I am trying to understand how UI thread's event queue works. I'm trying to run a code that can be broken into many parts but unfortunately it must run on the UI thread. So, in order to not block the UI thread and receive a ANR I was wondering if I can break that code in many Runnable objects and run them using runOnUiThread from another thread.
My question is, will this block the UI thread? If, for example, I have a piece of code that definitely runs in over 5 seconds and I break this code into, let's say 1000 Runnable objects, and add them to the event queue of the UI thread, will other events get processed by the UI thread between them?
Edit: I think I found a better way to express myself in case the above explanation is confusing.
The 1000 Runnable objects was just an example, in actual code I want to have at most 10.
Basically, I want 10 Runnable objects, each one initialising an Ad network on the UI thread. I want these Runnable objects to run one after another, not in parallel. Also, I want the UI thread to be able to process other events between running these objects, so that I don't get an ANR in case running all 10 run methods will take more than 5 seconds.
NOTE: I don't know why initialising Ad networks must be done on the UI thread, but it must, otherwise the app crashes. It also states in some of the networks' sdks' documentation that initialisation must happen on the UI thread. This is why I need to run them one after another on UI thread and I can't run them in parallel in the background.
Also, the app is actually a OpenGl game, so calls to running the Runnable objects will be made from a GL thread, not the main thread, so they will be added to the event queue, and not executed immediately.
Well, Runnable inside your runOnUiThread is just operation in Main Thread.
Imagine that some simple action like
textView.setText("example");
will block Main Thread for 5 ms. Usually you will not see it.
Now imagine that you have like 1000 same operations for 5 seconds. And every blocks Main Thread for 5 ms. Simple calculating 5ms * 1000 = 5000ms = 5 seconds. So it will block Main Thread permamently. But if you have 10 operations you will block only 50 ms, in other words its just 1% of load that you will not feel.
So possible amount of calls depends on size of View, how hard render is and how fast is device.
P.S. For adepts of AsyncTask - there is no real difference between runOnUiThread and AsyncTask because those 1000 Runnables will execute in Main Thread by the same way.
Even if I do same thing inside onCreate method of Activity that will block UI hard
Yes. Runnable executing on UI thread will block main thread.
Check if below approach is useful for you.
Create a Handler with Looper from Main :requestHandler
Create a Handler with Looper for main thread : responseHandler and override handleMessage method
post a Runnable task on requestHandler
Inside Runnable task, call sendMessage on responseHandler
This sendMessage result invocation of handleMessage in responseHandler.
Get attributes from the Message and process it, update UI
Sample code:
/* Handler */
Handler requestHandler = new Handler(Looper.getMainLooper());
final Handler responseHandler = new Handler(Looper.getMainLooper()) {
#Override
public void handleMessage(Message msg) {
//txtView.setText((String) msg.obj);
Toast.makeText(MainActivity.this,
"Adwork task is completed:"+(String)msg.obj,
Toast.LENGTH_LONG)
.show();
}
};
for ( int i=0; i<10; i++) {
// Start Adwork task
Runnable myRunnable = new Runnable() {
#Override
public void run() {
try {
/* Your business logic goes here */
// Send some result after computation
String text = "" + (++rId);
Message msg = new Message();
msg.obj = text.toString();
responseHandler.sendMessage(msg);
System.out.println(text.toString());
} catch (Exception err) {
err.printStackTrace();
}
}
};
requestHandler.post(myRunnable);
}
Useful articles:
handlerthreads-and-why-you-should-be-using-them-in-your-android-apps
android-looper-handler-handlerthread-i
What you are looking for is an AsyncTask.
These are designed to do some background processing, while the UI thread continues. It will NOT block the UI and will NOT cause ANR.
Within the AsyncTask, is an onPostExecute method, which allows you to post results back to the UI. So it is not completely detached from the UI Thread. And an onProgressUpdate for connection during the background processing
Yes, you will feel animation will stop or start shutter if You run heavy operations in Ui thread. It's also depends of how fast device is.
What I would suggest you is to break your code in two parts. One that can be done in background and second that need Ui thread and execute them in AsyncTask.
Use doInBackgroud method to execute heavy operation and onPostExecute to update UI.
Note: If you break code into 1000 threads and run them in one moment, then probably You will hit device queue limit. (Break them into 10-50 parts. )
Related
The following code leads to java.lang.IllegalThreadStateException: Thread already started when I called start() method second time in program.
updateUI.join();
if (!updateUI.isAlive())
updateUI.start();
This happens the second time updateUI.start() is called. I've stepped through it multiple times and the thread is called and completly runs to completion before hitting updateUI.start().
Calling updateUI.run() avoids the error but causes the thread to run in the UI thread (the calling thread, as mentioned in other posts on SO), which is not what I want.
