Can t create handler inside thread that has not called Looper prepare

Question

What does the following exception mean  how can I fix it   This is the code   Toast toast   Toast makeText mContext   Something   Toast LENGTH SHORT     This is the exception   java lang RuntimeException  Can t create handler inside thread that has not called Looper prepare        at android os Handler  lt init gt  Handler java 121       at android widget Toast  lt init gt  Toast java 68       at android widget Toast makeText Toast java 231

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first call Looper prepare   and then call Toast makeText   show   last call Looper loop   like   Looper prepare      to be able to make toast Toast makeText context   not connected   Toast LENGTH LONG  show   Looper loop

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I was running into the same issue when my callbacks would try to show a dialog     I solved it with dedicated methods in the Activity - at the Activity instance member level - that use runOnUiThread      public void showAuthProgressDialog         runOnUiThread new Runnable              Override         public void run                 mAuthProgressDialog   DialogUtil getVisibleProgressDialog SignInActivity this   Loading                             public void dismissAuthProgressDialog         runOnUiThread new Runnable              Override         public void run                 if  mAuthProgressDialog    null      mAuthProgressDialog isShowing                      return                            mAuthProgressDialog dismiss

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runOnUiThread new Runnable                 public void run                     Toast makeText mContext   Message   Toast LENGTH SHORT  show

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Java 8 new Handler Looper getMainLooper    post    - gt           Work in the UI thread       Kotlin Handler Looper getMainLooper    post         Work in the UI thread    GL

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Coroutine will do it perfectly CoroutineScope Job     Dispatchers Main  launch                           Toast makeText context   quot yourmessage quot  Toast LENGTH LONG  show

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You need to create toast on UI thread  Find the example below  runOnUiThread new Runnable       public void run         Toast makeText activity   quot YOUR MESSAGE quot   Toast LENGTH SHORT  show             For displaying Toast message please refer to this article

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I had the same problem and I fixed it simply by putting the Toast in onPostExecute   override function of the Asynctask lt   and it worked

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For Rxjava and RxAndroid User   public static void shortToast String msg        Observable just msg               observeOn AndroidSchedulers mainThread                 subscribe message - gt                    Toast makeText App getInstance    message  Toast LENGTH SHORT  show

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Here is the solution for Kotlin using Coroutine  Extend your class with CoroutineScope by MainScope    class BootstrapActivity    CoroutineScope by MainScope       Then simply do this  launch              whatever you want to do in the main thread        Don t forget to add the dependencies for coroutine  org jetbrains kotlinx kotlinx-coroutines-core   Versions kotlinCoroutines  org jetbrains kotlinx kotlinx-coroutines-android   Versions kotlinCoroutines

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I was getting this error until I did the following   public void somethingHappened final Context context        Handler handler   new Handler Looper getMainLooper         handler post          new Runnable                          Override             public void run                                 Toast makeText context   Something happened    Toast LENGTH SHORT  show                                       And made this into a singleton class   public enum Toaster       INSTANCE       private final Handler handler   new Handler Looper getMainLooper          public void postMessage final String message            handler post              new Runnable                      Override                 public void run                         Toast makeText ApplicationHolder INSTANCE getCustomApplication    message  Toast LENGTH SHORT                           show

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Using lambda  activity runOnUiThread    - gt  Toast makeText activity   quot Hello quot   Toast LENGTH SHORT  show

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Create Handler outside the Thread final Handler handler   new Handler             new Thread new Runnable                  Override             public void run                 try                   handler post new Runnable                              Override                         public void run                                 showAlertDialog p getProviderName    Token  p getProviderId    Amount                                                                                                  catch  Exception e                   Log d  quot ProvidersNullExp quot   e getMessage                                    start

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Reason for an error   Worker threads are meant for doing background tasks and you can t show anything on UI within a worker thread unless you call method like runOnUiThread  If you try to show anything on UI thread without calling runOnUiThread  there will be a java lang RuntimeException   So  if you are in an activity but calling Toast makeText   from worker thread  do this   runOnUiThread new Runnable         public void run               Toast toast   Toast makeText getApplicationContext     Something   Toast LENGTH SHORT  show                    The above code ensures that you are showing the Toast message in a UI thread since you are calling it inside runOnUiThread method  So no more java lang RuntimeException

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Handler handler2    HandlerThread handlerThread new HandlerThread  second thread    handlerThread start    handler2 new Handler handlerThread getLooper       Now handler2 will use a different Thread to handle the messages than the main Thread

