How to synchronize a static variable of a class ?

There are some ways(3 to my knowledge, but may be more), by which a static variable can be synchronized in java.

1) Use a synchronized static method. This synchronizes on the class object.

public class Counter {
    private static int count = 0;

    public static synchronized void incrementCount() {
        count++;
    }
} 

2) Explicitly synchronize on the class object, using synchronize on ClassName.class

public class Counter {
    private static int count = 0;

    public void incrementCount() {
        synchronize (Test.class) {
            count++;
        }
    }
} 

3) Synchronize on some other static object.

public class Counter {
    private static int count = 0;
    private static final Object countLockHelper = new Object();

    public void incrementCount() {
        synchronize (countLockHelper) {
            count++;
        }
    }
} 

Method 3 is best in many cases because the lock object is not exposed outside of your class. So if you create instance of these class, they will be synchronized on the same static object.

But if you just using some basic type like integer here in case of counter, consider using an AtomicInteger or another suitable class from the java.util.concurrent.atomic package:

public class Counter {

    private final static AtomicInteger count = new AtomicInteger(0); 

    public void incrementCount() {  
        count.incrementAndGet();
    }  
}