Errorstream in HttpUrlConnection

Question

I want to do a POST request to an HTTP Servlet I wrote myself. Good case (HTTP response Code 200) always works fine by using URL.openConnection() method. But when I receive a desired error response code (e.g. 400) then I thought I have to use HttpUrlConnection.getErrorStream(). But the ErrorStream object is null though I am sending data back from the servlet in error case (I want to evaluate this data to generate error messages). This is what my code looks like:

HttpURLConnection con = null;
        try {
            //Generating request String
            String request = "request="+URLEncoder.encode(xmlGenerator.getStringFromDocument(xmlGenerator.generateConnectRequest(1234)),"UTF-8");
            //Receiving HttpUrlConnection (DoOutput = true; RequestMethod is set to "POST")
            con = openConnection();
            if (con != null){
                PrintWriter pw = new PrintWriter(con.getOutputStream());
                pw.println(request);
                pw.flush();
                pw.close();
                InputStream errorstream = con.getErrorStream();

                BufferedReader br = null;
                if (errorstream == null){
                    InputStream inputstream = con.getInputStream();
                    br = new BufferedReader(new InputStreamReader(inputstream));
                }else{
                    br = new BufferedReader(new InputStreamReader(errorstream));
                }
                String response = "";
                String nachricht;
                while ((nachricht = br.readLine()) != null){
                    response += nachricht;
                }
            }
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

So my question is, why returns getErrorStream() null though status code is 400 (I can see it in the IOException that is thrown when it calls con.getInputStream())

Thanks

Answer 1

From the java documentation on getErrorStream():

Returns the error stream if the connection failed but the server sent useful data nonetheless. The typical example is when an HTTP server responds with a 404, which will cause a FileNotFoundException to be thrown in connect, but the server sent an HTML help page with suggestions as to what to do. This method will not cause a connection to be initiated. If the connection was not connected, or if the server did not have an error while connecting or if the server had an error but no error data was sent, this method will return null. This is the default.

So if you didn't get to the server (bad url for example) or the server didn't send anything in the response, getErrorStream() will return null.