Question

I’m trying to send some simple string to test data transfer between my servlet and an applet
(servlet -> applet, not applet -> servlet), using Json format and Gson library.
The resulting string in the applet should be exactly the same as the original message, but it isn’t.
I’m getting 9-character < !DOCTYPE string instead.

edit: It looks like the servlet returned HTML web page instead of JSON, didn’t it?
edit2: The message is correctly displayed in a browser, when lauching the servlet using “Run File” command in NetBeans.

Could you please take a look at my code:

Servlet:

//begin of the servlet code extract
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException
{
    PrintWriter out = response.getWriter();
    try
    {
        String action;
        action = request.getParameter("action");
        if ("Transfer".equals(action))
        {
            sendItToApplet(response);
        }
    }
    finally
    {
        out.close();
    }
}

public void sendItToApplet(HttpServletResponse response) throws IOException
{
    String messageToApplet = new String("my message from the servlet to the applet");
    String json = new Gson().toJson(messageToApplet);
    response.setContentType("application/json");
    response.setCharacterEncoding("UTF-8");

    Writer writer = null;
    writer = response.getWriter();
    writer.write(json);
    writer.close();
}
//end of the servlet code extract

Applet:

//begin of the applet code extract
public void getItFromServlet() throws MalformedURLException, IOException, ClassNotFoundException
{
    URL urlServlet = new URL("http://localhost:8080/Srvlt?action=Transfer");
    URLConnection connection = urlServlet.openConnection();
    connection.setDoInput(true);
    connection.setDoOutput(true);
    connection.setUseCaches(false);
    connection.setRequestProperty("Content-Type", "application/json");

    InputStream inputStream = connection.getInputStream();
    BufferedReader br = new BufferedReader(new InputStreamReader(inputStream));
    JsonReader jr = new JsonReader(br);
    String retrievedString = new Gson().fromJson(jr, String.class);
    inputStream.close();
    jtextarea1.setText(retrievedString); //jtextarea is to display the received string from the servlet in the applet
}
//end of the applet code extract

Answer 1

The problem is that you’re not sending JSON from your servlet, as you have figured out from your comments. And this is because … Gson is very confused as to what you’re trying to do.

If you test your JSON serialization (the output from toJson()) in your servlet, you’ll find that … it’s not doing anything and simply returning the contents of your String with quotes around it. JSON is based around a textual representation of an object (the class’ fields to values), and it certainly doesn’t want to do that with a String object; the default serialization there is to get the contents of the String to be put into the resulting JSON.

Edit to add: A typical use for Gson would be something like:

class MyClass {
    String message = "This is my message";
}

…

String json = new Gson().toJson(new MyClass());

The resulting JSON would be:

{“message”:”This is my message”}