Java Servlet runs with status code HTTP/500

Question

When I run this on Tomcat web server and try to POST the FORM with the right or wrong credentials it results to a HTTP/500 error.

The servlet code:

/*
 * Written by Leonardo van de Weteringh
 */
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet("/check")
public class check extends HttpServlet {

    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        final String Username = "Noord";
        final String Password = "Korea";

        String formUsername = request.getParameter("username");
        String formPassword = request.getParameter("password");

        if (formUsername.equals(Username) && formPassword.equals(Password)) {
            response.sendRedirect("/internet/ok.html");
        } else {
            RequestDispatcher view = request.getRequestDispatcher("/internet/fail.html");
            view.forward(request, response);
        }
    }
}

This is the Login Page code:

<!DOCTYPE html>
<html lang="en">
<head>
<title>Koryo Airlines DPRK</title>
<link rel="stylesheet" type="text/css" href="../../scripts/css/login.css">
<link rel="shortcut icon" type="image/png" href="../../content/images/favicon.png"/>
<meta name="viewport" content="width=device-width, initial-scale=1">
</head>
<body>
<div class=navbar>
<ul class=horinav>
  <li class=horinav><a href="../../.">Home</a></li>
  <li class=horinav><a href="../../news/news.html">News</a></li>
  <li class=horinav><a href="../../contact/contact.html">Contact</a></li>
  <li class=horinav><a href="../../about/about.html">About</a></li>
  <li class=horinav>
  <div class=dropdown>
    <button class="dropbtn">More...</button>
  <div class="dropdown-content">
    <a href="../../about/flyingkoryo.html">Flying Koryo</a>
    <a href="../../about/aboutkoryo.html">About Koryo</a>
    <a href="../../about/bagage.html">Bagage</a>
    <a href="../../about/checkin.html">Check-In Procedure</a>
    <a href="../../websites/koreatoday.html">Korea Today</a>
    <a href="../../websites/friendsofkorea.html">More Websites...</a>
  </div>  
  <li class=horinav style="float:right"><a href="../register/register.html">Register</a></li>
  <li class=horinav style="float:right"><a class="active" href="#">Login</a></li>
</ul>
</div>
<div class="row">
<div class="column">
<center><img src="../content/images/banner.jpg" alt="" height="200" width="50%"></center>
</div>
</div>

<h1> Koryo Airlines Login Portal</h1>

<form action="/check" method="post">
  <div class="imgcontainer">
    <img src="../../content/images/login.jpg" alt="Avatar" class="avatar">
  </div>
<center>
  <div class="container">
    <label for="username"><b>Username</b></label>
    <input type="text" name="username" placeholder="Gebruikersnaam" maxlength="20" required>
<br>
    <label for="password"><b>Password</b></label>
    <input type="password" name="password" placeholder="Wachtwoord" maxlength="20" required>

    <button type="submit">Login</button>

    <label>
      <input type="checkbox" checked="checked" name="remember"> Remember me
    </label>
  </div>
</center>
  <div class="container" style="background-color:#f1f1f1">
    <button onclick="window.location.href='../../index.html'" type="button" class="cancelbtn">Cancel</button>
    <span class="psw">Forgot <a href="./forgotpsw.html">password?</a></span>
  </div>

</form>

</body>
</html>

The error I get:

 java.lang.NullPointerException check.doPost(check.java:21)
 javax.servlet.http.HttpServlet.service(HttpServlet.java:660)
 javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
 org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)

Can somebody help me. This is what the Servlet needs to do:

Accept input from the form and POST it.
To check (validate) the input with a hard-coded username and password and see if there is any match.
Redirect to the page ok.html or the fail.html

Can you tell what is the code on line _21_ of the `check.java`? (`java.lang.NullPointerException check.doPost(check.java:21) `. — prasad_, Jan 15 '19 at 05:17
I think you are getting a `null` value for `formUsername` from this code: `String formUsername = request.getParameter("username");`. And it is resulting in the `NullPointerException` at: `if (formUsername.equals(Username)`. It is difficult to tell from the code you have shown what is wrong; the code looks alright at the first look. — prasad_, Jan 15 '19 at 05:50
Did you enter _user name_ and _password_ values in the HTML form? If you don't enter values and submit the form you will get that error. — prasad_, Jan 15 '19 at 05:56
One way to check if your form submit is working and the parameters are sent to the servlet is do these three steps: (1) remove the form method="post" and it will default to "get". (2) When you do the form submit you can see the request parameters and values in the browser address bar like this `http://example.com/path/to/page?username=John&password=Doe`; note the query string has the request parameter names and their values. What do you see there? (3) Add a `doGet` method to the servlet to verify the posting of the form from the changed HTML. — prasad_, Jan 15 '19 at 06:08

score 0 · Answer 1 · answered Jan 15 '19 at 01:19

0

Maybe your variable（formUsername or formPassword） is null value , So your program gives a null pointer。

You should check if you can get the name and password from the request.

answered Jan 15 '19 at 01:19

lixinglin

61
1
4

score 0 · Answer 2 · answered Jan 15 '19 at 12:07

Thank prasad_ for his last comment to change the POST to GET to check the variables which are given. They did not seem to match in the HTML form. I changed the

<input type="text" name="username" placeholder="Gebruikersnaam" maxlength="20" required>

to

<input type="text" name="Username" placeholder="Gebruikersnaam" maxlength="20" required>

and

<input type="password" name="password" placeholder="Wachtwoord" maxlength="20" required>

to

<input type="password" name="Password" placeholder="Wachtwoord" maxlength="20" required>

All the other changes remain untouched and it worked again. The credentials when entered OK are redirecting to ok.html

But, your servlet code shows the parameter names as "username" and "password"? — prasad_, Jan 15 '19 at 16:03

score -1 · Answer 3 · answered Jan 15 '19 at 03:13

One thing I realized your servlet code is not overriding the default doPost method. You should have something like this normally

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
                   throws ServletException, IOException

and other thing I can say check if you are adding javax.servlet.api-3.0.1.jar (there might be latest version)

Java Servlet runs with status code HTTP/500

3 Answers3