Question

Is it possible to create a static member in a class that would react (if its wait method is called), correctly for threads which do not share the same instance of the class (would they be able to notify each other with notifyAll using the static member field)?

Answer 1

You can store a single instance in a static field of a class that any number of threads can wait upon. Where the instance is stored does not matter. The key is that all threads have access to the same instance–be it from a static field/method or a singleton service object or a static local variable.

public class MakeMeWait {
    private static Object semaphore = new Object();

    public static void waitPlease() {
        semaphore.wait();
    }

    public static void wakePlease() {
        semaphore.notifyAll();
    }
}

While this example uses the built-in Java wait and notifyAll methods, you’re far better off using the java.util.concurrent package rather than rolling your own multithreading solutions.