Why Thread.sleep() doesn't work accordingly in JavaFX?

Question

When i am using JavaFX, the sleep function won't work accordingly. Like in this code:

public class Controller {

@FXML private Label label;
@FXML private Button b1;

public void write() throws InterruptedException
{
    label.setText("FIRST TIME");
    for(int i=1;i<=5;i++)
    {
        System.out.println("Value "+i);
        label.setText("Value "+i);
        Thread.sleep(2000);
    }
    label.setText("LAST TIME");
}

when the Button b1 is pressed the write function is called. Now, in the console "Value + i" is being printed after 2 seconds. But that time the text of Label l1 doesn't change and finally it only changes to "LAST TIME". What is wrong in here ?

Answer 1

After having read the links proposed in the comments, you may want to remove the long process (delay) from the fx thread.
You can do it by invoking another thread :

public void write() {

    label.setText("FIRST TIME");

    new Thread(()->{ //use another thread so long process does not block gui
        for(int i=1;i<=6;i++)   {
            String text;
            if(i == 6 ){
                text = "LAST TIME";
            }else{
                 final int j = i;
                 text = "Value "+j;
            }

            //update gui using fx thread
            Platform.runLater(() -> label.setText(text));
            try {Thread.sleep(2000);} catch (InterruptedException ex) { ex.printStackTrace();}
        }

    }).start();
}

Or better use fx animation tools like :

private int i = 0; // a filed used for counting 

public void write() {

    label.setText("FIRST TIME");

    PauseTransition pause = new PauseTransition(Duration.seconds(2));
    pause.setOnFinished(event ->{
        label.setText("Value "+i++);
        if (i<=6) {
            pause.play();
        } else {
            label.setText("LAST TIME");
        }
    });
    pause.play();
}

Answer 2

I would try to create new thread used for time delay.

package sample;

import javafx.application.Platform;
import javafx.fxml.FXML;
import javafx.scene.control.Button;
import javafx.scene.control.Label;

public class Controller implements Runnable{


    private volatile boolean isRunning = false;
    private Thread timer;

    private int delay = 2000;

    @FXML
    private Label label;

    @FXML
    private Button button;


    private void start(){
        timer = new Thread(this, "timer");
        isRunning = true;
        timer.start();
    }

    private void stop(){
        isRunning = false;
    }

    private void interrupt(){
        isRunning = false;
        timer.interrupt();
    }

    @Override
    public void run() {
        int counter = 0;
        while (isRunning) {
            try {
                ++counter;
                String text = "MyText" + counter;
                Platform.runLater(() -> label.setText(text));

                if (counter == 5) {
                    stop();
                }


                Thread.currentThread().sleep(delay);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        }
    }
}

It is essential to update the label under Platform.runLater() - the JavaFx main thread is the only thread permitted to update JavaFx objects.