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.<init>(Handler.java:121)
at android.widget.Toast.<init>(Toast.java:68)
at android.widget.Toast.makeText(Toast.java:231)
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.
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 Activity.runOnUiThread(). Straightforward if you have an Activity:
#WorkerThread
void workerThread() {
myActivity.runOnUiThread(() -> {
// This is where your UI code goes.
}
}
You could also post to the main looper. This works great if all you have is a Context.
#WorkerThread
void workerThread() {
ContextCompat.getMainExecutor(context).execute(() -> {
// This is where your UI code goes.
}
}
Deprecated:
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.
It's convenient, but can leak contexts if not used correctly. It's been officially deprecated, and you shouldn't use it anymore.
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<PojoObject> getObservableItems() {
return Observable.create(subscriber -> {
for (PojoObject pojoObject: pojoObjects) {
subscriber.onNext(pojoObject);
}
subscriber.onCompleted();
});
}
Use this Observable like this -
getObservableItems().
subscribeOn(Schedulers.io()).
observeOn(AndroidSchedulers.mainThread()).
subscribe(new Observer<PojoObject> () {
#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<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;
}
});
Toast.makeText() can only be called from Main/UI thread. Looper.getMainLooper() helps you to achieve it:
JAVA
new Handler(Looper.getMainLooper()).post(new Runnable() {
#Override
public void run() {
// write your code here
}
});
KOTLIN
Handler(Looper.getMainLooper()).post {
// write your code here
}
An advantage of this method is that you can run UI code without Activity or Context.
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 );
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.
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.
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:)
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();
}
}
);
}
}
runOnUiThread(new Runnable() {
public void run() {
Toast.makeText(mContext, "Message", Toast.LENGTH_SHORT).show();
}
});
Wonderful Kotlin solution:
runOnUiThread {
// Add your ui thread code here
}
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()
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);
}
});
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.
For Rxjava and RxAndroid User:
public static void shortToast(String msg) {
Observable.just(msg)
.observeOn(AndroidSchedulers.mainThread())
.subscribe(message -> {
Toast.makeText(App.getInstance(), message, Toast.LENGTH_SHORT).show();
});
}
Coroutine will do it perfectly
CoroutineScope(Job() + Dispatchers.Main).launch {
Toast.makeText(context, "yourmessage",Toast.LENGTH_LONG).show()}
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();
}
});
}
Java 8
new Handler(Looper.getMainLooper()).post(() -> {
// Work in the UI thread
});
Kotlin
Handler(Looper.getMainLooper()).post{
// Work in the UI thread
}
GL
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.
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
&& myActivity.this.processingWaitDialog.isShowing()) {
myActivity.this.processingWaitDialog.dismiss();
myActivity.this.processingWaitDialog = null;
}
}
}); // .runOnUiThread(new Runnable()
...
Using lambda:
activity.runOnUiThread(() -> Toast.makeText(activity, "Hello", Toast.LENGTH_SHORT).show());
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
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<Void, Void, Void> {
#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.
I had the same problem and I fixed it simply by putting the Toast in onPostExecute() override function of the Asynctask<> and it worked.
You need to create toast on UI thread. Find the example below.
runOnUiThread(new Runnable() {
public void run() {
Toast.makeText(activity, "YOUR_MESSAGE", Toast.LENGTH_SHORT).show();
}
});
For displaying Toast message please refer to this article
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}
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("ProvidersNullExp", e.getMessage());
}
}
}).start();
Recently, I encounter this problem - It was happening because I was trying to call a function that was to do some UI stuff from the constructor. Removing the initialization from the constructor solved the problem for me.
I got the same problem and this code is working fine for me now.
As an example this is my code to do a task in the background and UI thread.
Observe how the looper is used:
new Thread(new Runnable() {
#Override
public void run() {
Looper.prepare();
// your Background Task here
runOnUiThread(new Runnable() {
#Override
public void run() {
// update your UI here
Looper.loop();
}
});
}
}).start();
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();
}
}
I have the following below code.
//This is global for the activity.
Handler handler = new Handler()
{
#Override
public void handleMessage(Message msg)
{
//update the UI
}
};
In my onResume() I am starting a new thread that runs a runnable.
protected void onResume()
{
super.onResume();
Thread t = new Thread(runnable);
t.start();
}
My runnable is as follows :
Runnable runnable = new Runnable()
{
public void run()
{
// some networking stuff
// some db stuff
// whatever
handler.sendEmptyMessage(0);
handler.postDelayed(new Runnable()
{
public void run()
{
new Thread(runnable).start();
}
}, 30000);
}
}
And I have this inside onPause() :
protected void onPause()
{
super.onPause();
handler.removeCallbacks(runnable);
}
At the end I call the handler.sendEmptyMessage(0); so that the handleMessage(Message msg) gets called and I do the UI changes, and I repeat the task but starting a new runnable, which starts a new thread that runs the same runnable as this.
Clarification questions :
I am starting a new Thread inside my onResume(), which means that the runnable is not running on the UI Thread, however, the handler is being created on the UI thread and is naturally attached to the UI thread. How is the UI is being changed flawlessly ?
