Regarding android server error 500

Question

I am trying to send request to my my web application servlet deployed in glass fish sever and receive json response using volley HTTP library. I managed to get the response from several servlets but but with one servlet I m getting error BasicNetwork.performRequest: Unexpected response code 500 . I tried with switching HTTP methods, setting content type as "application/json" in servlet and implement the getHeaders() method and may solutions given in SO regarding this issue. But I couldnt solve this problem yet.

My Volley request is given below :

JsonObjectRequest jsonObjectRequest = new JsonObjectRequest(Request.Method.GET, url,
        new Response.Listener<JSONObject>() {
            @Override
            public void onResponse(JSONObject response) {
                Log.i("RESPONSE",response.toString());
                //Type type = new TypeToken<CartItem>() {}.getType();
                //CartItem item = new Gson().fromJson(response.toString(), type);
                //cartItem = item;
            }
        }, new Response.ErrorListener() {
    @Override
    public void onErrorResponse(VolleyError error) {
        Log.i("Volley_error", error.toString());
    }
});

My json response in the browser looks like below :

{
    "items": [{
        "p_id": 1,
        "p_name": "Multi Checked Shirt",
        "p_price": 1234.0,
        "p_size": "M",
        "p_img": "uploads/products/Dorothy-Perkins-Multi-Checked-Shirt-4510-9254471-1-pdp_slider_l.jpg",
        "p_qnty": 1
    }, {
        "p_id": 15,
        "p_name": "Rust Solid Coloured Pant",
        "p_price": 3500.0,
        "p_size": "S",
        "p_img": "uploads/products/Dorothy-Perkins-Rust-Solid-Coloured-Pant-971120977.jpg",
        "p_qnty": 2
    }],
    "cart_total": 8234.0,
    "total_items": 3
}

My Servlet code :

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    if (request.getSession(false).getAttribute("cart") != null) {

        ShoppingCart cart = (ShoppingCart) request.getSession(false).getAttribute("cart");
        List<CartItem> cartItems = cart.getShoppingList();
        List<Double> tot = new ArrayList<>();
        CartModel model = new CartModel();
        List<CartItemModel> cartItemModels = new ArrayList<>();
        for (CartItem item : cartItems) {

            CartItemModel itemModel = new CartItemModel(item.getProduct_id(),
                    cart.getNameofProduct(item.getProduct_id()),
                    cart.getPriceofProduct(item.getProduct_id(), item.getSize()),
                    item.getSize(),
                    path + cart.getImageofProduct(item.getProduct_id()),
                    item.getQnty());
            cartItemModels.add(itemModel);
            tot.add(cart.getPriceofProduct(item.getProduct_id(), item.getSize()) * item.getQnty());

        }
        model.setCart_total(cart.grandTotal(tot));
        model.setTotal_items(cart.getTotalItemsOfTheCart());
        model.setItems(cartItemModels);
        Type type = new TypeToken<CartModel>() {
        }.getType();
        String element = new Gson().toJson(model, type);
        response.getWriter().write(element);
    }
}

I know it is not a good idea to modify the Volley library code but I tried that too.

if (statusCode < 200 || statusCode > 299) { throw new IOException();}

to

if (statusCode < 200 || statusCode > 505) {throw new IOException();}

Then I got Error :

JSONException: Value <!DOCTYPE of type java.lang cannot be converted to JSONObject

I am new to android . Any help would be appreciable. Thank you.

UPDATE :

Here is how when I access url with HttpRequester :

Answer 1

Based on

JSONException: Value <!DOCTYPE

Your servlet is returning HTML, not plaintext JSON, so it can't be parsed.

Since you are getting a 500 Internal server error, that HTML content is the real error here. If you access that URL from your browser, or using a tool such as Postman, you'll be able to see the stacktrace of your servlet.

Once you fix that, you may want to set the header for Content-Type to application/json in either the request or response to force json to be written.

For full details of a solution, see How to send JSON back with Java