Can't get async task to work

Question

I was calling a class which was performing network operations on the main thread, causing my app to blow up on more recent devices. So I've tried moving the call to the class into a AsyncTask inner class in my main activity. However now i'm getting null reference expections.

Here's my AsyncTask:

      private class RetreiveAmazonNodesXML extends AsyncTask {

        private Exception exception;

        @Override
        protected Object doInBackground(Object... params) {
               try {
                    childrenBrowseNodesXml = new Amazon(browseNodeId, locality);

                } catch (Exception e) {
                    this.exception = e;
                }
            return null;
        }


     }

And here's where I call it in my activity:

 RetreiveAmazonNodesXML test = new RetreiveAmazonNodesXML();
 test.execute();
 parseXmlFile(childrenBrowseNodesXml.getBrowseNodesXML());

childrenBrowseNodesXml isn't getting updated and returning null. I know my Amazon class works fine so its something im doing with AsyncTask, but I have no idea?

have a look at my example here about how to execute an Async task and how to get its result in a callback function: http://stackoverflow.com/questions/13713751/json-object-null-android-asynctask/13713852#13713852 — lahsrah, Apr 17 '13 at 10:38
Your code prevents you to see if there's an exception thrown. Put some log in your catch Exception (or better, [remove this](http://stackoverflow.com/questions/21938/is-it-really-that-bad-to-catch-a-general-exception)). — m0skit0, Apr 17 '13 at 10:38
Pass generic tag, class RetreiveAmazonNodesXML extends AsyncTask — Vishesh Chandra, Apr 17 '13 at 10:41

SBotirov · Answer 1 · 2013-04-17T11:51:16.020

Use callback for get result from async task. Here interface callback class:

public interface Callback {
    public void onSuccess(Object data);
    public void onError(String errorMsg);
}

And create instance this class and implement its:

final Callback resCallback = new Callback() {
    @Override
public void onSuccess(Object data) {
    parseXmlFile(data);
}

@Override
public void onError(String errorMsg) {
    //show error with Alert or Toast
}
};

And create asynctask class with your callback:

RetreiveAmazonNodesXML test = new RetreiveAmazonNodesXML(resCallback);
test.execute(yourObjectsParams);

Write asynctask class like this:

private class RetreiveAmazonNodesXML extends AsyncTask {
    private Callback responderCallback;
    private Exception exception;

    public GeneralHttpTask(Callback callback){
        this.responderCallback = callback;
    }

    @Override
    protected Object doInBackground(Object... params) {
           try {
                Amazon childrenBrowseNodesXml = new Amazon(browseNodeId, locality);
                return childrenBrowseNodesXml;
            } catch (Exception e) {
                this.exception = e;
            }
        return null;
    }

    @Override
    protected void onPostExecute(Object result) {
        if(result != null) {
            responderCallback.onSuccess(result);
        } else {
            responderCallback.onError(exception);
        }
    }
}

Thanks for your answer. Just wondering what's the benifit of a callback interface when AsyncTask provides onPostExecute()? — KingFu, Apr 17 '13 at 11:42
Example you want use general AsyncTask class in more activities yes, then you need create a general AsyncTask class and send its url and callback, when callback come then you make use result. If you want separately create AsyncTask class — SBotirov, Apr 17 '13 at 11:49

score 1 · Accepted Answer · answered Apr 17 '13 at 10:40

It is because you're trying to use the value that AsyncTask hasn't returned, as AsyncTask is running asyncronously.

You should put parseXmlFile(childrenBrowseNodesXml.getBrowseNodesXML()); into your AsyncTask's onPostExecute() method, like this:

  private class RetreiveAmazonNodesXML extends AsyncTask {

    private Exception exception;

    @Override
    protected Object doInBackground(Object... params) {
           try {
                childrenBrowseNodesXml = new Amazon(browseNodeId, locality);

            } catch (Exception e) {
                this.exception = e;
            }
        return null;
    }

    @Override
    protected void onPostExecute(Object obj) {

           parseXmlFile(childrenBrowseNodesXml.getBrowseNodesXML());

    }

 }

Also, return null in doInBackground method may not be a good manner, the stuff that doInBackground returns will be passed as a parameter to onPostExecute() method automatically by AsyncTask.

For more about the AsyncTask, please refer to the Android Developers: AsyncTask

this did the trick thanks! should of known it would be something to do with multi-threading messing my code up again. Had to add more code to onPostExecute, which has now resulted in a problem with a spinner not updating...eurgh! — KingFu, Apr 17 '13 at 11:40

score 0 · Answer 3 · answered Apr 17 '13 at 10:39

The problem is, that you create a background thread (AsyncTask) that fills the childrenBrowseNodesXml after a while (when it's actually executed), but you try to use it immediately in your activity code.

Move your parseXMLFile to onPostExecute(Void result) method of AsyncTask instead.

Can't get async task to work

3 Answers3