问异步任务和从url NullPointerException解析JSONObject

EN

public class JSONParser {
    static InputStream is = null;
    static JSONObject jObj = null;
    static String json = "";
    // constructor
    public JSONParser() {
    }
    public JSONObject getJSONFromUrl(String url) {
        // Making HTTP request
        try {
            // defaultHttpClient
            HttpParams params = new BasicHttpParams();
            params.setParameter(CoreProtocolPNames.PROTOCOL_VERSION, HttpVersion.HTTP_1_1);
            HttpClient httpClient = new DefaultHttpClient(params);
            HttpPost httpPost = new HttpPost(url);
            HttpResponse httpResponse = httpClient.execute(httpPost);
            HttpEntity httpEntity = httpResponse.getEntity();
            is = httpEntity.getContent();
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (ClientProtocolException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(
                    is, "iso-8859-1"), 8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
            is.close();
            json = sb.toString();
        } catch (Exception e) {
            Log.e("Buffer Error", "Error converting result " + e.toString());
        }
        // try parse the string to a JSON object
        try {
            jObj = new JSONObject(json);
        } catch (JSONException e) {
            Log.e("JSON Parser", "Error parsing data " + e.toString());
        }
        // return JSON String
        return jObj;
    }
}

public class MainActivity extends ActionBarActivity {

    private TextView tv;
    private static final String urllisteemplois = "the url that contains the json";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv = (TextView) findViewById(R.id.text);
        JParse jParse = new JParse(urllisteemplois);
        jParse.execute();
        String result = jParse.concat();
        tv.setText(result);
    }
}

public class JParse extends AsyncTask<String, String, JSONObject> {
    private String url;
    private JSONObject json;

    public JParse(String url) {
        this.url=url;
    }


    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }

    @Override
    protected JSONObject doInBackground(String... args) {
        JSONParser jParser = new JSONParser();
        JSONObject json2 = jParser.getJSONFromUrl(url);
        return json2;
    }

    @Override
    protected void onPostExecute(JSONObject json2) {
        this.json=json2;
    }

    public String concat()
    {
        String result = this.json.toString()+"hello";
        return result;
    }
}

01-20 03:18:43.797    2298-2298/com.aziz.bdk.ryadencule E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.aziz.bdk.ryadencule, PID: 2298
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.aziz.bdk.ryadencule/com.aziz.bdk.ryadencule.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String org.json.JSONObject.toString()' on a null object reference