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Need Cookbook JavaScript to Servlet Example

I need a simple example on how pass and retrieve information from a servlet using java script.   There's a couple of related examples on EE, but I just don't get the whole picture.

Perhaps a simple cookbook example would make more sense.

Servlet takes values a and b and returns a + b

function callAddServlet(a, b){
    // pseudo code ,  How do I pass A and B to servlet and return c?
    c = a + b;
    return c;

What does the servlet look like that take 'a' and 'b' and returns 'a + b'?

How do i call the servlet from the JavaScript function.
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2 Solutions
 One way would be to use Ajax:

  The pseudo code for this would be:

  - create a request object
  - connect to servlet, sending values
  - wait for response

  The servlet would simply take the specified parameters, operate on them, and respond with the result.

  // Ajax routines for calling a servlet
  var xmlHttp;

  // Name: createXMLHttpRequest()
  // Role: Create the XML Request Object used by the Ajax routines
  function createXMLHttpRequest() {
    if ( window.ActiveXObject ) {
      xmlHttp = new ActiveXObject( "Microsoft.XMLHTTP" );
    } else if ( window.XMLHttpRequest ) {
      xmlHttp = new XMLHttpRequest();
  // Name: getInfo()
  // Role: Issue the request to the Addition servlet
  function getInfo( a, b ) {
    var queryString = '/servlet/Addition?A=' + a + '&B=' + b;
    xmlHttp.onreadystatechange = handleStateChange;
    xmlHttp.open( 'GET', queryString, true );
    xmlHttp.send( null );

  // Name: handleStateChange()
  // Role: Callback routine invoked when XML Request state changes
  function handleStateChange() {
    var field = document.getElementById( 'state' );
    field.value += xmlHttp.readyState + ' ';
    if ( xmlHttp.readyState == 4 ) {
        field.value += '  RC = ' + xmlHttp.status;
      if ( xmlHttp.status == 200 ) {

  // Name: parseResults()
  // Role: Routine used to process results of Addition
  function parseResults() {
    document.getElementById( "result" ).value = xmlHttp.responseText;
Alternative using the javascript prototype ajax object(http://prototype.conio.net/)

<script src="prototype-1.4.0.js"></script>
function getInfo(a, b) {
var myAjax = new Ajax.Request(
                  'yourservleturl?a=' + a + '&b=' + b,  
                        method: 'get',
                        onComplete: function(response) {$F("result") = response.responseText;}

The servlet would simply take the a and b values from the querystring, add them and return the value(pseudo code below):
write querystiring(a) + querystring(b);
tmonteitAuthor Commented:
Morcalvin, I think i get it...  does getInfo need a return value?

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No.  the onComplete parameter will run an anonymous function that will populate an element value with the response from the servlet.  You could also populate a span or a div element with an id of "result" by doing the following:
$("result").innerHTML = response.responseText;
tmonteitAuthor Commented:
For the other half of the question, What would the servlet look like that returns for example:  2 + 3 = 5?

Stating the obvious...  Does the code that calls 'getInfo' then become:

var mySum = getInfo(a, b);

No.  The server side code depends on the type of language you are using.  PHP, perl, ruby, python, asp, asp.net, etc.  It's entirely up to you.  All it needs to do is add up the two values and print the data out to the data to the browser.
getInfo has no return value.  response.responseText would be the value returned from your server side code(in your case the sum of a and b).  So if you wanted to use that value in some other function, you would need to pass the value to that function like so
...ajax code...
onComplete: function(response) {myFunction(response.responseText)}
...more ajax code..

function myFunction(mySum) {
         alert('The server added up a and b and the sum is + ' mySum);
tmonteitAuthor Commented:
Sorry, I'm getting confused over this 'missiing return value'.  Say its a simple Java Servlet

I pretty much have this code working.. passing in the a and b from the ajax.    Return of sum back to Javascript still baffles me.

public ActionForward executeAdd(ActionMapping mapping,
                  ActionForm form,
                  HttpServletRequest request,
                  HttpServletResponse response)
      AddForm addForm = (AddForm) form;
      Integer a = addForm.getA();
      Integer b = addForm.getB();

      Integer c = a + b;
   // Now what?

tmonteitAuthor Commented:
At the point I call getInfo, where do I get the return value?

var c = getInfo(a, b)   ??

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