This example is copied from a book on Android. As you can see from my question, I am new to Android and trying to understand. This application should crash but it does not (I am updating UI from another thread. Which is not allowed.It should cause a crash. It does not. Why?). My code is:
final ProgressDialog dialogue = ProgressDialog.show(this, "title", "message");
new Thread(new Runnable() {
#Override
public void run() {
// TODO Auto-generated method stub
try {
Thread.sleep(7000);
dialogue.dismiss();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}).start();
This is done in OnCreate function. I am confused with line - dialogue.dismiss(); Isn't that updating UI (dismissing dialogue) from another thread? Why does this app not cause segmentation fault?
Thanks.
You cant dismiss() it in run method because it is non UI thread.and if you want to dismiss then use Handler.And its better to use AsyncTask
The Code is correct only man.You are starting a thread using the .start function and after that the run function called and then after 7 seconds the the dialogue will dismiss.The dialogue.dismiss() is used to dismiss the dialogue.If you will not call dismiss(),the progress bar never dismissed.You can check by commenting the line Thread.sleep(7000).
Related
tl:dr at the bottom. I am taking a Java course on Udemy and our instructor is demonstrating a situation where background threads can interfere with each other and crash the program. In this case pushing either of the two buttons initiates a method that starts a background Thread, waits, then updates a label. He explains it something along the lines as the threads overwriting each other causing the program to crash with an IllegalStateException. What I don't understand is Threads overwriting each other is not uncommon and just makes the program behave in ways not intended and doesn't usually completely crash the program.
In this case the threads are both trying to change the label to the same String.
Why is this throwing an IllegalStateException instead of just causing the usual interference? After all more than one thread trying to update an object doesn't necessarily crash the program.
tl:dr Why does more than one thread modifying a Label object throw an IllegalStateException in this case but in other multi-threaded programs you just get the usual thread interference? The method of interest is the onButtonClicked() method.
I have tried catching the IllegalStateException to see if I can call Thread.currentThread().getName() but the catch block(both of them actually) seem to get ignored.
public class Controller {
#FXML
private TextField nameField;
#FXML
private Button helloButton;
#FXML
private Button byeButton;
#FXML
private CheckBox ourCheckBox;
#FXML
private Label ourLabel;
#FXML
public void initialize() {
helloButton.setDisable(true);
byeButton.setDisable(true);
}
#FXML
public void onButtonClicked(ActionEvent event) {
if (event.getSource().equals(helloButton)) {
System.out.println("hello, " + nameField.getText());
} else if (event.getSource().equals(byeButton)) {
System.out.println("Bye, " + nameField.getText());
}
Runnable task = new Runnable() {
#Override
public void run() {
try {
Thread.sleep(5000);
ourLabel.setText("We did something");
} catch (InterruptedException e) {
System.out.println("Interuppted Exception " + e.getMessage());
} catch (IllegalStateException e) {
System.out.println("Illegal State Exception: " + e.getMessage() + " "
+ Thread.currentThread().getName());
}
}
};
new Thread(task).start();
if (ourCheckBox.isSelected()) {
nameField.clear();
byeButton.setDisable(true);
helloButton.setDisable(true);
}
}
#FXML
public void handleKeyReleased() {
String text = nameField.getText();
boolean disableButtons = text.isEmpty() || text.trim().isEmpty();
byeButton.setDisable(disableButtons);
helloButton.setDisable(disableButtons);
}
#FXML
public void handleChange() {
System.out.println("The checkbox is " +
(ourCheckBox.isSelected() ? "checked" : "not checked"));
}
}
Updating active scene graph elements (such as label text) in your own threads has the potential to cause a race condition (please read and understand the Software section of the link) in the internal state of the JavaFX system. The results of which are unpredictable and a corruption or ”crash” of the JavaFX system cannot be ruled out. Perhaps nothing bad will happen, but perhaps something bad will.
Some calls to JavaFX APIs will detect when potential race condition may occur (when they are being invoked by a thread which is not the JavaFX thread), and fail fast by throwing an IllegalStateException. Other JavaFX APIs may not bother to check the calling thread and may not throw an IllegalStateException, thus allowing the potential race condition to occur. Either way, the users calling code is wrong, it should not be trying to modify the active scene graph off of the JavaFX application thread.
JavaFX code which manipulates the active scene graph (elements or properties of elements currently being displayed in a rendered scene) is only ever expected to occur on a single thread (the JavaFX application thread). In that way a race condition cannot occur.
If you want to feedback information from another thread to the UI, then you can either use the Platform.runLater API, which will execute a piece of code at some time in the future on the JavaFX thread, or make use of utilities in the javafx.concurrent package.
I have a problem while working with JavaFX and Threads. Basically I have two options: working with Tasks or Platform.runLater. As I understand Platform.runLater should be used for simple/short tasks, and Task for the longer ones. However, I cannot use any of them.
When I call Thread, it has to pop up a captcha dialog in a middle of task. While using Task, it ignores my request to show new dialog... It does not let me to create a new stage.
On the other hand, when I use Platform.runLater, it lets me show a dialog, however, the program's main window freezes until the pop up dialog is showed.
I need any kind of solution for this. If anyone knows how to deal with this or had some similar experience and found a solution I am looking forward to hearing from you!
As puce says, you have to use Task or Service for the things that you need to do in background. And Platform.runLater to do things in the JavaFX Application thread from the background thread.
You have to synchronize them, and one of the ways to do that is using the class CountDownLatch.
