How perform validation and display error message in same form in JSP?

Question

How can I display an error message in the very same JSP when a user submits a wrong input? I do not intend to throw an exception and show an error page.

Answer 1

I'm not quite sure what you mean by "display error message". If you have a standard error handling, then you can always check for options:

<%
    if(wrong option selected)
        throw new Exception("Invalid option selected");
%>

Of course, this is just the notion; preferably, you'd have your own exception class. But then again, I'm not quite sure what you're after.

Answer 2

Easiest would be to have placeholders for the validation error messages in your JSP.

The JSP /WEB-INF/foo.jsp:

<form action="${pageContext.request.contextPath}/foo" method="post">
    <label for="foo">Foo</label>
    <input id="foo" name="foo" value="${fn:escapeXml(param.foo)}">
    <span class="error">${messages.foo}</span>
    <br />
    <label for="bar">Bar</label>
    <input id="bar" name="bar" value="${fn:escapeXml(param.bar)}">
    <span class="error">${messages.bar}</span>
    <br />
    ...
    <input type="submit">
    <span class="success">${messages.success}</span>
</form>

In the servlet where you submit the form to, you can use a Map<String, String> to get hold of the messages which are to be displayed in JSP.

The Servlet @WebServlet("foo"):

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.getRequestDispatcher("/WEB-INF/foo.jsp").forward(request, response);
}

@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    Map<String, String> messages = new HashMap<String, String>();
    request.setAttribute("messages", messages); // Now it's available by ${messages}

    String foo = request.getParameter("foo");
    if (foo == null || foo.trim().isEmpty()) {
        messages.put("foo", "Please enter foo");
    } else if (!foo.matches("\\p{Alnum}+")) {
        messages.put("foo", "Please enter alphanumeric characters only");
    }

    String bar = request.getParameter("bar");
    if (bar == null || bar.trim().isEmpty()) {
        messages.put("bar", "Please enter bar");
    } else if (!bar.matches("\\d+")) {
        messages.put("bar", "Please enter digits only");
    }

    // ...

    if (messages.isEmpty()) {
        messages.put("success", "Form successfully submitted!");
    }

    request.getRequestDispatcher("/WEB-INF/foo.jsp").forward(request, response);
}

In case you create more JSP pages and servlets doing less or more the same, and start to notice yourself that this is after all a lot of repeated boilerplate code, then consider using a MVC framework instead.

See also:

• Our Servlets wiki page
• What is the best practice for validating parameters in JSP?

Answer 3

I see tag "form-validation", so maybe you should just use JavaScript and client form validation? If you need validation with JSP, handle form data, and redisplay the form with an error message or accept form data, if it's correct.