[FIXED] Servlet to JSP always passing null value

Issue

I just started using JSP and Servlet, so I encountered a really basic problem. I'm trying to make a request from JSP to servlet, where I set a parameter and then forward the answer from servlet back to the jsp. Here is the code from my JSP:

<% String s = (String)request.getAttribute("name");
   out.println(s);
%>

Here is my code from servlet:

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    try (PrintWriter out = response.getWriter()) {
        request.setAttribute("name", new String("aa"));
        this.getServletContext().getRequestDispatcher("/index.jsp").forward(request,response);
    }
}

So in the end, the servlet has the value, but my jsp doesn't.

Solution

Here you have already declared a String type but you cast it as String also, this is redundant.

<% String s = (String)request.getAttribute("name");
   out.println(s);
%>

Also, there's a difference between <%= %> and <% %>. If you want to output the variable into your jsp use the one with the equals (<%= %>). The second line of your scriptlet code would also generate an error. The code you write in your servlet doesn't just continue on the JSP, it's not how it works.

if you want to output the name attribute just do this:

<%= request.getAttribute("name") %>

However since 2010 scriptlets are discouraged (outdated technology).. We use EL and JSTL instead. You should be able to just output the variable like this:

${name}

In your Servlet all you need to do is this:

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

  String name = "Jane"; //create a string
  request.setAttribute("name", name); //set it to the request

  RequestDispatcher rs = request.getRequestDispatcher("index.jsp"); //the page you want to send your value
  rs.forward(request,response); //forward it

}

EDIT

You asked,

Is there a way to trigger the servlet let s say on a click of a button or something like that?

Yes, there are multiple ways to do it and it really depends on how you want it setup. An easy way to trigger the servlet on a button click is like this. *(Assuming you have a servlet mapped onto /Testing):

<a href="/Testing">Trigger Servlet<a>

Another way could be with a form:

<form action="Testing" method="get">
<input type="hidden" name="someParameterName" value="you can send values like this">
<button type="submit">Do some magic</button>
</form>

There's also AJAX (which involves javascript). But this is fairly advanced and i don't recommend doing it until you are familiar with normal synchronous http behaviour.

Answered By - Jonathan Laliberte
Answer Checked By - Terry (JavaFixing Volunteer)