Can a Thread be started only once? If so than what do I do if I want to run the thread again? This particular thread is doing some calculation in the background, if I don't do it in the thread than it's done in the UI thread and the user has an unreasonably long wait.
From the Java API Specification for the Thread.start method:
It is never legal to start a thread
more than once. In particular, a
thread may not be restarted once it
has completed execution.
Furthermore:
Throws:
IllegalThreadStateException - if the thread was already started.
So yes, a Thread can only be started once.
If so than what do I do if I want to
run the thread again?
If a Thread needs to be run more than once, then one should make an new instance of the Thread and call start on it.
Exactly right. From the documentation:
It is never legal to start a thread
more than once. In particular, a
thread may not be restarted once it
has completed execution.
In terms of what you can do for repeated computation, it seems as if you could use SwingUtilities invokeLater method. You are already experimenting with calling run() directly, meaning you're already thinking about using a Runnable rather than a raw Thread. Try using the invokeLater method on just the Runnable task and see if that fits your mental pattern a little better.
Here is the example from the documentation:
Runnable doHelloWorld = new Runnable() {
public void run() {
// Put your UI update computations in here.
// BTW - remember to restrict Swing calls to the AWT Event thread.
System.out.println("Hello World on " + Thread.currentThread());
}
};
SwingUtilities.invokeLater(doHelloWorld);
System.out.println("This might well be displayed before the other message.");
If you replace that println call with your computation, it might just be exactly what you need.
EDIT: following up on the comment, I hadn't noticed the Android tag in the original post. The equivalent to invokeLater in the Android work is Handler.post(Runnable). From its javadoc:
/**
* Causes the Runnable r to be added to the message queue.
* The runnable will be run on the thread to which this handler is
* attached.
*
* #param r The Runnable that will be executed.
*
* #return Returns true if the Runnable was successfully placed in to the
* message queue. Returns false on failure, usually because the
* looper processing the message queue is exiting.
*/
So, in the Android world, you can use the same example as above, replacing the Swingutilities.invokeLater with the appropriate post to a Handler.
No, we cannot start Thread again, doing so will throw runtimeException java.lang.IllegalThreadStateException.
>
The reason is once run() method is executed by Thread, it goes into dead state.
Let’s take an example-
Thinking of starting thread again and calling start() method on it (which internally is going to call run() method) for us is some what like asking dead man to wake up and run. As, after completing his life person goes to dead state.
public class MyClass implements Runnable{
#Override
public void run() {
System.out.println("in run() method, method completed.");
}
public static void main(String[] args) {
MyClass obj=new MyClass();
Thread thread1=new Thread(obj,"Thread-1");
thread1.start();
thread1.start(); //will throw java.lang.IllegalThreadStateException at runtime
}
}
/*OUTPUT in run() method, method completed. Exception in thread
"main" java.lang.IllegalThreadStateException
at java.lang.Thread.start(Unknown Source)
*/
check this
The just-arrived answer covers why you shouldn't do what you're doing. Here are some options for solving your actual problem.
This particular thread is doing some
calculation in the background, if I
don't do it in the thread than it's
done in the UI thread and the user has
an unreasonably long wait.
Dump your own thread and use AsyncTask.
Or create a fresh thread when you need it.
Or set up your thread to operate off of a work queue (e.g., LinkedBlockingQueue) rather than restarting the thread.
What you should do is create a Runnable and wrap it with a new Thread each time you want to run the Runnable.
It would be really ugly to do but you can Wrap a thread with another thread to run the code for it again but only do this is you really have to.
It is as you said, a thread cannot be started more than once.
Straight from the horse's mouth: Java API Spec
It is never legal to start a thread
more than once. In particular, a
thread may not be restarted once it
has completed execution.
If you need to re-run whatever is going on in your thread, you will have to create a new thread and run that.
To re-use a thread is illegal action in Java API.
However, you could wrap it into a runnable implement and re-run that instance again.
Yes we can't start already running thread.
It will throw IllegalThreadStateException at runtime - if the thread was already started.
What if you really need to Start thread:
Option 1 ) If a Thread needs to be run more than once, then one should make an new instance of the Thread and call start on it.
Can a Thread be started only once?
Yes. You can start it exactly once.
If so than what do I do if I want to run the thread again?This particular thread is doing some calculation in the background, if I don't do it in the thread than it's done in the UI thread and the user has an unreasonably long wait.
Don't run the Thread again. Instead create Runnable and post it on Handler of HandlerThread. You can submit multiple Runnable objects. If want to send data back to UI Thread, with-in your Runnable run() method, post a Message on Handler of UI Thread and process handleMessage
Refer to this post for example code:
Android: Toast in a thread
It would be really ugly to do but you can Wrap a thread with another thread to run the code for it again but only do this is you really have to.
I have had to fix a resource leak that was caused by a programmer who created a Thread but instead of start()ing it, he called the run()-method directly. So avoid it, unless you really really know what side effects it causes.
