Server-Sent Event Java Servlet backend

DefaultListener.java

public interface DefaultListener {
	public void onNotification(NotificationEvent event);
}

NotificationChannel.java

import java.io.IOException;
import java.io.PrintWriter;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

import javax.inject.Inject;
import javax.servlet.AsyncContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(urlPatterns = {"/notify"}, asyncSupported = true)
public class NotificationChannel extends HttpServlet implements DefaultListener{
	private static final long serialVersionUID = -2827663265593547983L;
	private ConcurrentMap<String, AsyncContext> contexts = new ConcurrentHashMap<>();
	@Inject
	NotificationListener listener;

	@Override
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException{
		//initialize for server-sent events
		response.setContentType("text/event-stream");
		response.setCharacterEncoding("UTF-8");
		
		final String userId = request.getParameter("userId"); //retrieve the user Id
		if (userId == null){
			response.sendError(HttpServletResponse.SC_BAD_REQUEST);
			return;
		}
		
		//to clear threads and allow for asynchronous execution
		final AsyncContext asyncContext = request.startAsync(request, response);
		asyncContext.setTimeout(0); 
		
		//add context to list for later use
		if (contexts == null || contexts.isEmpty() || !contexts.containsKey(userId)){
			if (contexts == null){
				contexts = new ConcurrentHashMap<String, AsyncContext>();
			}
			contexts.put(userId, asyncContext);
		}
		
		listener.addListener(this);	
		sendNotification(userId, "connected");
	}
	
	protected void sendNotification(String userId, String notification){
		try{
			if(contexts.containsKey(userId)){
				AsyncContext asyncContext = contexts.get(userId);
				PrintWriter writer = asyncContext.getResponse().getWriter();
				writer.write("data: " + notification + "\n\n");
				writer.flush();
			}
		}catch (Exception e){
			e.printStackTrace();
			//try again before disconnecting the user
			try{
				if(contexts.containsKey(userId)){
					AsyncContext asyncContext = contexts.get(userId);
					PrintWriter writer = asyncContext.getResponse().getWriter();
					writer.write("data: " + notification + "\n\n");
					writer.flush();
				}
			}catch (Exception e2){
				e2.printStackTrace();
				AsyncContext asyncContext = contexts.remove(userId);
				asyncContext.complete();
			}
		}
	}

	@Override
	public void onNotification(NotificationEvent event) {
		this.sendNotification(event.getUserId(), event.getJSONEvent());
	}
	
}

NotificationEvent.java

import org.json.JSONException;
import org.json.JSONObject;

public class NotificationEvent {
	private String type;
	private String message;
	private String toUserId;
	
	public NotificationEvent(String type, String message, String toUserId){
		this.type = type;
		this.message = message;
		this.toUserId = toUserId;
	}
	
	public NotificationEvent(String type, String toUserId){
		this.type = type;
		this.message = "";
		this.toUserId = toUserId;
	}
	
	public String getType(){
		return this.type;
	}
	
	public String getMessage(){
		return this.message;
	}
	
	public String getUserId(){
		return this.toUserId;
	}
	
	public String getJSONEvent(){
		JSONObject obj = new JSONObject();
		try {
			obj.put("type", type);
			obj.put("message", message);
		} catch (JSONException e) {
			e.printStackTrace();
		}
		return obj.toString();
	}
	
}

NotificationListener.java

import java.util.ArrayList;
import java.util.List;

import javax.ejb.Asynchronous;
import javax.ejb.Stateful;
import javax.enterprise.context.ApplicationScoped;
import javax.enterprise.event.Observes;

@Stateful
@ApplicationScoped
public class NotificationListener {
	
	List<DefaultListener> listeners = new ArrayList<>();
	
	public void addListener(DefaultListener l){
		listeners.add(l);
	}
	
	@Asynchronous
	public void listenForNotifications(@Observes NotificationEvent event){
		//this method is called by the container whenever a NotificationEvent is fired
		for (DefaultListener l : listeners){
			l.onNotification(event);
		}
	}
	
	@Asynchronous
	public void checkForUpdates(String userId){
		//manually check the database for updates
		
	}
	
}

notify.js

(function(){
	console.log("notify.js");
	var eventSource = new EventSource("notify?userId=123456");//need a way to access userId from server, then apply it here
	eventSource.onopen = function(e){
		console.log("Connection Opened");
	};
	eventSource.onmessage = function(e){
		console.log("Message: " + e.data);
	};
	eventSource.onerror = function(e){
		console.log("Error");
	};
	
})();