Concept behind putting wait notify methods in Object class

Question

I am just having hard time to understand concept behind putting wait   in Object class   For this questions sake consider if wait   and notifyAll   are in Thread class   class Reader extends Thread       Calculator c      public Reader Calculator calc            c   calc             public void run             synchronized c                                   line 9         try               System out println  Waiting for calculation                   c wait              catch  InterruptedException e                 System out println  Total is      c total                        public static void main String    args            Calculator calculator   new Calculator            new Reader calculator  start            new Reader calculator  start            new Reader calculator  start            calculator start             class Calculator extends Thread       int total      public void run             synchronized this                          Line 31             for int i 0 i lt 100 i                      total    i                             notifyAll                         My Question is that what difference it could have made  In line 9 we are acquiring lock on object c and then performing wait which satisfy the condition for wait that we need to acquire lock on the object before we use wait and so is case for notifyAll we have acquired lock on object of Calculator at line 31

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In simple terms  the reasons are as follows    Object has monitors  Multiple threads can access one Object  Only one thread can hold object monitor at a time for synchronized methods blocks  wait    notify   and notifyAll   method being in Object class allows all the threads created on that object to communicate with other Locking   using synchronized or Lock API   and Communication  wait   and notify    are two different concepts     If Thread class contains wait    notify   and notifyAll   methods  then it will create below problems    Thread communication problem Synchronization on object won   t be possible  If each thread will have monitor  we won   t have any way of achieving synchronization Inconsistency in state of object    Refer to this article for more details

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The other answers to this question all miss the key point that in Java  there is one mutex associated with every object   I m assuming you know what a mutex or  lock  is   This is not the case in most programming languages which have the concept of  locks   For example  in Ruby  you have to explicitly create as many Mutex objects as you need   I think I know why the creators of Java made this choice  although  in my opinion  it was a mistake   The reason has to do with the inclusion of the synchronized keyword  I believe that the creators of Java  naively  thought that by including synchronized methods in the language  it would become easy for people to write correct multithreaded code -- just encapsulate all your shared state in objects  declare the methods that access that state as synchronized  and you re done  But it didn t work out that way     Anyways  since any class can have synchronized methods  there needs to be one mutex for each object  which the synchronized methods can lock and unlock   wait and notify both rely on mutexes  Maybe you already understand why this is the case    if not I can add more explanation  but for now  let s just say that both methods need to work on a mutex  Each Java object has a mutex  so it makes sense that wait and notify can be called on any Java object  Which means that they need to be declared as methods of Object   Another option would have been to put static methods on Thread or something  which would take any Object as an argument  That would have been much less confusing to new Java programmers  But they didn t do it that way  It s much too late to change any of these decisions  too bad

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This is just my 2 cents on this question   not sure if this holds true in its entirety    Each Object has a monitor and waitset --  set of threads  this is probably more at OS level   This means the monitor and the waitset can be seen as private members of an object  Having wait   and notify   methods in Thread class would mean giving public access to the waitset or using get-set methods to modify the waitset  You wouldnt want to do that because thats bad designing   Now given that the Object knows the thread s waiting for its monitor  it should be the job of the Object to go and awaken those threads waiting for it rather than an Object of thread class going and awakening each one of them  which would be possible only if the thread class object is given access to the waitset   However  it is not the job of a particular thread to go and awaken each of the waiting threads   This is exactly what would happened if all these methods were inside the Thread class   Its job is just to release the lock and move ahead with its own task  A thread works independently and does not need to know whihc other threads are waiting for the objects monitor  it is an unnecessary detail for the thread class object   If it started awakening each thread on its own   it is moving from away its core functionality and that is to carry out its own task  When you think about a scene where there might 1000 s of threads   you can assume how much of a performance impact it can create  Hence  given that Object Class knows who is waiting on it  it can carry out the job awakening the waiting threads and the thread which sent notify   can carry out with its further processing    To give an analogy  perhaps not the right one but cant think of anything else   When we have a power outage  we call a customer representative of that company because she knows the right people to contact to fix it  You as a consumer are not allowed to know who are the engineers behind it and even if you know  you cannot possibly call each one of them and tell them of your troubles  that is not ur duty  Your duty is to inform them about the outage and the CR s job is to go and notify awaken  the right engineers for it    Let me know if this sounds right     I do have the ability to confuse sometimes with my words

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Wait and notify method always called on object so whether it may be Thread object or simple object  which does not extends Thread class  Given Example will clear your all the doubts    I have called wait and notify on class ObjB and that is the Thread class so we can say that wait and notify are called on any object   public class ThreadA       public static void main String   args           ObjB b   new ObjB            Threadc c   new Threadc b            ThreadD d   new ThreadD b           d setPriority 5           c setPriority 1           d start            c start             class ObjB       int total      int count            for int i 0  i lt 100   i                 total    i                    return total           class Threadc extends Thread      ObjB b      Threadc ObjB objB           b  objB            int total       Override     public void run            System out print  Thread C run method            synchronized b               total   b count                System out print  Thread C notified called                 b notify                       class ThreadD extends Thread      ObjB b      ThreadD ObjB objB           b  objB            int total       Override     public void run            System out print  Thread D run method            synchronized b               System out println  Waiting for b to complete                   try                   b wait                    System out print  Thread C B value is    b total                                      catch  InterruptedException e                        e printStackTrace