I don't know if it is good practice but when I let run() be called inside the run() method it throws no error and actually does exactly what I wanted.
I know it is not starting a thread again, but maybe this comes in handy for you.
public void run() {
LifeCycleComponent lifeCycleComponent = new LifeCycleComponent();
try {
NetworkState firstState = lifeCycleComponent.getCurrentNetworkState();
Thread.sleep(5000);
if (firstState != lifeCycleComponent.getCurrentNetworkState()) {
System.out.println("{There was a NetworkState change!}");
run();
} else {
run();
}
} catch (SocketException | InterruptedException e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
Thread checkingNetworkStates = new Thread(new LifeCycleComponent());
checkingNetworkStates.start();
}
Hope this helps, even if it is just a little.
Cheers
So, I am working in android studio and trying to set a pause between two methods. In the first one, a gif is loaded and in the second one a pop-up message appears. I've tried to use Thread.sleep and the TimeUnit as bellow but both of them execute in a way that the time is waited before running everything else. In another words, my code down bellow waits 4 seconds and then runs gameGif() and gameFinish() simultaneously. Here is my piece of code:
gameGif();
{
try
{
TimeUnit.SECONDS.sleep(4);
}
catch(InterruptedException ex)
{}
}
gameFinish();
This is working for me
gameGif();
Handler handler = new Handler();
handler.postDelayed(new Runnable() {
#Override
public void run() {
gameFinish();
}
}, 4000);
What you are doing is setting the intention to update the UI. The actual UI updates are performed by the Android Looper separate to your code. In fact, by calling wait(), you're actually blocking the rendering thread from doing its update.
Using a Handler is the correct way to do it, as Matus posts. The Handler will organise the scheduling with the Looper and will still allow the Looper to run (i.e. is non-blocking). When the timeout elapses it will call the Runnable
The one thing you need to be careful with this approach, is to call the Handler on the UI loop (i.e. from one of the 'normal' Android methods). If you call it from your own Thread it won't interact with the Looper correctly.
I've a runnable instance that self-schedules itself again at the end of its run method:
private class MyRunnable implements Runnable {
private volatile boolean cancelled = false;
private Handler handler;
public MyRunnable(Handler h){
handler = h;
}
#Override
public void run(){
//Do stuff
if(!cancelled){
//Preemtion possible here?
handler.postDelayed(this, 1000);
}
}
public void selfStart(){
cancelled = false;
handler.removeCallbacks(this);
handler.post(this);
}
public void selfCancel(){
cancelled = true;
handler.removeCallbacks(this);
}
}
The runnable is first scheduled in the main thread calling selfStart from an activity's onStart.
At the same time, the runnable can be externally cancelled (calling selfCancel) from the activity's onStop and also from a Broadcast Receiver.
AFAIK Runnable.run, Activity.onStop and BroadcastReceiver.onReceive run in the same thread (the main one), so at first glance I thought there would be no thread-safety issues.
But it looks like sometimes, the runnable is being preemted in the middle of its run call, then it is cancelled from the activity or receiver, and then it resumes and re-schedules itself again.
Is this possible?
UPDATE:
I'll try to explain better the issue. The class shown above is intended to run tasks periodically in the main thread. In the "do stuff" comment there's actually code that updates a TextView with a value passed to the MyRunnable constructor. The activity cancels the current runnable and starts a new one when certain intents are received. Despite the current runnable is always requested to cancel itself before the new one is created, sometimes it is being left running along with the new one, so the text view is showing alternating values. This is not the intended behavior.
I thought if the runnable was currently running in the main thread, it would run until completion, and then other runnables or event handlers would be taken out of the queue and executed if needed, but no pending event or runnable could be "half executed".
There are two kinds of tasks running in the main thread that are related to the problem:
R1: The MyRunnable self-scheduling task. Runs and then it self-posts itself again with a delay of 1s.
R2: The event handlers that request cancellation of the current MyRunnable instance and create a new R1'. These happen randomly and are executed only once.
I've contemplated two scenarios. The first one:
R1 is already running in the main thread.
R2 arrives and is enqueued in the main thread.
R1 finishes running and posts itself again.
R2 runs and removes callbacks for R1.
R1 should never run again.
And the second one:
R1 is not running but is scheduled.
R2 arrives and removes callbacks for R1.
R1 should never run again.
Theoretically, if there's no preemtion, and there's only a single thread, how comes sometimes there are two R1s in the main thread?
As you have no synchronization on selfStart or selfCancel this is entirely possible.
An unreleated note, selfCancel could be called on a separate thread after your if statement in your run method has checked the value of cancelled. MyRunnable would then get one more call to run, which would end immediately as it's been cancelled.
My suggestion would be to move the //Do stuff inside the canceled check.
