Sharing a variable between multiple different threads

Question

I want to share a variable between multiple threads like this   boolean flag   true  T1 main   new T1    T2 help   new T2    main start    help start      I d like to share flag between main and help thread where these are two different Java classes I ve created  Is any way to do this  Thanks

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Making it static could fix this issue  Reference to the main thread in other thread and making that variable visible

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To make it visible between the instances of T1 and T2 you could make the two classes contain a reference to an object that contains the variable   If the variable is to be modified when the threads are running  you need to consider synchronization  The best approach depends on your exact requirements  but the main options are as follows    make the variable volatile  turn it into an AtomicBoolean  use full-blown synchronization around code that uses it

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Both T1 and T2 can refer to a class containing this variable  You can then make this variable volatile  and this means that Changes to that variable are immediately visible in both threads   See this article for more info      Volatile variables share the visibility features of synchronized but   none of the atomicity features  This means that threads will   automatically see the most up-to-date value for volatile variables    They can be used to provide thread safety  but only in a very   restricted set of cases  those that do not impose constraints between   multiple variables or between a variable s current value and its   future values    And note the pros cons of using volatile vs more complex means of sharing state

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In addition to the other suggestions - you can also wrap the flag in a control class and make a final instance of it in your parent class   public class Test     class Control       public volatile boolean flag   false        final Control control   new Control       class T1 implements Runnable        Override     public void run           while    control flag                           class T2 implements Runnable        Override     public void run           while    control flag                           private void test         T1 main   new T1        T2 help   new T2         new Thread main  start        new Thread help  start           public static void main String   args  throws InterruptedException       try         Test test   new Test          test test          catch  Exception e          e printStackTrace

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Using static will not help your case   Using synchronize locks a variable when it is in use by another thread   You should use volatile keyword to keep the variable updated among all threads      Using volatile is yet another way  like synchronized  atomic wrapper  of making class thread safe  Thread safe means that a method or class instance can be used by multiple threads at the same time without any problem

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AtomicBoolean The succinct Answer by NPE sums up your three options  I ll add some example code for the second item listed there  AtomicBoolean  You can think of the AtomicBoolean class as providing some thread-safety wrapping around a boolean value  If you instantiate the AtomicBoolean only once  then you need not worry about the visibility issue in the Java Memory Model that requires volatile as a solution  the first item in that other Answer   Also  you need not concern yourself with synchronization  the third item in that other Answer  because AtomicBoolean performs that function of protecting multi-threaded access to its internal boolean value  Let s look at some example code  Firstly  in modern Java we generally do not address the Thread class directly  We now have the Executors framework to simplify handling of threads  This code below is using Project Loom technology  coming to a future version of Java  Preliminary builds available now  built on early-access Java 16  This makes for simpler coding  with ExecutorService being AutoCloseable for convenient use with try-with-resources syntax  But Project Loom is not related to the point of this Answer  it just makes for simpler code that is easier to understand as    structured concurrency     The idea here is that we have three threads  the original thread  plus a ExecutorService that will create two more threads  The two new threads both report the value of our AtomicBoolean  The first new thread does so immediately  while the other waits 10 seconds before reporting  Meanwhile  our main thread sleeps for 5 seconds  wakes  changes the AtomicBoolean object   s contained value  and then waits for that second thread to wake and complete its work the report on the now-altered AtomicBoolean contained value  While we are installing seconds between each event  this is merely for dramatic demonstration  The real point is that these threads could coincidently try to access the AtomicBoolean simultaneously  but that object will protect access to its internal boolean value in a thread-safe manner  Protecting against simultaneous access is the job of the Atomic    classes  try           ExecutorService executorService   Executors newVirtualThreadExecutor             AtomicBoolean flag   new AtomicBoolean  true            This task  when run  will immediately report the flag      Runnable task1       - gt  System out println   quot First task reporting flag    quot    flag get      quot    quot    Instant now              This task  when run  will wait several seconds  then report the flag  Meanwhile  code below waits a shorter time before  changing  the flag      Runnable task2       - gt            try   Thread sleep  Duration ofSeconds  10        catch   InterruptedException e     e printStackTrace              System out println   quot Second task reporting flag    quot    flag get      quot    quot    Instant now                  executorService submit  task1        executorService submit  task2            Wait for first task to complete  so sleep here briefly  But wake before the sleeping second task awakens      try   Thread sleep  Duration ofSeconds  5        catch   InterruptedException e     e printStackTrace          System out println   quot INFO - Original thread waking up  and setting flag to false   quot    Instant now          flag set  false         At this point  with Project Loom technology  the flow-of-control blocks until the submitted tasks are done     Also  the  ExecutorService  is automatically closed shutdown by this point  via try-with-resources syntax  System out println   quot INFO - Tasks on background threads are done  The  AtomicBoolean  and threads are gone  quot    Instant now       Methods such as AtomicBoolean get and AtomicBoolean set are built to be thread-safe  to internally protect access to the boolean value nested within  Read up on the various other methods as well  When run  First task reporting flag   true  2021-01-05T06 42 17 367337Z INFO - Original thread waking up  and setting flag to false  2021-01-05T06 42 22 367456Z Second task reporting flag   false  2021-01-05T06 42 27 369782Z INFO - Tasks on background threads are done  The  AtomicBoolean  and threads are gone 2021-01-05T06 42 27 372597Z   Pro Tip  When engaging in threaded code in Java  always study the excellent book  Java Concurrency in Practice by Brian Goetz et al

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You can use lock variables  quot a quot  and  quot b quot  and synchronize them for locking the  quot critical section quot  in reverse order  Eg  Notify  quot a quot  then Lock  quot b quot    quot PRINT quot   Notify  quot b quot  then Lock  quot a quot   Please refer the below the code   public class EvenOdd        static int a   0       public static void main String   args             EvenOdd eo   new EvenOdd             A aobj   eo new A            B bobj   eo new B             aobj a   Lock lock1          aobj b   Lock lock2           bobj a   Lock lock2          bobj b   Lock lock1           Thread t1   new Thread aobj           Thread t2   new Thread bobj            t1 start            t2 start               static class Lock           final static Object lock1   new Object            final static Object lock2   new Object               class A implements Runnable            Object a          Object b           public void run                 while  EvenOdd a  lt  10                    try                       System out println   EvenOdd a    quot  A  quot                        synchronized  a                            a notify                                              synchronized  b                            b wait                                            catch  InterruptedException e                        e printStackTrace                                                         class B implements Runnable            Object a          Object b           public void run                 while  EvenOdd a  lt  10                     try                       synchronized  b                            b wait                            System out println   EvenOdd a    quot  B  quot                                              synchronized  a                            a notify                                            catch  InterruptedException e                        e printStackTrace                                                       OUTPUT   1 A  2 B  3 A  4 B  5 A  6 B  7 A  8 B  9 A  10 B

[java] Sharing a variable between multiple different threads

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