I am trying to implement real-time notification system using Servlet and ServerSentEvents

This is the following code I have implemented

@WebServlet("/notification")
public class NotificationSSE extends HttpServlet {

    @Override
    public void init() throws ServletException {
        this.getServletContext().setAttribute("is_message", new Object());
        super.init();
    }

    @Override
    protected void doGet(HttpServletRequest reuest, HttpServletResponse response) throws ServletException, IOException {
        Object obj = null;
        try {
            obj = this.getServletContext().getAttribute("is_message");

            response.setContentType("text/event-stream");
            response.setCharacterEncoding("UTF-8");

            PrintWriter writer = response.getWriter();

            MessagesHolder.users.add(response);
            
            synchronized (obj) {
                obj.wait();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

    }

This is the MessageHolder which will be responsible to send message

I have a plan to change the Vector to a HashMap which would link the user and send the Message only to the particular user

public class MessagesHolder {

    public static Vector<HttpServletResponse> users = new Vector<>();

    public void sendMessage(Message message) {
        try {
            for (HttpServletResponse user : users) {
                PrintWriter writer = user.getWriter();
                writer.write("data: " + message.toJson() + "nn");
                writer.flush();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

This is a servlet which is responsible for receiving the messages post by the user and stores it in the database

@WebServlet("/send-message")
public class SendMessage extends HttpServlet {

    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        try {

            String messageSubject = req.getParameter("message_subject");
            String messageContent = req.getParameter("message_content");
            String messageSendTime = req.getParameter("message_sendTime");
            String[] receivers = req.getParameterValues("receivers[]");
            String user_id = req.getSession().getAttribute("user_id").toString();
            try (
                    Connection conn = DriverManager.getConnection(DBUtil.getUrl(), DBUtil.getUser(),
                            DBUtil.getPassword());
                    PreparedStatement insertMessage = conn.prepareStatement(
                            "INSERT INTO Message(message_title, message_content, sender_id, send_time) VALUES (?, ?, ?, ?)",
                            Statement.RETURN_GENERATED_KEYS);
                    PreparedStatement insertNotify = conn
                            .prepareStatement("INSERT INTO Notification (message_id, receiver_id) VALUES (?,?)");
                    PrintWriter out = resp.getWriter();) {
                insertMessage.setString(1, messageSubject);
                insertMessage.setString(2, messageContent);
                insertMessage.setString(3, user_id);
                insertMessage.setString(4,
                        messageSendTime.split("T")[0] + " " + messageSendTime.split("T")[1] + ":00");
                insertMessage.executeUpdate();
                Message message = new Message(user_id, messageSubject, messageContent);
                ResultSet generatedKeys = insertMessage.getGeneratedKeys();
                if (generatedKeys.next()) {
                    int i = 0;
                    Vector<String> receiverList = new Vector<>();
                    long messageId = generatedKeys.getLong(1);
                    insertNotify.setLong(1, messageId);
                    while (i < receivers.length) {
                        insertNotify.setString(2, receivers[i]);
                        insertNotify.addBatch();
                        receiverList.add(receivers[i]);
                        i++;
                    }
                    insertNotify.executeBatch();
                    
                    out.write("sent succssfully");
                    MessagesHolder messagesHolder = new MessagesHolder();
                    messagesHolder.sendMessage(message);
                    
                    out.flush();
                }
            } catch (Exception ex) {
                ex.printStackTrace();
            }

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

The Message Object

public class Message {
    public String senderId;
    public String title;
    public String message;
    public Status status;

    public Message(String senderId, String title, String message) {
        this.senderId = senderId;
        this.title = title;
        this.message = message;
        this.status = Status.SENT;
    }

    @Override
    public String toString() {
        return "Message [senderId=" + senderId + ", title=" + title + ", message=" + message + ", status=" + status
                + "]";
    }

    public JSONObject toJson(){
        JSONObject obj = new JSONObject();
        obj.put("senderId", senderId);
        obj.put("title", title);
        obj.put("message", message);
        obj.put("status", status);
        return obj;
        
    }
}

Is there any better way to implement this because I read that It’s not recommended to use wait() inside request

• What are other ways I could implement the server sent events to achieve the notification sending.

• I don’t know much about Http2 keep-alive connection, Is there any possibility with that I can create a channel send the event streams with that

• Also need a clarification that is there a possibility that could I implement this with ServletListeners by any chance and send the notification