Android - Can't create handler inside thread that has not called Looper.prepare()

Question

I am developing a simple application using Google Maps V2 API just to get basics and I am facing this error:

09-09 21:21:41.154: E/AndroidRuntime(3796): FATAL EXCEPTION: Thread-441
09-09 21:21:41.154: E/AndroidRuntime(3796): java.lang.ExceptionInInitializerError
09-09 21:21:41.154: E/AndroidRuntime(3796):  at cz.vongrad.dataSourceLayer.DataMapper.loadLocation(DataMapper.java:111)
09-09 21:21:41.154: E/AndroidRuntime(3796):  at cz.vongrad.dataSourceLayer.DBFacade.loadLocations(DBFacade.java:32)
09-09 21:21:41.154: E/AndroidRuntime(3796):  at cz.vongrad.domainLayer.Controller.loadLocations(Controller.java:94)
09-09 21:21:41.154: E/AndroidRuntime(3796):  at cz.vongrad.locator.MapActivity$1.run(MapActivity.java:199)
09-09 21:21:41.154: E/AndroidRuntime(3796): Caused by: java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
09-09 21:21:41.154: E/AndroidRuntime(3796):  at android.os.Handler.<init>(Handler.java:121)
09-09 21:21:41.154: E/AndroidRuntime(3796):  at android.os.AsyncTask$InternalHandler.<init>(AsyncTask.java:607)
09-09 21:21:41.154: E/AndroidRuntime(3796):  at android.os.AsyncTask$InternalHandler.<init>(AsyncTask.java:607)
09-09 21:21:41.154: E/AndroidRuntime(3796):  at android.os.AsyncTask.<clinit>(AsyncTask.java:190)
09-09 21:21:41.154: E/AndroidRuntime(3796):  ... 4 more
09-09 21:21:41.164: E/EmbeddedLogger(239): App crashed!

DataMapper:

class LoadLocations extends AsyncTask<String, Void, ArrayList<Friend>>{

        private JSONObject jSONObject;


        @Override
        protected ArrayList<Friend> doInBackground(String... args) {

            List<NameValuePair> params = new ArrayList<NameValuePair>();
            params.add(new BasicNameValuePair("user_name", args[1]));

            jSONObject = jSONParser.makeHttpRequest(args[0], "POST", params);
            Log.d(TAG, jSONObject.toString());

            ArrayList<Friend> tempFriends = new ArrayList<Friend>();

            try {
                if(jSONObject.getInt("success") == 1){

                    JSONArray jsonArray = jSONObject.getJSONArray("users");


                    for (int i = 0; i < jsonArray.length(); i++) {
                        JSONObject o = jsonArray.getJSONObject(i);

                        tempFriends.add(new Friend(o.getString("user_name"), new LatLng(o.getDouble("loc_y"), o.getDouble("loc_x")), false));
                    }

                }
            } catch (JSONException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

            return tempFriends;
        }

    }

public ArrayList<Friend> loadLocation(String URL, String userName) throws InterruptedException, ExecutionException {
        return new LoadLocations().execute(URL, userName).get();

    }

MapActivity:

public class MapActivity extends Activity implements LocationSource, LocationListener {
@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
}
private Thread loadFriendLocations = new Thread()
    {

        @Override
        public void run() {
            Log.d(TAG, "LastKnownLocation: " + locationManager.getLastKnownLocation(LocationManager.GPS_PROVIDER));
            try {
                while(true) {
                    controller.loadLocations(LOAD_LOCATIONS);
                    Log.d(TAG, "Loading new locations");
                    Message msg = uIHandler.obtainMessage();
                    msg.what = ADD_MAP_PIN;
                    uIHandler.sendMessage(msg);

                    sleep(10000);                   
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (ExecutionException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    };

    final Handler uIHandler = new Handler(){
      @Override
      public void handleMessage(Message msg) {

        if(msg.what==ADD_MAP_PIN){

            for (Friend friend : controller.getFriends()) {

                    addMapPin(friend);


            }
        }
        super.handleMessage(msg);
      }
    };

The weird thing is that I can run this app on my emulator (API 16) and it runs just great but when I try to run it on a physical device (API 15), it gets this error. The AsyncTask is run in separate class, so it might cause this problem. I would appreciate any suggestions. Thanks in advance.

Answer 1

I had a similar issue, problem was that I was having series of Thread from Main Thread. Actually Main UI Thread started a service, which ran on a service and subsequently started another thread, which finally had a FileObserver (another thread), I had to communicate to UI thread by popping a toast message. I tried for several hours, and tried below, it worked as a charm.

//Let this be the code in your n'th level thread from main UI thread
Handler h = new Handler(Looper.getMainLooper());
h.post(new Runnable() {
  public void run() {
    Toast.makeText(context, "Your message to main thread", Toast.LENGTH_SHORT).show();
  }
});

The main key here is the getMainLooper() function of Looper class, which will provide you the Looper against the Main UI thread.

Answer 2

You cannot execute an AsyncTask from a background thread. See the "Threading Rules" section of the AsyncTask documentation.

Answer 3

Do this in your do in background...

runOnUiThread(new Runnable() {
    public void run() 
    {
        if (items.equals("null")) 
        {
            Toast.makeText(getApplicationContext(), "No Event Found",Toast.LENGTH_LONG).show();
        }                   
        else {                       
        }   
    }
});