I have a servlet in jsp named login servlet which checks the credentials if the password matchs or not. code is as follows:

LoginServlet.java

package com.servlet;

import java.io.IOException;

import jakarta.servlet.RequestDispatcher; import jakarta.servlet.ServletException; import jakarta.servlet.annotation.WebServlet; import jakarta.servlet.http.HttpServlet; import jakarta.servlet.http.HttpServletRequest; import jakarta.servlet.http.HttpServletResponse; @WebServlet("/login") public class LoginServlet extends HttpServlet { private static final long serialVersionUID = 1L; public LoginServlet() { super(); } protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { } protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String responsePage = "welcome.jsp"; String userName = request.getParameter("userName"); String password = request.getParameter("password"); String message=""; if (UserCache.users.containsKey(userName)) { User user = UserCache.users.get(userName); if (!user.getPassword().equals(password)) { responsePage="index.jsp"; message="Invalid Credentials"; }else { message=user.getName(); } } RequestDispatcher requestDispatcher = request.getRequestDispatcher(responsePage); request.setAttribute("message", message); requestDispatcher.forward(request, response); }

}

and code for jsp files i am having

-------------------------------------------index.jsp--------------------------------------------

<%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%>

Sign In
User Name
Password
Register
<%=request.getAttribute("message")==null?"":request.getAttribute("message")%>

--------------------------------------------------------------------Register.jsp-------------------------------

<%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%>

Registration
Name
User Name
Password

------------------------------------------------------------welcome.jsp-------------------------------------------------

<%@ page language="java" contentType="text/html; charset=ISO-8859-1" pageEncoding="ISO-8859-1"%>

Welcome <%=request.getAttribute("message") %> Logout

My question is I want to add an alert using javascript if the user didn't enter any username and starts clicking on the login button. please help me where to add