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

There are several ways to synchronize access to a static variable.

1. 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++;
       }
   } 
   

2. Explicitly synchronize on the class object.

   public class Test {
       private static int count = 0;
   
       public void incrementCount() {
           synchronized (Test.class) {
               count++;
           }
       }
   } 
   

3. 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.

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();
    }  
}

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.