@RequestMapping(value = "/addUser", method = RequestMethod.POST)
public String addUser(@RequestParam(value="firstname") String firstname,
                      @RequestParam(value="lastname") String lastname,
                      @RequestParam(value="username") String username,
                      @RequestParam(value="password") String password) throws Exception {
    String success = add.addUser(firstname, lastname, username, password);
    return toJson(success);
}

var data = {
        firstname: firstname,
        lastname: lastname,
        username : username,
        password : password
    };
    $.ajax({
        type: 'POST',
        url: "<url removed>/import/addUser",
        success: function(data) {
            callbackSuccess(JSON.stringify(data));
        },
        error: function(data) {
            callbackFail(data);
        },
        data: data,
        dataType: "json"
    });

String url = "<url removed>/import/addUser";
        InputStream inputStream = null;
        String result = "";
                try {
                    // 1. create HttpClient
                    HttpClient httpclient = new DefaultHttpClient();

                    // 2. make POST request to the given URL
                    HttpPost httpPost = new HttpPost(url);
                    String json = "";
                    // 3. build jsonObject
                    JSONObject jsonObject = new JSONObject();
                    jsonObject.accumulate("firstname", userFirstName);
                    jsonObject.accumulate("lastname", userLastName);
                    jsonObject.accumulate("username", emailStr);
                    jsonObject.accumulate("password", passwordStr);

                    // 4. convert JSONObject to JSON to String
                    json = jsonObject.toString();

                    // 5. set json to StringEntity
                    StringEntity se = new StringEntity(json);

                    // 6. set httpPost Entity
                    httpPost.setEntity(se);
                    Log.i("Sending", json);
                    // 7. Set some headers to inform server about the type of the content
                    httpPost.setHeader("Accept", "application/json");
                    httpPost.setHeader("Content-type", "application/json");

                    // 8. Execute POST request to the given URL
                    HttpResponse httpResponse = httpclient.execute(httpPost);

                    // 9. receive response as inputStream
                    inputStream = httpResponse.getEntity().getContent();

                    // 10. convert inputstream to string
                    if(inputStream != null)
                        result = convertInputStreamToString(inputStream);
                    else
                        result = "Did not work!";

                Log.i("Response", result);