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.