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You re calling it from a worker thread  You need to call Toast makeText    and most other functions dealing with the UI  from within the main thread  You could use a handler  for example   Look up Communicating with the UI Thread in the documentation  In a nutshell      Set this up in the UI thread   mHandler   new Handler Looper getMainLooper           Override     public void handleMessage Message message               This is where you do your work in the UI thread             Your worker tells you in the message what to do            void workerThread            And this is how you call it from the worker thread      Message message   mHandler obtainMessage command  parameter       message sendToTarget        Other options   You could use an AsyncTask  that works well for most things running in the background  It has hooks that you can call to indicate the progress  and when it s done   You could also use Activity runOnUiThread

User · Answer

i use the following code to show message from non main thread  context      FunctionalInterface public interface IShowMessage       Context getContext         default void showMessage String message            final Thread mThread   new Thread                  Override             public void run                     try                       Looper prepare                        Toast makeText getContext    message  Toast LENGTH LONG  show                        Looper loop                      catch  Exception error                        error printStackTrace                        Log e  IShowMessage   error getMessage                                                        mThread start              then use as the following   class myClass implements IShowMessage     showMessage  your message       Override     public Context getContext             return getApplicationContext

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Try this  when you see runtimeException due to Looper not prepared before handler   Handler handler   new Handler Looper getMainLooper       handler postDelayed new Runnable        Override   public void run          Run your task here        1000

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that s what i did   new Handler Looper getMainLooper    post new Runnable          Override     public void run             Toast                   Visual components are  locked  to changes from outside threads  So  since the toast shows stuff on the main screen that is managed by the main thread  you need to run this code on that thread  Hope that helps

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You need to call Toast makeText      from the UI thread   activity runOnUiThread new Runnable       public void run         Toast makeText activity   Hello   Toast LENGTH SHORT  show              This is copy-pasted from another  duplicate  SO answer

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This is because Toast makeText   is calling from a worker thread  It should be call from main UI thread like this      runOnUiThread new Runnable           public void run             Toast toast   Toast makeText mContext   Something   Toast LENGTH SHORT

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The answer by ChicoBird worked for me  The only change I made was in the creation of the UIHandler where I had to do   HandlerThread uiThread   new HandlerThread  UIHandler      Eclipse refused to accept anything else  Makes sense I suppose   Also the uiHandler is clearly a class global defined somewhere  I still don t claim to understand how Android is doing this and what is going on but I am glad it works  Now I will proceed to study it and see if I can understand what Android is doing and why one has to go through all these hoops and loops  Thanks for the help ChicoBird

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Toast  AlertDialogs needs to run on UI thread  you can use Asynctask to use them properly in android development but some cases we need to customize the time outs  so we use Threads  but in threads we cannot use Toast Alertdialogs like we using in AsyncTask So we need separate Handler  for popup those   public void onSigned         Thread thread   new Thread             Override         public void run                 try                  sleep 3000                   Message message   new Message                    message what   2                  handler sendMessage message                 catch  Exception e                   e printStackTrace                                       thread start        in Above example i want to sleep my thread in 3sec and after i want to show a Toast message for that in your mainthread implement handler   handler   new Handler            public void handleMessage Message msg               switch msg what                 case 1                Toast makeText getActivity    cool  Toast LENGTH SHORT  show                  break                          super handleMessage msg                 I used switch case here  because if you need to show different message in same way  you can use switch case within Handler class   hope this will help you

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To display a dialog or a toaster in a thread  the most concise way is to use the Activity object   For example   new Thread new Runnable          Override     public void run             myActivity runOnUiThread new Runnable                 public void run                     myActivity this processingWaitDialog   new ProgressDialog myActivity this getContext                     myActivity this processingWaitDialog setProgressStyle ProgressDialog STYLE SPINNER                   myActivity this processingWaitDialog setMessage  abc                    myActivity this processingWaitDialog setIndeterminate true                   myActivity this processingWaitDialog show                                      expenseClassify serverPost                  new AsyncOperationCallback                         public void operationCompleted Object sender                            myActivity runOnUiThread new Runnable                                 public void run                                     if  myActivity this processingWaitDialog    null                                           amp  amp  myActivity this processingWaitDialog isShowing                                          myActivity this processingWaitDialog dismiss                                        myActivity this processingWaitDialog   null                                                                                                  runOnUiThread new Runnable

