Example using projectreactor.io : create a Flux to enable clients to subscribe to your method call.

Example.java

import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;

import reactor.core.publisher.Flux;
import reactor.core.publisher.FluxSink;

/**
 * Created by ton on 10/11/16.
 */
public class Example {

    private List<FluxSink<String>> handlers = new ArrayList<>();

    public Flux<String> getMessagesAsStream() {
        Flux<String> result = Flux.create(sink -> {
                handlers.add(sink);
            sink.setCancellation(() -> handlers.remove(sink));
        });

        return result;
    }

    public void handleMessage(String message) {
        handlers.forEach(han -> han.next(message));
    }

    public static void main(String[] args) {
        Example example = new Example();
        example.getMessagesAsStream().subscribe(req -> System.out.println("req = " + req));
        example.getMessagesAsStream().subscribe(msg -> System.out.println(msg.toUpperCase()));
        example.handleMessage("een");
        example.handleMessage("twee");
        example.handleMessage("drie");
    }
}