Here is an example:
Service<Void> service = new Service<Void>() {
#Override
protected Task<Void> createTask() {
return new Task<Void>() {
#Override
protected Void call() throws Exception {
//Background work
final CountDownLatch latch = new CountDownLatch(1);
Platform.runLater(new Runnable() {
#Override
public void run() {
try{
//FX Stuff done here
}finally{
latch.countDown();
}
}
});
latch.await();
//Keep with the background work
return null;
}
};
}
};
service.start();
Use a Worker (Task, Service) from the JavaFX Application thread if you want to do something in the background.
http://docs.oracle.com/javafx/2/api/javafx/concurrent/package-summary.html
Use Platform.runLater from a background thread if you want to do something on the JavaFX Application thread.
http://docs.oracle.com/javafx/2/api/javafx/application/Platform.html#runLater%28java.lang.Runnable%29
It's too late to answer but for those who have the error, here is the solution XD
You can use one Thread.
Use the lambda expression for the runnable in the thread and the runlater.
Thread t = new Thread(() -> {
//Here write all actions that you want execute on background
Platform.runLater(() -> {
//Here the actions that use the gui where is finished the actions on background.
});
});
t.start();
You can user directly this code
Don't forget you can't send non-final variable in thread .
you can send final variable in thread
//final String me="ddddd";
new Thread(new Runnable() {
#Override
public void run() {
// me = me + "eee";
//...Your code....
}
}).start();
Use in
your code
try/catch
I've been reading about AsyncTasks and Hanlders and Loopers but I still can't figure out where I'm going wrong in my code. I'm trying to run code that will look over a Tic Tac Toe grid and determine the best move for the computer. I want this code to run in the background 'cause it can take some time and then I can update the UI level with a textbox that says something like "I'm Thinking". I've tried this a bunch of different ways, none with success.
private class PostTask extends AsyncTask<String, Integer, String> {
private Board _b;
private Welcome.Player _opp;
private int _depth;
#Override
protected void onPreExecute() {
super.onPreExecute();
}
protected void SetVars(Board b, Player p, int depth){
_b = b;
_opp = p;
_depth = depth;
}
#Override
protected String doInBackground(String... params) {
Looper.prepare();
try{
_bestMove = _b.GetBestMove(_opp,_depth);
}
catch(Exception err){
_bestMove = -1;
}
return "All done";
}
#Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
}
#Override
protected void onPostExecute(String result) {
super.onPostExecute(result);
if(_bestMove == -1){
TextView tv = (TextView) findViewById(R.id.tv_Score);
tv.setText("Had and error, couldn't make a move.");
}
FollowUpComputerMove(this);
}
The above code will work for exactly 5 moves and then it crashes. When I watch in the debugger I see new theads being created named Thread<#> AsyncTask #1. Once I get to five of those AsyncTasks it goes back to try and grab the first AsyncTask and crashes. When it crashes I'm shown the ThreadPoolExecutor.class file.
I've also read that I shouldn't be using both the AsyncTask and the Looper objects together so I've tried taking the Loooper.prepare() statement out, but then my AsyncTask fails immediately with the error message:
Can't create handler inside thread that has not called Looper.prepare() - AsyncTask inside a dialog
I've read repeatedly that you shouldn't be trying to update the UI from an AsyncTask and that often the above error is because of that, but GetBestMove isn't updating the UI thread. When I trace through to see where the error comes, it fails when calling a constructor saying it can't find the class.
Can anyone point me in the right direction? My end goal is to use one main thread and only one background thread, and just keep re-using the background thread whenever the computer needs to make a move. I know that the recursive method GetBestMove works when I run this program in a single-thread manner. But the screen freezes for too long on some moves as the method is being run. Thank you so much.
-NifflerX
Apologies for answering my own question, but the issue I was facing had nothing to do with recursion. The class I was calling was extending the class Activity, and while trying to call that from an AsyncTask the program was erroring out. When I removed the extends Activity from the class definition it started working again. Sorry for the post.
-NifflerX
This example is copied from a book on Android. As you can see from my question, I am new to Android and trying to understand. This application should crash but it does not (I am updating UI from another thread. Which is not allowed.It should cause a crash. It does not. Why?). My code is:
final ProgressDialog dialogue = ProgressDialog.show(this, "title", "message");
new Thread(new Runnable() {
#Override
public void run() {
// TODO Auto-generated method stub
try {
Thread.sleep(7000);
dialogue.dismiss();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}).start();
This is done in OnCreate function. I am confused with line - dialogue.dismiss(); Isn't that updating UI (dismissing dialogue) from another thread? Why does this app not cause segmentation fault?
Thanks.
the dismiss() method can be run safetly on any thread as described in the Android documentation.
public void dismiss ()
Since: API Level 1 Dismiss this dialog, removing it from the screen.
This method can be invoked safely from any thread. Note that you
should not override this method to do cleanup when the dialog is
dismissed, instead implement that in onStop().
Could you let me know the reason for this error in SWT
"org.eclipse.swt.SWTException" Invalid Thread access ?
And How to fix such errors.
It happens when you try to act upon an interface item from a thread that's not the UI thread.
To run a code on the UI thread you have to use a Runnable and ask the display thread to run it. This way:
Display.getDefault().syncExec( new Runnable() {
#Override
public void run() {
// Do your job here
}
} );
As stated by the syncExec method javadoc,
the thread which calls this method is suspended until the runnable completes.
Also, you might check the asyncExec method.
In SWT you can access GUI resources only from the display thread. For example when setting the text in a org.eclipse.swt.widgets.Text control you must already be in the display thread or call
final Text text = ...;
Display.getCurrent().syncExec(new Runnable() {
#Override
public void run() {
text.setText("test");
}
});