java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare();

I have an Android app running a thread. I want a Toast message to show with a message.

When I do this, I get the below exception:

Logcat trace:

FATAL EXCEPTION: Timer-0 
 java.lang.RuntimeException: Can't create handler inside thread that has not 
    called Looper.prepare()

 at android.os.Handler.<init>(Handler.java:121)
 at android.widget.Toast$TN.<init>(Toast.java:322)
 at android.widget.Toast.<init>(Toast.java:91)
 at android.widget.Toast.makeText(Toast.java:238) 

Is there a work around for pushing Toast messages from threads to the User Interface?

I got this exception because I was trying to make a Toast popup from a background thread.
Toast needs an Activity to push to the user interface and threads don't have that.
So one workaround is to give the thread a link to the parent Activity and Toast to that.

Put this code in the thread where you want to send a Toast message:

parent.runOnUiThread(new Runnable() {
    public void run() {
        Toast.makeText(parent.getBaseContext(), "Hello", Toast.LENGTH_LONG).show();
    }
});

Keep a link to the parent Activity in the background thread that created this thread. Use parent variable in your thread class:

private static YourActivity parent;

When you create the thread, pass the parent Activity as a parameter through the constructor like this:

public YourBackgroundThread(YourActivity parent) {
    this.parent = parent;
}

Now the background thread can push Toast messages to the screen.

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<Void, Void, Void> {

    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;
        }
    });

Here's what I've been doing:

  public void displayError(final String errorText) {
    Runnable doDisplayError = new Runnable() {
        public void run() {
            Toast.makeText(getApplicationContext(), errorText, Toast.LENGTH_LONG).show();
        }
    };
    messageHandler.post(doDisplayError);
}

That should allow the method to be called from either thread.

Where messageHandler is declared in the activity as ..

Handler messageHandler = new Handler();

From http://developer.android.com/guide/components/processes-and-threads.html :

> Additionally, the Android UI toolkit is not thread-safe. So, you > must not manipulate your UI from a worker thread—you must do all > manipulation to your user interface from the UI thread. Thus, there > are simply two rules to Android's single thread model: > > 1. Do not block the UI thread > 2. Do not access the Android UI toolkit from outside the UI thread

You have to detect idleness in a worker thread and show a toast in the main thread.

Please post some code, if you want a more detailed answer.

After code publication :

In strings.xml

<string name="idleness_toast">"You are getting late do it fast"</string>

In YourWorkerThread.java

Toast.makeText(getApplicationContext(), getString(R.string.idleness_toast), 
    Toast.LENGTH_LONG).show();

Don't use AlertDialog, make a choice. AlertDialog and Toast are two different things.

runOnUiThread(new Runnable(){
   public void run() {
     Toast.makeText(getApplicationContext(), "Status = " + message.getBody() , Toast.LENGTH_LONG).show();
   }
 });

---

this works for me

You can simply use BeginInvokeOnMainThread(). It invokes an Action on the device main (UI) thread.

Device.BeginInvokeOnMainThread(() => { displayToast("text to display"); });

It is simple and works perfectly for me!

EDIT : Works if you're using C# Xamarin

I got this error in a JobService from the following code:

    BluetoothLeScanner bluetoothLeScanner = getBluetoothLeScanner();
    if (BluetoothAdapter.STATE_ON == getBluetoothAdapter().getState() && null != bluetoothLeScanner) {
        // ...
    } else {
        Logger.debug(TAG, "BluetoothAdapter isn't on so will attempting to turn on and will retry starting scanning in a few seconds");
        getBluetoothAdapter().enable();
        (new Handler()).postDelayed(new Runnable() {
            @Override
            public void run() {
                startScanningBluetooth();
            }
        }, 5000);
    }

The service crashed:

2019-11-21 11:49:45.550 729-763/? D/BluetoothManagerService: MESSAGE_ENABLE(0): mBluetooth = null
    
    --------- beginning of crash
2019-11-21 11:49:45.556 8629-8856/com.locuslabs.android.sdk E/AndroidRuntime: FATAL EXCEPTION: Timer-1
    Process: com.locuslabs.android.sdk, PID: 8629
    java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
        at android.os.Handler.<init>(Handler.java:203)
        at android.os.Handler.<init>(Handler.java:117)
        at com.locuslabs.sdk.ibeacon.BeaconScannerJobService.startScanningBluetoothAndBroadcastAnyBeaconsFoundAndUpdatePersistentNotification(BeaconScannerJobService.java:120)
        at com.locuslabs.sdk.ibeacon.BeaconScannerJobService.access$500(BeaconScannerJobService.java:36)
        at com.locuslabs.sdk.ibeacon.BeaconScannerJobService$2$1.run(BeaconScannerJobService.java:96)
        at java.util.TimerThread.mainLoop(Timer.java:555)
        at java.util.TimerThread.run(Timer.java:505)

So I changed from Handler to Timer as follows:

   (new Timer()).schedule(new TimerTask() {
                @Override
                public void run() {
                    startScanningBluetooth();
                }
            }, 5000);

Now the code doesn't throw the RuntimeException anymore.