time  If you are not sure which to use  then use notifyAll Please see explanation that follows   Read very carefully and understand  Please send me an email if you have any questions   Look at producer consumer  assumption is a ProducerConsumer class with two methods   IT IS BROKEN  because it uses notify  - yes it MAY work - even most of the time  but it may also cause deadlock - we will see why   public synchronized void put Object o        while  buf size    MAX SIZE            wait       called if the buffer is full  try catch removed for brevity            buf add o       notify       called in case there are any getters or putters waiting    public synchronized Object get            Y  this is where C2 tries to acquire the lock  i e  at the beginning of the method      while  buf size    0            wait       called if the buffer is empty  try catch removed for brevity             X  this is where C1 tries to re-acquire the lock  see below            Object o   buf remove 0       notify       called if there are any getters or putters waiting     return o      FIRSTLY   Why do we need a while loop surrounding the wait   We need a while loop in case we get this situation   Consumer 1  C1  enter the synchronized block and the buffer is empty  so C1 is put in the wait set  via the wait call   Consumer 2  C2  is about to enter the synchronized method  at point Y above   but Producer P1 puts an object in the buffer  and subsequently calls notify  The only waiting thread is C1  so it is woken and now attempts to re-acquire the object lock at point X  above    Now C1 and C2 are attempting to acquire the synchronization lock  One of them  nondeterministically  is chosen and enters the method  the other is blocked  not waiting - but blocked  trying to acquire the lock on the method   Let s say C2 gets the lock first  C1 is still blocking  trying to acquire the lock at X   C2 completes the method and releases the lock  Now  C1 acquires the lock  Guess what  lucky we have a while loop  because  C1 performs the loop check  guard  and is prevented from removing a non-existent element from the buffer  C2 already got it    If we didn t have a while  we would get an IndexArrayOutOfBoundsException as C1 tries to remove the first element from the buffer   NOW   Ok  now why do we need notifyAll   In the producer consumer example above it looks like we can get away with notify  It seems this way  because we can prove that the guards on the wait loops for producer and consumer are mutually exclusive  That is  it looks like we cannot have a thread waiting in the put method as well as the get method  because  for that to be true  then the following would have to be true   buf size      0 AND buf size      MAX SIZE  assume MAX SIZE is not 0   HOWEVER  this is not good enough  we NEED to use notifyAll  Let s see why      Assume we have a buffer of size 1  to make the example easy to follow   The following steps lead us to deadlock  Note that ANYTIME a thread is woken with notify  it can be non-deterministically selected by the JVM - that is any waiting thread can be woken  Also note that when multiple threads are blocking on entry to a method  i e  trying to acquire a lock   the order of acquisition can be non-deterministic  Remember also that a thread can only be in one of the methods at any one time - the synchronized methods allow only one thread to be executing  i e  holding the lock of  any  synchronized  methods in the class  If the following sequence of events occurs - deadlock results   STEP 1  - P1 puts 1 char into the buffer  STEP 2  - P2 attempts put - checks wait loop - already a char - waits  STEP 3  - P3 attempts put - checks wait loop - already a char - waits  STEP 4  - C1 attempts to get 1 char  - C2 attempts to get 1 char - blocks on entry to the get method - C3 attempts to get 1 char - blocks on entry to the get method  STEP 5  - C1 is executing the get method - gets the char  calls notify  exits method - The notify wakes up P2 - BUT  C2 enters method before P2 can  P2 must reacquire the lock   so P2 blocks on entry to the put method - C2 checks wait loop  no more chars in buffer  so waits - C3 enters method after C2  but before P2  checks wait loop  no more chars in buffer  so waits  STEP 6  - NOW  there is P3  C2  and C3 waiting  - Finally P2 acquires the lock  puts a char in the buffer  calls notify  exits method  STEP 7  - P2 s notification wakes P3  remember any thread can be woken  - P3 checks the wait loop condition  there is already a char in the buffer  so waits  - NO MORE THREADS TO CALL NOTIFY and THREE THREADS PERMANENTLY SUSPENDED   SOLUTION  Replace notify with notifyAll in the producer consumer code  above

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Waking up all does not make much significance here   wait notify and notifyall  all these are put after owning the object s monitor  If a thread is in the waiting stage and notify is called  this thread will take up the lock and no other thread at that point can take up that lock  So concurrent access can not take place at all  As far as i know any call to wait notify and notifyall can be made only after taking the lock on the object  Correct me if i am wrong

