Question

I have a thread which get some data from Internet. It seams that it is executed correctly and data is retrieved. However if I call a method which should return data it leaves me with null. From that I drew a conclusion that thread is somehow stopped just before finning.

Here is the code:

private class getHash extends AsyncTask<String, Void, String>{
    @Override
    protected String doInBackground(String... params) {
        String str = null;
        try {
            // Create a URL for the desired page
            URL url = new URL(params[0]);

            // Read all the text returned by the server
            InputStream is =  url.openStream();
            InputStreamReader isr = new InputStreamReader(is);
            BufferedReader in = new BufferedReader(isr);
            str = in.readLine();
            is.close();
            isr.close();
            in.close();
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        hash = str; //If I set a global variable here it gets passed without a hitch
        return str;
    }
    @Override
    protected void onPostExecute(String result) {
        hash = result; // If I comment the line above and live this one I left with a null
    }
}

EDIT:
As requested adding code where the thread was called:

            getHash hashThread =  new getHash();
            hashThread.execute(new String[] {"http://www.full.path/to/the/file.hash"});


            if(hash != null && !hash.equals(localHash)){
....

Answer 1

Whatever launched the AsyncTask

{
 ....
 getHash hashThread =  new getHash(this);
 hashThread.execute(new String[] {"http://www.full.path/to/the/file.hash"});
 return; // ok now we just have to wait for it to finish ... can't read it until then
}

// Separate callback method
public void onHashComplete(String hash) {

   if(hash != null && !hash.equals(localHash)) {
      ....
   }
   ....
 }

Now in your GetHash class

public String doInBackground(String[] params) {
    .... // don't set hash here ... it will work but you will probably read it at the wrong time.
    return str;
}

public void onPostExecute(String str) {
    onHashComplete(str); // or just do all the work in here since it is a private inner class
}

….

Hopefully that helps. Remember doInBackground() happens on the AsyncTask thread, onPostExecute() executes on the main Thread. Whatever thread called execute() is should also be the main thread. Because of the way the main thread works, you can’t expect the onPostCreate() to occur until whatever callback that it was using to call execute() in the first place, finishes. So that is why I add the return.