How do I trigger a servlet method on page load

Question

I have entries in a text file which I want to load into a drop down box.

I want this to happen on loading a jsp page. I could easily do this if I had a button that triggered the passing of a request object to the servlet. How do I send a request object on page load - do I need to use Javascript or is this something I can do with just jps.

Answer 1

Where you can populate it on the serverside. By that I mean that when you create the select box on your JSP, populate it then. For example:

<select id="sel1" name="animal">
<c:forEach var="animal" items="${model.animals}">
<option value="<c:out value="${animal.id}"/><c:out value="${animal.name}"/></option>
</c:forEach>
</select>

If that isn't possible, practical or desired, you'll need to use some form of AJAX method. Personally I use jQuery for this. For example:

<select id="sel1" name="animal">
</select>

<script type="text/javascript">
$(function() {
  $.get('/server/getanimals', function(data, textStatus) {
    var sel = $("#sel1");
    for (var i=0; i<data.length; i++) {
      sel.append('<option value="' + data[i].id + '">' + data[i].name + '</option>');
    }
  }, 'json');
});
</script>

The above calls /server/getanimals on page ready. It is expecting to be returned a JSON object with a list of animals that it then uses to populate the select box.

There are lots of ways to skin this particular cat.

Answer 2

You can use Servlet's doGet() method to preprocess data on GET requests. The doPost() is to be used to postprocess data on POST requests (when you submit the form).

Example:

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    List<Option> options = optionDAO.list();
    request.setAttribute("options", options);
    request.getRequestDispatcher("page.jsp").forward(request, response);
}

Where the Option class is just your own javabean class containing the properties value and label. You can also use a List<Map<String, String>> instead if you like.

In JSP you can use the JSTL c:forEach to iterate over a list

<select name="foo">
    <c:forEach items="${options}" var="option">
        <option value="${option.value}">${option.label}</option>
    </c:forEach>
</select>

Finally map the servlet in web.xml and invoke the request so that it matches its url-pattern, e.g. http://example.com/contextroot/page.