How to use wait and notify in Java without IllegalMonitorStateException

Question

I have 2 matrices and I need to multiply them and then print the results of each cell  As soon as one cell is ready I need to print it  but for example I need to print the  0  0  cell before cell  2  0  even if the result of  2  0  is ready first  So I need to print it by order  So my idea is to make the printer thread wait until the multiplyThread notifies it that the correct cell is ready to be printed and then the printerThread will print the cell and go back to waiting and so on    So I have this thread that does the multiplication   public void run          int countNumOfActions   0     How many multiplications have we done     int maxActions   randomize       Maximum number of actions allowed      for  int i   0  i  lt  size  i                         result rowNum  colNum    result rowNum  colNum    row i    col i           countNumOfActions               Reached the number of allowed actions         if  countNumOfActions  gt   maxActions                        countNumOfActions   0              maxActions   randomize                yield                           isFinished rowNum  colNum    true      notify        Thread that prints the result of each cell   public void run         int j   0     Columns counter     int i   0     Rows counter     System out println  The result matrix of the multiplication is          while  i  lt  creator getmThreads   length                synchronized  this                        try                                this wait                               catch  InterruptedException e1                                                 if  creator getmThreads   i  j  getIsFinished   i  j     true                        if  j  lt  creator getmThreads   i  length                                System out print creator getResult   i  j                          j                              else                               System out println                    j   0                  i                    System out print creator getResult   i  j                                          Now it throws me these exceptions   Exception in thread  Thread-9  java lang IllegalMonitorStateException     at java lang Object notify Native Method      at multiplyThread run multiplyThread java 49  Exception in thread  Thread-6  Exception in thread  Thread-4  java lang IllegalMonitorStateException     at java lang Object notify Native Method      at multiplyThread run multiplyThread java 49  java lang IllegalMonitorStateException     at java lang Object notify Native Method      at multiplyThread run multiplyThread java 49  Exception in thread  Thread-5  java lang IllegalMonitorStateException     at java lang Object notify Native Method      at multiplyThread run multiplyThread java 49  Exception in thread  Thread-8  java lang IllegalMonitorStateException     at java lang Object notify Native Method      at multiplyThread run multiplyThread java 49  Exception in thread  Thread-7  java lang IllegalMonitorStateException     at java lang Object notify Native Method      at multiplyThread run multiplyThread java 49  Exception in thread  Thread-11  java lang IllegalMonitorStateException     at java lang Object notify Native Method      at multiplyThread run multiplyThread java 49  Exception in thread  Thread-10  java lang IllegalMonitorStateException     at java lang Object notify Native Method      at multiplyThread run multiplyThread java 49  Exception in thread  Thread-12  java lang IllegalMonitorStateException     at java lang Object notify Native Method      at multiplyThread run multiplyThread java 49    line 49 in multiplyThread is the  notify     I think I need to use the synchronized differently but I am not sure how   If anyone can help this code to work I will really appreciate it

User · Accepted Answer

To be able to call notify   you need to synchronize on the same object   synchronized  someObject        someObject wait          different thread   object    synchronized  someObject        someObject notify

User · Answer

notify   needs to be synchronized as well

User · Answer

Simple use if you want How to execute threads alternatively  -  public class MyThread       public static void main String   args            final Object lock   new Object            new Thread    - gt                try                   synchronized  lock                        for  int i   0  i  lt   5  i                              System out println Thread currentThread   getName            A                            lock notify                            lock wait                                                          catch  Exception e                 T1   start             new Thread    - gt                try                   synchronized  lock                        for  int i   0  i  lt   5  i                              System out println Thread currentThread   getName            B                            lock notify                            lock wait                                                          catch  Exception e                 T2   start                 response  -   T1 A T2 B T1 A T2 B T1 A T2 B T1 A T2 B T1 A T2 B T1 A T2 B

User · Answer

This looks like a situation for producer-consumer pattern  If you   re using java 5 or up  you may consider using blocking queue java util concurrent BlockingQueue  and leave the thread coordination work to the underlying framework api implementation  See the example from java 5  http   docs oracle com javase 1 5 0 docs api java util concurrent BlockingQueue html or java 7  same example   http   docs oracle com javase 7 docs api java util concurrent BlockingQueue html