It is supposed for handler.removeCallbacks(runnable), however, whenever I minized the app, the runnable still keeps on running every 30 seconds. (this might be because it is on a new thread, that is not associated with the Handler created). How can I make it stop ?
public class MainActivity extends Activity {
public static final int UPDATE = 1;
public static final int WORK = 2;
private Handler uiHandler = new Handler() {
#Override
public void handleMessage(Message msg) {
switch (msg.what) {
case UPDATE:
// Perform UI updates here
....
// UI Updates done, schedule WORK in 30 seconds:
this.sendMessageDelayed(this.obtainMessage(WORK), 30000);
break;
case WORK:
new Thread(doWork).start();
break;
default:
super.handleMessage(msg);
}
}
};
private WeakReference<Handler> handlerRef = new WeakReference<Handler>( uiHandler );
private Runnable doWork = new Runnable() {
#Override
public void run() {
// This will run on a different thread.
// If UI is still around, tell it to update
Handler ui = handlerRef.get();
if( ui != null )
ui.sendEmptyMessage(MainActivity.UPDATE);
}
};
#Override
protected void onPause() {
uiHandler.removeMessages(WORK);
super.onPause();
}
#Override
protected void onResume() {
super.onResume();
// Resume UI updates in 500ms, allowing UI to settle
uiHandler.sendMessageDelayed(uiHandler.obtainMessage(WORK), 500);
}
....
}
This pattern uses a single Handler on the UI thread. Background work is done in the Runnable, which the ui Handler will post to a new Thread, so avoiding NetworkOnMainThreadException and -- more importantly -- unresponsive UI. Further, a new update is scheduled thirty seconds after the background process has completed, to avoid taxing the system with long-running updates. The background thread uses a WeakReference to the ui Handler, so if the Activity is killed while the thread is working, it won't send UI updates to it.
Is possible to raise an AlertDialog from a background thread using a reference to getApplicationContext()?
I'm trying with that code but it doesn't work
new Thread(){
public void run(){
new AlertDialog.Builder(appcontext)
.setMessage("Test")
.setPositiveButton("Ok", null)
.show();
}
}.start();
Thanks in advance
No, you do not want to do this. Android does not permit UI work on any thread but the UI-thread because UI code is not thread safe. See "Painless Threading"1.
You can call Activity.runOnUiThread(Runnable) (on the specific activity) from another thread to force the code to run on the UI thread. You can also call View.post(Runnable) (on the specific view) to cause the action to be queued onto the UI thread. For more details on those options and others, see the above mentioned article.
However, Android also provides something called AsyncTask which is specifically designed for running some stuff on a separate thread, and some on the UI thread. This automatically uses Android's threadpool, and if you do not have any reason to use an explicit separate thread, is an easy, clean way to go:
From the Android docs:
private class DownloadFilesTask extends AsyncTask<URL, Integer, Long> {
// Runs on a ThreadPool thread
protected Long doInBackground(URL... urls) {
int count = urls.length;
long totalSize = 0;
for (int i = 0; i < count; i++) {
totalSize += Downloader.downloadFile(urls[i]);
// Sends data to onProgressUpdate to run on the UI thread
publishProgress((int) ((i / (float) count) * 100));
}
return totalSize;
}
// Runs on the UI thread!
protected void onProgressUpdate(Integer... progress) {
setProgressPercent(progress[0]);
}
// Runs on the UI thread!
protected void onPostExecute(Long result) {
showDialog("Downloaded " + result + " bytes");
}
}
you cannot access UI elements from a thread, you must create a handler and call it from your thread.
1- handler: to handle UI from other thread
private Handler handler= new Handler() {
public void handleMessage(Message msg){
/*put your code here to update on UI*/
}
};
2- in you thread call this:
Thread t = new Thread(new Runnable() {
#Override
public void run() {
handler.sendEmptyMessage(0);
}
}); //thread
t.start();
Simplest Solution is to use an AsyncTask.
try the following code
private class LaunchDialog extends AsyncTask<Void,Void,Void>{
Context context;
public LaunchDialog(Context ctx){
context = ctx;
}
#Override
protected ArrayList<CategoryObj> doInBackground(Void... params) {
//do the task to be done on NON-UI thread , or NON-Blocking thread
// publishProgress(null);
}
#Override
protected void onProgressUpdate(Void... v){
//stuff done on UI thread , can be invoked from doInBackground
}
#Override
protected void onPostExecute(Void x){
//stuff to be done after task executes(done on UI thread)
new AlertDialog.Builder(context)
.setMessage("Test")
.setPositiveButton("Ok", null)
.show();
}
#Override
protected void onPreExecute(){
//stuff to be done before task executes (done on UI thread)
}
}
to start the thread just do
new LaunchDialog(this).execute();
read article about painless threading here -- http://developer.android.com/resources/articles/painless-threading.html
You'll have to get access to the UI thread, any of these: View.post(), Activity.runOnUiThread(), or obtaining and sending a Message.
You cannot change UI in background thread. There is another thread to perform UI activities
runOnUiThread(new Runnable() {
#Override
public void run() {
//perform UI operations
}
});