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To summarize the excellent detailed explanations above  and in the simplest way I can think of  this is due to the limitations of the JVM built-in monitor  which 1  is acquired on the entire synchronization unit  block or object  and 2  does not discriminate about the specific condition being waited notified on about    This means that if multiple threads are waiting on different conditions and notify   is used  the selected thread may not be the one which would make progress on the newly fulfilled condition - causing that thread  and other currently still waiting threads which would be able to fulfill the condition  etc    not to be able to make progress  and eventually starvation or program hangup    In contrast  notifyAll   enables all waiting threads to eventually re-acquire the lock and check for their respective condition  thereby eventually allowing progress to be made    So notify   can be used safely only if any waiting thread is guaranteed to allow progress to be made should it be selected  which in general is satisfied when all threads within the same monitor check for only one and the same condition - a fairly rare case in real world applications

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notify   will wake up one thread while notifyAll   will wake up all  As far as I know there is no middle ground  But if you are not sure what notify   will do to your threads  use notifyAll    Works like a charm everytime

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Taken from blog on Effective Java   The notifyAll method should generally be used in preference to notify    If notify is used  great care must be taken to ensure liveness    So  what i understand is  from aforementioned blog  comment by  Yann TM  on accepted answer and Java docs     notify     JVM awakens one of the waiting threads on this object  Thread selection is made arbitrarily without fairness  So same thread can be awakened again and again  So system s state changes but no real progress is made  Thus creating a livelock  notifyAll     JVM awakens all threads and then all threads race for the lock on this object  Now  CPU scheduler selects a thread which acquires lock on this object  This selection process would be much better than selection by JVM  Thus  ensuring liveness

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Here s a simpler explanation   You re correct that whether you use notify   or notifyAll    the immediate result is that exactly one other thread will acquire the monitor and begin executing   Assuming some threads were in fact blocked on wait   for this object  other unrelated threads aren t soaking up all available cores  etc   The impact comes later   Suppose thread A  B  and C were waiting on this object  and thread A gets the monitor  The difference lies in what happens once A releases the monitor  If you used notify    then B and C are still blocked in wait    they are not waiting on the monitor  they are waiting to be notified  When A releases the monitor  B and C will still be sitting there  waiting for a notify     If you used notifyAll    then B and C have both advanced past the  wait for notification  state and are both waiting to acquire the monitor  When A releases the monitor  either B or C will acquire it  assuming no other threads are competing for that monitor  and begin executing

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notify   wakes up the first thread that called wait   on the same object   notifyAll   wakes up all the threads that called wait   on the same object   The highest priority thread will run first

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I think it depends on how resources are produced and consumed  If 5 work objects are available at once and you have 5 consumer objects  it would make sense to wake up all threads using notifyAll   so each one can process 1 work object   If you have just one work object available  what is the point in waking up all consumer objects to race for that one object  The first one checking for available work will get it and all other threads will check and find they have nothing to do   I found a great explanation here  In short       The notify   method is generally used   for resource pools  where there   are an arbitrary number of  consumers    or  workers  that take resources  but   when a resource is added to the pool    only one of the waiting consumers or   workers can deal with it  The   notifyAll   method is actually used in   most other cases  Strictly  it is   required to notify waiters of a   condition that could allow multiple   waiters to proceed  But this is often   difficult to know  So as a general   rule  if you have no particular   logic for using notify    then you   should probably use notifyAll      because it is often difficult to know   exactly what threads will be waiting   on a particular object and why

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From Joshua Bloch  the Java Guru himself in Effective Java 2nd edition    Item 69  Prefer concurrency utilities to wait and notify

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Note that with concurrency utilities you also have the choice between signal   and signalAll   as these methods are called there   So the question remains valid even with java util concurrent   Doug Lea brings up an interesting point in his famous book  if a notify   and Thread interrupt   happen at the same time  the notify might actually get lost   If this can happen and has dramatic implications notifyAll   is a safer choice even though you pay the price of overhead  waking too many threads most of the time

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There are three states for a thread   WAIT - The thread is not using any CPU cycle BLOCKED - The thread is blocked trying to acquire a monitor  It might still be using the CPU cycles RUNNING - The thread is running   Now  when a notify   is called  JVM picks one thread and move them to the BLOCKED state and hence to the RUNNING state as there is no competition for the monitor object  When a notifyAll   is called  JVM picks all the threads and move all of them to BLOCKED state  All these threads will get the lock of the object on a priority basis  Thread which is able to acquire the monitor first will be able to go to the RUNNING state first and so on