User · Answer

You have properly guarded your code block when you call wait   method by using synchronized this     But you have not taken same precaution when you call notify   method without using guarded block   synchronized this  or synchronized someObject   If you refer to oracle documentation page on Object class  which contains wait    notify    notifyAll   methods  you can see below precaution in all these three methods     This method should only be called by a thread that is the owner of this object s monitor   Many things have been changed in last 7 years and let s have look into other alternatives to synchronized in below SE questions   Why use a ReentrantLock if one can use synchronized this    Synchronization vs Lock  Avoid synchronized this  in Java

User · Answer

You can only call notify on objects where you own their monitor  So you need something like  synchronized threadObject       threadObject notify

User · Answer

we can call notify to resume the execution of waiting objects as  public synchronized void guardedJoy            This guard only loops once for each special event  which may not        be the event we re waiting for      while  joy            try               wait              catch  InterruptedException e               System out println  Joy and efficiency have been achieved         resume this by invoking notify on another object of same class  public synchronized notifyJoy         joy   true      notifyAll

User · Answer

For this particular problem  why not store up your various results in variables and then when the last of your thread is processed you can print in whatever format you want  This is especially useful if you are gonna be using your work history in other projects

User · Answer

Let s say you have  black box  application with some class named BlackBoxClass that has method doSomething      Further  you have observer or listener named onResponse String resp  that will be called by BlackBoxClass after unknown time   The flow is simple    private String mResponse   null        BlackBoxClass bbc   new BlackBoxClass       bbc doSomething         override public void onResponse String resp                 mResponse   resp             Lets say we don t know what is going on with BlackBoxClass and when we should  get answer but you don t want to continue your code till you get answer or in other word get onResponse call  Here enters  Synchronize helper    public class SyncronizeObj   public void doWait long l       synchronized this           try               this wait l             catch InterruptedException e                       public void doNotify         synchronized this            this notify             public void doWait         synchronized this           try               this wait              catch InterruptedException e                          Now we can implement what we want   public class Demo    private String mResponse   null        SyncronizeObj sync   new SyncronizeObj     public void impl     BlackBoxClass bbc   new BlackBoxClass       bbc doSomething        if mResponse    null         sync doWait               at this momoent you sure that you got response from  BlackBoxClass because   onResponse method released your  wait   In other cases if you don t want wait too         long  for example wait data from socket  you can use doWait time                 override public void onResponse String resp                 mResponse   resp        sync doNotify

User · Answer

Do you need to thread this at all   I m wondering how big your matrices are  and whether there s any benefit in having one thread print whilst the other does the multiplication    Perhaps it would be worth measuring this time before doing the relatively complex threading work    If you do need to thread it  I would create  n  threads to perform the multiplication of the cells  perhaps  n  is the number of cores available to you   and then use the ExecutorService and Future mechanism to dispatch multiple multiplications simultaneously    That way you can optimise the work based on the number of cores  and you re using the higher level Java threading tools  which should make life easier   Write the results back into a receiving matrix  and then simply print this once all your Future tasks have completed

User · Answer

I ll right simple example show you the right way  to use wait and notify in Java  So I ll create two class named ThreadA  amp  ThreadB  ThreadA will call ThreadB    public class ThreadA       public static void main String   args           ThreadB b   new ThreadB      lt ----Create Instance for seconde class         b start      lt --------------------Launch thread          synchronized b               try                  System out println  Waiting for b to complete                       b wait      lt -------------WAIT until the finish thread for class B finish              catch InterruptedException e                   e printStackTrace                               System out println  Total is      b total                        and for Class ThreadB   class ThreadB extends Thread      int total       Override     public void run            synchronized this               for int i 0  i lt 100   i                     total    i                            notify      lt ----------------Notify the class wich wait until my    finish    and tell that I m finish

User · Answer

While using the wait and notify or notifyAll methods in Java the following things must be remembered    Use notifyAll instead of notify if you expect that more than one thread will be waiting for a lock    The wait and notify methods must be called in a synchronized context  See the link for a more detailed explanation     Always call the wait   method in a loop because if multiple threads are waiting for a lock and one of them got the lock and reset the condition  then the other threads need to check the condition after they wake up to see whether they need to wait again or can start processing    Use the same object for calling wait   and notify   method  every object has its own lock so calling wait   on object A and notify   on object B will not make any sense

[java] How to use wait and notify in Java without IllegalMonitorStateException?

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