AsyncTask的doInBackground（）方法中的NullPointer异常

Question

I'm trying to store the user details i got from Facebook api into a database using php.I used a JSONParser class to do the POST which returns a JSON object.When i call the makeHttpRequest method in JSONParser class from AsyncTask in another class,i get a null pointer exception.I checked whether any of the arguments i pass is null,but they aren't and can't understand why am i not able to get a response .I tried to find the answer from similar questions asked in SO, but it went in vain.Kindly help me.Thanks

This is the AsyncTask in the class which is trying to invoke makeHTTPRequest method in JSONparser class:

private class CreateUser extends AsyncTask<String, String, String> {

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

          //  Log.d("email", email);
            pDialog = new ProgressDialog(FbProfileActivity.this);
            pDialog.setMessage("Creating User...");
            pDialog.setIndeterminate(false);
            pDialog.setCancelable(true);
            pDialog.show();
        }

        @Override
        protected String doInBackground(String... args) {
            // TODO Auto-generated method stub
             // Check for success tag
            int success;

           try {
                // Building Parameters
       //       Log.d("param_username", username);
                List<NameValuePair> params = new ArrayList<NameValuePair>();
                params.add(new BasicNameValuePair("username", username));
            //    params.add(new BasicNameValuePair("age", age));
               params.add(new BasicNameValuePair("gender",gender));
                params.add(new BasicNameValuePair("email", email));
               // params.add(new BasicNameValuePair("location", location));
                Log.d("params","params" + params);


              Log.d("JsonParser's URL", "params" +REGISTER_URL );
                Log.d("request!", "starting",new Exception());                  //Posting user data to script 
                JSONObject json = jsonParser.makeHttpRequest(REGISTER_URL, "POST", params);

                // full json response
              //  Log.d("Registering attempt", json.toString());

                // json success element
                success = json.getInt(TAG_SUCCESS);
                if (success == 1) {
                    Log.d("User Created", json.toString());                 
                    finish();
                    return json.getString(TAG_MESSAGE);
                }else{
                    Log.d("Registering Failure!", json.getString(TAG_MESSAGE));
                    return json.getString(TAG_MESSAGE);

                }
            } catch (JSONException e) {
                e.printStackTrace();
            }

            return null;

        }

        protected void onPostExecute(String file_url) {
            // dismiss the dialog once product deleted
            pDialog.dismiss();
            if (file_url != null){
                Toast.makeText(FbProfileActivity.this, file_url, Toast.LENGTH_LONG).show();
            }

}
}

And this is the line where i get nullpointer Exception:

JSONObject json = jsonParser.makeHttpRequest(REGISTER_URL, "POST", params);

This is the makeHttpRequest method from the JSONParser class:

public JSONObject makeHttpRequest(String url, String method,
        List<NameValuePair> params) {
  Log.d(TAG,"makeHttpRequest() is called");
    // Making HTTP request
    try {

        // check for request method
        if(method == "POST"){
            // request method is POST
            // defaultHttpClient
            DefaultHttpClient httpClient = new DefaultHttpClient();
            HttpPost httpPost = new HttpPost(url);
            httpPost.setEntity(new UrlEncodedFormEntity(params));

            HttpResponse httpResponse = httpClient.execute(httpPost);
            HttpEntity httpEntity = httpResponse.getEntity();
            is = httpEntity.getContent();

        }else if(method == "GET"){
            // request method is GET
            DefaultHttpClient httpClient = new DefaultHttpClient();
            String paramString = URLEncodedUtils.format(params, "utf-8");
            url += "?" + paramString;
            HttpGet httpGet = new HttpGet(url);

            HttpResponse httpResponse = httpClient.execute(httpGet);
            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;

}

Stack Trace:

02-06 19:28:13.720: D/request!(2247): starting
02-06 19:28:13.720: D/request!(2247): java.lang.Exception
02-06 19:28:13.720: D/request!(2247):   at       com.example.letschatindia.FbProfileActivity$CreateUser.doInBackground(FbProfileActivity.java:448)
02-06 19:28:13.720: D/request!(2247):   at com.example.letschatindia.FbProfileActivity$CreateUser.doInBackground(FbProfileActivity.java:1)
02-06 19:28:13.720: D/request!(2247):   at android.os.AsyncTask$2.call(AsyncTask.java:288)
02-06 19:28:13.720: D/request!(2247):   at java.util.concurrent.FutureTask.run(FutureTask.java:237)
02-06 19:28:13.720: D/request!(2247):   at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
02-06 19:28:13.720: D/request!(2247):   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
02-06 19:28:13.720: D/request!(2247):   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
02-06 19:28:13.720: D/request!(2247):   at java.lang.Thread.run(Thread.java:841)
02-06 19:28:13.721: W/dalvikvm(2247): threadid=19: thread exiting with uncaught exception (group=0x4165fd40)
02-06 19:28:13.723: E/AndroidRuntime(2247): FATAL EXCEPTION: AsyncTask #5
02-06 19:28:13.723: E/AndroidRuntime(2247): Process: com.example.letschatindia, PID: 2247
02-06 19:28:13.723: E/AndroidRuntime(2247): java.lang.RuntimeException: An error occured while executing doInBackground()
02-06 19:28:13.723: E/AndroidRuntime(2247):     at android.os.AsyncTask$3.done(AsyncTask.java:300)
02-06 19:28:13.723: E/AndroidRuntime(2247):     at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
02-06 19:28:13.723: E/AndroidRuntime(2247):     at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
02-06 19:28:13.723: E/AndroidRuntime(2247):     at java.util.concurrent.FutureTask.run(FutureTask.java:242)
02-06 19:28:13.723: E/AndroidRuntime(2247):     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
02-06 19:28:13.723: E/AndroidRuntime(2247):     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
02-06 19:28:13.723: E/AndroidRuntime(2247):     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
02-06 19:28:13.723: E/AndroidRuntime(2247):     at java.lang.Thread.run(Thread.java:841)
02-06 19:28:13.723: E/AndroidRuntime(2247): Caused by: java.lang.NullPointerException
02-06 19:28:13.723: E/AndroidRuntime(2247):     at com.example.letschatindia.FbProfileActivity$CreateUser.doInBackground(FbProfileActivity.java:449)
02-06 19:28:13.723: E/AndroidRuntime(2247):     at com.example.letschatindia.FbProfileActivity$CreateUser.doInBackground(FbProfileActivity.java:1)
02-06 19:28:13.723: E/AndroidRuntime(2247):     at android.os.AsyncTask$2.call(AsyncTask.java:288)
02-06 19:28:13.723: E/AndroidRuntime(2247):     at java.util.concurrent.FutureTask.run(FutureTask.java:237)

Answer 1

我认为jsonParser为null，必须在使用它之前将其实例化。

Answer 2

Your jsonParser is null. I can't see where you are instantiating it, so check your code to avoid using it before the object is created.