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.