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Waiting queue and blocked queue You can assume there are two kinds of queues associated with each lock object  One is blocked queue containing thread waiting for the monitor lock  other is waiting queue containing thread waiting to be notified   Thread will be put into waiting queue when they call Object wait   Each time the lock is available  the scheduler choose one thread from blocked queue to execute  When notify is called  there will only be one thread in waiting queue are put into blocked queue to contend for the lock  while notifyAll will put all thread in waiting queue into blocked queue  Now can you see the difference  Although in both case there will only be one thread get executed  but with notifyAll  other threads still get a change to be executed Because they are in the blocked queue  even if they failed to contend the lock  some guidline I basically recommend use notifyAll all the time althrough there may be a little performance penalty  And use notify only if    Any waked thread can make the programe proceed  performance is important   For example   xagyg  s answer gives a example which notify will cause deadlock  In his example  both producer and consumer are related with the same lock object  So when a producer calls notify  either a producer or a consumer can be notified  But if a producer is woken up it can not make the programe proceed because the buffer is already full So a deadlock happens  There are two ways to solve it    use notifyALl as  xagyg suggests  Make procuder and consumer related with different lock object and procuder can only wake up consumer  consumer can only wake up producer  In that case  no matter which consumer is waked  it can consumer the buffer and make the programe proceed

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notify   lets you write more efficient code than notifyAll     Consider the following piece of code that s executed from multiple parallel threads    synchronized this        while busy     a loop is necessary here         wait        busy   true        synchronized this        busy   false      notifyAll        It can be made more efficient by using notify      synchronized this        if busy       replaced the loop with a condition which is evaluated only once         wait        busy   true        synchronized this        busy   false      notify        In the case if you have a large number of threads  or if the wait loop condition is costly to evaluate  notify   will be significantly faster than notifyAll    For example  if you have 1000 threads then 999 threads will be awakened and evaluated after the first notifyAll    then 998  then 997  and so on  On the contrary  with the notify   solution  only one thread will be awakened    Use notifyAll   when you need to choose which thread will do the work next    synchronized this        while idx    last 1      wait until it s my turn         wait          synchronized this        last   idx      notifyAll        Finally  it s important to understand that in case of notifyAll    the code inside synchronized blocks that have been awakened will be executed sequentially  not all at once  Let s say there are three threads waiting in the above example  and the fourth thread calls notifyAll    All three threads will be awakened but only one will start execution and check the condition of the while loop  If the condition is true  it will call wait   again  and only then the second thread will start executing and will check its while loop condition  and so on

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Take a look at the code posted by  xagyg     Suppose two different threads are waiting for two different conditions  The first thread is waiting for buf size      MAX SIZE  and the second thread is waiting for buf size      0     Suppose at some point buf size   is not equal to 0  JVM calls notify   instead of notifyAll    and the first thread is notified  not the second one       The first thread is woken up  checks for buf size   which might return MAX SIZE  and goes back to waiting  The second thread is not woken up  continues to wait and does not call get

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However  if I do understand the difference between these methods right   only one thread is always selected for further monitor acquisition    That is not correct   o notifyAll   wakes all of the threads that are blocked in o wait   calls   The threads are only allowed to return from o wait   one-by-one  but they each will get their turn     Simply put  it depends on why your threads are waiting to be notified  Do you want to tell one of the waiting threads that something happened  or do you want to tell all of them at the same time   In some cases  all waiting threads can take useful action once the wait finishes  An example would be a set of threads waiting for a certain task to finish  once the task has finished  all waiting threads can continue with their business  In such a case you would use notifyAll   to wake up all waiting threads at the same time   Another case  for example mutually exclusive locking  only one of the waiting threads can do something useful after being notified  in this case acquire the lock   In such a case  you would rather use notify    Properly implemented  you could use notifyAll   in this situation as well  but you would unnecessarily wake threads that can t do anything anyway     In many cases  the code to await a condition will be written as a loop   synchronized o        while    IsConditionTrue              o wait              DoSomethingThatOnlyMakesSenseWhenConditionIsTrue and MaybeMakeConditionFalseAgain        That way  if an o notifyAll   call wakes more than one waiting thread  and the first one to return from the o wait   makes leaves the condition in the false state  then the other threads that were awakened will go back to waiting

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notify will notify only one thread which are in waiting state  while notify all will notify all the threads in the waiting state now all the notified threads and all the blocked threads are eligible for the lock  out of which only one will get the lock and all others  including those who are in waiting state earlier  will be in blocked state

[java] Java: notify() vs. notifyAll() all over again

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