CODEWITHSUNDEEP
androidhandler
tyczj
tyczj

Reputation: 74046

Cannot create handler in main thread?

A user is getting an error on their phone when executing the code below, I am checking to see if the user is currently on a call or not before I run an AsyncTask making a Web Service call. The reason I am doing this is because some people are losing internet connection when on a call so i check every 5 seconds to see if they are on a call or not. If they are not then AsyncTask gets called

do{
        Handler handler = new Handler();
        handler.postDelayed(new Runnable(){

            @Override
            public void run() {

                 onCall = ((TelephonyManager) context.getSystemService(Context.TELEPHONY_SERVICE))
                        .getCallState() != TelephonyManager.CALL_STATE_IDLE;
                 if(!onCall){
                     new CallRegWS().execute();
                 }  
            }
        },5000);
    }while(onCall);

I do not have the full stack trace only this showing me the error

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

obviously not too helpful but that is all I have to go on. they keep getting that error so it must be happening when I create the handler.

All this is done in the main thread so I dont know what the problem is, any insight on how I could fix this?

Update this is called from a service and the method is in a separate class

Upvotes: 1

Views: 3861

Answers (3)

Martin Cazares
Martin Cazares

Reputation: 13705

First of all you have to know that, Handlers automatically attach to the thread where are created as specified in the documentation:

Handler Class Overview

A Handler allows you to send and process Message and Runnable objects associated with a thread's MessageQueue. Each Handler instance is associated with a single thread and that thread's message queue. When you create a new Handler, it is bound to the thread / message queue of the thread that is creating it -- from that point on, it will deliver messages and runnables to that message queue and execute them as they come out of the message queue. Handler | Android Developers

Now, taking that on count and since the current thread don't have a message queue, in order to work properly the thread needs an actual "Queue", and that's exactly what the system is telling you to do, by calling Looper.prepare() you are creating a Message Looper within the thread you created because by default Threads do not have a Message looper as mentioned in documentation:

Looper Class Overview

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. Looper | Android Developers

In other words and in a simple manner, the Main UI DOES have a Message Queue already prepared, and that's why you don't see that error when calling new Handler() within "onCreate, onStart, onResume etc...", but for any worker thread created the system needs a queue to send the messages to when using handlers, although the code doesn't show it, i bet that code is being executed in a worker thread either normal Thread or doInBackground method of an AsyncTask and that's what causes the error, to avoid it remember to always call Looper.prepare before creating a handler from a worker thread.

Hope this Helps.

Regards!

Upvotes: 1

LuxuryMode
LuxuryMode

Reputation: 33771

Try Handler handler = new Handler(Looper.getMainLooper());

Also, see this post regarding this error: http://levinotik.com/loopers-handlers-runtimeexceptions-explained/

Upvotes: 7

Matt Wolfe
Matt Wolfe

Reputation: 9294

How do you know this is running on the Main thread? And when and where is this getting called from? Instead of creating your handler right before you use it, create it in the onCreate method of your Activity or your Service (depending on what this is).Then in your onDestroy null the value out.

Upvotes: 1

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