Question

I have a JSP with some fields. When I fill in the fields and click the send button, I have to go check that the data in database exists; if it does not, I display a popup alerting the user that the data does not exist in the database. If they choose to continue anyway it goes on the screen; if not it returns to the starting point.

I’m stuck at where I display the popup, since the server can not display the popup on the client

Answer 1

Just let Servlet store the condition in request scope and let JSP print the Javascript code conditionally.

Servlet:

boolean exist = yourDAO.exist(parameters);
request.setAttribute("exist", exist);
request.getRequestDispatcher("page.jsp").forward(request, response);

JSP (using JSTL):

<c:if test="${!exist}">
    <script>
        if (confirm('Data does not exist, do you want to continue?')) {
            // Do whatever you want when user want to continue ("goes on screen").
        } else {
            // Do whatever you want when user don't want to continue ("returns to starting point").
        }
    </script>
</c:if>