This avoids the race regardless of assumptions about which thread things are running on.
#Override
public void run(){
if(!cancelled){
//Do stuff
handler.post(this);
}
}
In general for maintainability, try to write code that works correctly regardless of the thread it is being run on. You never know when somebody will call selfCancel() on some other thread later thinking it is okay, when you have assumed they won't do that.
Well as others have said, there's no way a runnable can be preempted in a single thread. I also thought this idea was absurd. Shame on me for coming up with that nonsense.
There was nothing wrong with the runnables themselves. They were launched in the activity's onStart, and cancelled from Intents received by the activity, or in the activity's onStop. An this is the root of the problem: assuming onStart and onStop would run in a predictable order. Sometimes when coming back to the activity, a second onStart was executed before the first activity's onStop. Two tasks were running and the thing messed up to a point where the first task was never terminated.
Ensuring no task is launched without previous termination of the current one solved the issue.
In my activity, my app downloads a list of something from a website via an Http Connection. Before this list of things is displayed on the screen, I have a Loading... TextView with a little spinning ProgressBar to indicate that the data is currnetly being downloaded.
I noticed, that if I do any type of Thread.sleep() during the process of fetching the data from the web, it freezes the spinning ProgressBar.
Even if I put the method in it's own Handler Runnable so that it's on it's own thread, the animation freezing still occurs.
Is there anything I can do about this?
Yes, use AsyncTask.
A Handler is usually attached to the UI thread. Posting Runnables to it doesn't change the fact that they will run on the UI thread.
Any application should do long running tasks on a seperate thread to the UI thread as any blocking calls will not allow the UI to update.
As the other poster said, using Runnables alone do not mean the code will be executed on a seperate thread. In this case the run() method will execute on whatever thread your Handler object was created on. Either subclass Thread or pass your runnabe to new thread object with public Thread (Runnable runnable).
Using AsyncTasks, Loaders or Threads will help.
Also read Designing for Responsiveness and Multithreading For Performance. Following the above approaches will help you avoid ANRs
The reason for such a behaviour is that:
When you do thread.sleep() inside a runnable, it is the UI thread which executes that runnable rather than the respective other thread which you created. Basically what happens is that when you post any runnable through the handler of the UI thread, UI thread will poll from its queue and execute the runnable; in this case the runnable will do sleep.
Hence no change in progress bar
I have an app and want to put the main thread to sleep for 1500ms, so I can show a ProgressDialog, but I don't know how to do it. It's possible to stop an app main thread? I can achieve the same goal using another thread right?
Stopping the main thread is usually a bad idea as it would also stop all UI interaction processing.
Try making the progress dialog modal. This will prevent the user from doing anything on your UI until the dialog is gone. I think this will achieve what you want to do and is compatible with how Android works
not a good idea to do this, but if you have to use this
try {
Thread.sleep(1500);
} catch (InterruptedException e) {
//handle
}
Don't stop main UI thread! It will freeze UI. I can imagine that you show progress dialog during some background operation. Once this operation is complete just post something to update UI via handler.
The point of progress dialog is to interact with user while something long is executing. Otherwise you would not even need background operation and progress dialog. You would just do you operation in main UI thread and wait for UI to unfreeze.
What you're asking for should be unnecessary and as Carsten said, is a bad idea. It's a cardinal rule that you both never interrupt the UI thread and also only update elements of the UI on that thread. While a Dialog and any of it's subclasses are shown, nothing in the background will receive any input, so while it's up... though the main thread is running, nothing should be happening besides what is going on in the Dialog.
What you're probably looking to do is use something like an AsyncTask() to show your ProgressDialog, do some work, then dismiss it when that work is done.
To do something like this, you'll want to use "Runnables" along with a "Handler." As others mentioned, you don't want to stop the main UI thread. Since there is only one, you won't be showing ANY updates if you make it sleep or wait. See a small sample below of running code on another thread.
The main thing to take out of this is that sleeping the main thread means stopping any and all visual updates.
// Class Variables
private Handler mHandler;
#Override
public void onCreate(){
// Create a Handler Object
// In Layman's terms, Handlers can run processes off the main
// User interface thread.
mHandler = new Handler();
// Post a "Runnable" after a delay in milliseconds
// On a separate thread, the progressDialog_Runnable
// will call the function "createProgressDialog"
mHandler.postDelayed(progressDialog_Runnable, 250);
}
// Runnables (Process to run on separate thread)
private Runnable progressDialog_Runnable;
{
progressDialog_Runnable = new Runnable(){
#Override
public void run() {
createProgressDialog();
}
};
}
// Method you create to make the progress dialog
private void createProgressDialog(){
//... Run this code on a separate thread via Runnable
}
Sleep 10s on a thread in Kotlin
try {
Thread.sleep(10000)
} catch (e: InterruptedException) {
//handle
}