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Wonderful Kotlin solution   runOnUiThread          Add your ui thread code here

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I ran into the same problem  and here is how I fixed it   private final class UIHandler extends Handler       public static final int DISPLAY UI TOAST   0      public static final int DISPLAY UI DIALOG   1       public UIHandler Looper looper                super looper               Override     public void handleMessage Message msg                switch msg what                    case UIHandler DISPLAY UI TOAST                        Context context   getApplicationContext                Toast t   Toast makeText context   String msg obj  Toast LENGTH LONG               t show                      case UIHandler DISPLAY UI DIALOG                TBD         default              break                     protected void handleUIRequest String message        Message msg   uiHandler obtainMessage UIHandler DISPLAY UI TOAST       msg obj   message      uiHandler sendMessage msg       To create the UIHandler  you ll need to perform the following       HandlerThread uiThread   new HandlerThread  UIHandler        uiThread start        uiHandler   new UIHandler  HandlerThread  uiThread getLooper       Hope this helps

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UPDATE - 2016  The best alternative is to use RxAndroid  specific bindings for RxJava  for the P in MVP to take charge fo data   Start by returning Observable from your existing method   private Observable lt PojoObject gt  getObservableItems         return Observable create subscriber - gt             for  PojoObject pojoObject  pojoObjects                subscriber onNext pojoObject                     subscriber onCompleted                Use this Observable like this -  getObservableItems    subscribeOn Schedulers io     observeOn AndroidSchedulers mainThread     subscribe new Observer lt PojoObject gt            Override     public void onCompleted                Print Toast on completion             Override     public void onError Throwable e           Override     public void onNext PojoObject pojoObject               Show Progress               ----------------------------------------------------------------------------------------------------------------------------------  I know I am a little late but here goes  Android basically works on two thread types namely UI thread and background thread  According to android documentation -      Do not access the Android UI toolkit from outside the UI thread to fix this problem  Android offers several ways to access the UI thread from other threads  Here is a list of methods that can help    Activity runOnUiThread Runnable    View post Runnable    View postDelayed Runnable  long    Now there are various methods to solve this problem    I will explain it by code sample   runOnUiThread  new Thread         public void run                 myactivity this runOnUiThread new Runnable                         public void run                                   Do your UI operations like dialog opening or Toast here                                   start      LOOPER     Class used to run a message loop for a thread  Threads by default do   not have a message loop associated with them  to create one  call   prepare   in the thread that is to run the loop  and then loop   to   have it process messages until the loop is stopped    class LooperThread extends Thread       public Handler mHandler       public void run             Looper prepare             mHandler   new Handler                 public void handleMessage Message msg                       process incoming messages here                                   Looper loop              AsyncTask     AsyncTask allows you to perform asynchronous work on your user   interface  It performs the blocking operations in a worker thread and   then publishes the results on the UI thread  without requiring you to   handle threads and or handlers yourself    public void onClick View v        new CustomTask   execute  Void   null       private class CustomTask extends AsyncTask lt Void  Void  Void gt         protected Void doInBackground Void    param              Do some work         return null             protected void onPostExecute Void param              Print Toast or open dialog           Handler     A Handler allows you to send and process Message and Runnable objects   associated with a thread s MessageQueue    Message msg   new Message      new Thread         public void run                 msg arg1 1          handler sendMessage msg           start       Handler handler   new Handler new Handler Callback           Override     public boolean handleMessage Message msg            if msg arg1  1                          Print Toast or open dialog                           return false

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Toast makeText   should only be called from Main UI thread  Looper getMainLooper   helps you to achieve it  ANDROID new Handler Looper getMainLooper    post new Runnable          Override     public void run             Toast toast   Toast makeText mContext   quot Something quot   Toast LENGTH SHORT           toast show               KOTLIN Handler Looper getMainLooper    post           Toast makeText mContext   quot Something quot   Toast LENGTH SHORT  show      An advantage of this method is that you can use it without Activity or Context

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This usually happens when something on the main thread is called from any background thread  Lets look at an example   for instance   private class MyTask extends AsyncTask lt Void  Void  Void gt       Override protected Void doInBackground Void    voids            textView setText  Any Text            return null            In the above example   we are setting text on the textview which is in the main UI thread from doInBackground   method   which operates only on a worker thread

[android] Can't create handler inside thread that has not called Looper.prepare()

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