How to synchronize a static variable among threads running different instances of a class in Java

Question

I know that using the synchronize keyword before a method brings synchronization to that object  That is  2 threads running the same instance of the object will be synchronized      However  since the synchronization is at the object level  2 threads running different instances of the object will not be synchronized  If we have a static variable in a Java class that is called by the method  we would like it to be synchronized across instances of the class  The two instances are running in 2 different threads     Can we achieve synchronization in the following way     public class Test          private static int count   0       private static final Object lock  new Object           public synchronized void foo                synchronized lock                    count                         Is it true that since we have defined an object lock that is static and we are using the keyword synchronized for that lock  the static variable count is now synchronized across instances of class Test

User · Answer

If you re simply sharing a counter  consider using an AtomicInteger or another suitable class from the java util concurrent atomic package   public class Test        private final static AtomicInteger count   new AtomicInteger 0         public void foo               count incrementAndGet

User · Answer

You can synchronize your code over the class  That would be simplest       public class Test                  private static int count   0           private static final Object lock  new Object               public synchronized void foo                        synchronized Test class                            count                                     Hope you find this answer useful

User · Answer

There are several ways to synchronize access to a static variable    Use a synchronized static method   This synchronizes on the class object   public class Test       private static int count   0       public static synchronized void incrementCount             count              Explicitly synchronize on the class object   public class Test       private static int count   0       public void incrementCount             synchronized  Test class                count                        Synchronize on some other static object   public class Test       private static int count   0      private static final Object countLock   new Object         public void incrementCount             synchronized  countLock                count                          Method 3 is the best in many cases because the lock object is not exposed outside of your class

User · Answer

Yes it is true     If you create two instance of your class  Test t1   new Test    Test t2   new Test      Then t1 foo and t2 foo both synchronize on the same static object and hence block each other

User · Answer

We can also use ReentrantLock to achieve the synchronization for static variables  public class Test        private static int count   0      private static final ReentrantLock reentrantLock   new ReentrantLock         public void foo               reentrantLock lock            count   count   1          reentrantLock unlock

[java] How to synchronize a static variable among threads running different instances of a class in Java?

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