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I am just having hard time to understand concept behind putting wait   in object class For this questions sake consider as if wait   and notifyAll   are in thread class  In the Java language  you wait   on a particular instance of an Object     a monitor assigned to that object to be precise   If you want to send a signal to one thread that is waiting on that specific object instance then you call notify   on that object   If you want to send a signal to all threads that are waiting on that object instance  you use notifyAll   on that object  If wait   and notify   were on the Thread instead then each thread would have to know the status of every other thread   How would thread1 know that thread2 was waiting for access to a particular resource   If thread1 needed to call thread2 notify   it would have to somehow find out that thread2 was waiting   There would need to be some mechanism for threads to register the resources or actions that they need so others could signal them when stuff was ready or available  In Java  the object itself is the entity that is shared between threads which allows them to communicate with each other   The threads have no specific knowledge of each other and they can run asynchronously   They run and they lock  wait  and notify on the object that they want to get access to   They have no knowledge of other threads and don t need to know their status   They don t need to know that it is thread2 which is waiting for the resource     they just notify on the resource and whomever it is that is waiting  if anyone  will be notified  In Java  we then use objects as synchronization  mutex  and communication points between threads   We synchronize on an object to get mutex access to an important code block and to synchronize memory   We wait on an object if we are waiting for some condition to change     some resource to become available   We notify on an object if we want to awaken sleeping threads     locks should be final objects so the object instance we are synchronizing on     never changes private final Object lock   new Object           ensure that the thread has a mutex lock on some key code synchronized  lock                   i need to wait for other threads to finish with some resource        this releases the lock and waits on the associated monitor     lock wait                   i need to signal another thread that some state has changed and they can        awake and continue to run     lock notify       There can be any number of lock objects in your program     each locking a particular resource or code segment   You might have 100 lock objects and only 4 threads   As the threads run the various parts of the program  they get exclusive access to one of the lock objects   Again  they don t have to know the running status of the other threads  This allows you to scale up or down the number of threads running in your software as much as you want   You find that the 4 threads is blocking too much on outside resources  then you can increase the number   Pushing your battered server too hard then reduce the number of running threads   The lock objects ensure mutex and communication between the threads independent of how many threads are running

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wait and notify operations work on implicit lock  and implicit lock is something that make inter thread communication possible  And all objects have got their own copy of implicit object  so keeping wait and notify where implicit lock lives is a good decision    Alternatively wait and notify could have lived in Thread class as well  than instead of wait   we may have to call Thread getCurrentThread   wait    same with notify   For wait and notify operations there are two required parameters  one is thread who will be waiting or notifying other is implicit lock of the object   both are these could be available in Object as well as thread class as well  wait   method in Thread class would have done the same as it is doing in Object class  transition current thread to waiting state wait on the lock it had last acquired   So yes i think wait and notify could have been there in Thread class as well but its more like a design decision to keep it in object class

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the wait   method will release the lock on the specified object and waits when it can retrieve the lock   the notify    notifyAll   will check if there are threads waiting to get the lock of an object and if possible will give it to them   The reason why the locks are a part of the objects is because the resources  RAM  are defined by Object and not Thread    The easiest method to understand this is that Threads can share Objects  in the example is calculator shared by all threads   but objects can not share Attributes   like primitives  even references itself to Objects are not shared  they just point to the same location   So in orde to make sure only one thread will modify a object  the synchronized locking system is used

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wait - wait method tells the current thread to give up monitor and go to sleep   notify - Wakes up a single thread that is waiting on this object s monitor   So you see wait   and notify   methods work at the monitor level  thread which is currently holding the monitor is asked to give up that monitor through wait   method and through notify method  or notifyAll  threads which are waiting on the object s monitor are notified that threads can wake up   Important point to note here is that monitor is assigned to an object not to a particular thread  That s one reason why these methods are in Object class  To reiterate threads wait on an Object s monitor  lock  and notify   is also called on an object to wake up a thread waiting on the Object s monitor

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Answer to your first question is As every object in java has only one lock monitor  andwait   notify   notifyAll   are used for monitor sharing thats why they are part of Object class rather than Threadclass

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For better understanding why wait   and notify   method belongs to Object class  I ll give you a real life example   Suppose a gas station has a single toilet  the key for which is kept at the service desk  The toilet is a shared resource for passing motorists  To use this shared resource the prospective user must acquire a key to the lock on the toilet  The user goes to the service desk and acquires the key  opens the door  locks it from the inside and uses the facilities   Meanwhile  if a second prospective user arrives at the gas station he finds the toilet locked and therefore unavailable to him  He goes to the service desk but the key is not there because it is in the hands of the current user  When the current user finishes  he unlocks the door and returns the key to the service desk  He does not bother about waiting customers  The service desk gives the key to the waiting customer  If more than one prospective user turns up while the toilet is locked  they must form a queue waiting for the key to the lock  Each thread has no idea who is in the toilet   Obviously in applying this analogy to Java  a Java thread is a user and the toilet is a block of code which the thread wishes to execute  Java provides a way to lock the code for a thread which is currently executing it using the synchronized keyword  and making other threads that wish to use it wait until the first thread is finished  These other threads are placed in the waiting state  Java is NOT AS FAIR as the service station because there is no queue for waiting threads  Any one of the waiting threads may get the monitor next  regardless of the order they asked for it  The only guarantee is that all threads will get to use the monitored code sooner or later   Finally the answer to your question  the lock could be the key object or the service desk  None of which is a Thread   However  these are the objects that currently decide whether the toilet is locked or open  These are the objects that are in a position to notify that the bathroom is open     notify     or ask people to wait when it is locked wait

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These methods works on the locks and locks are associated with Object and not Threads  Hence  it is in Object class   The methods wait    notify   and notifyAll   are not only just methods  these are synchronization utility and used in communication mechanism among threads in Java   For more detailed explanation  please visit   http   parameshk blogspot in 2013 11 why-wait-notify-and-notifyall-methods html

[java] Concept behind putting wait(),notify() methods in Object class

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