implementing Server Sent Events using java Servlet for sending notification by calling wait inside the HttpServlet

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() +"\n\n");                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;    }}

Client Side js

let eventSource = new EventSource("notification");const list = document.querySelector("ul");eventSource.onopen = (event) => {    list.append("Connection created");    };eventSource.onerror = (event) => {    console.log("Error occured");  console.log(event);};eventSource.onmessage = function (event) {  let listItem = document.createElement("li");  listItem.innerText = event.data;  list.append(listItem);      console.log(event, event.data);};

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