CODEWITHSUNDEEP
javamultithreadingjavafxjava.util.concurrent
DazedNConfused
DazedNConfused

Reputation: 199

Javafx thread join on traffic Light UI. How do I synchronize threads ?

I have a javafx UI that simulates a traffic light. I have three threads for each of the lights which lights up each of their respective lights while turning off the others. I start the threads and use join() so that each thread can light up their respective lights before another thread starts.

When I use the Thread.join(), the UI freezes but I know the threads are running properly because the of test string i put inside the threads executes and waits for appropriate time before another one executes.

Code is as follows -

main method

public void start(Stage primaryStage) {

    lightBoard.setArcWidth(10);
    lightBoard.setArcHeight(10);
    lightBoard.setFill(Color.DARKGREY);

    circleGreen.setFill(Color.WHITE);

    circleYellow.setFill(Color.WHITE);

    circleRed.setFill(Color.WHITE);

    Group group = new Group();
    group.getChildren().addAll(lightBoard, circleGreen, circleYellow, circleRed);
    BorderPane light = new BorderPane(group);
    light.setPadding( new Insets(20, 20, 20, 20) );

    GridPane layoutControls = new GridPane();
    layoutControls.setAlignment(Pos.CENTER);
    Button btnStart = new Button("Start");
    Button btnStop = new Button("Stop");
    layoutControls.add(btnStart, 0, 0);
    layoutControls.add(btnStop, 1, 0);

    BorderPane root = new BorderPane();
    root.setCenter(light);
    root.setBottom(layoutControls);

    Scene scene = new Scene(root, 380, 250);

    primaryStage.setTitle("Traffic Light Simulator");
    primaryStage.setScene(scene);
    primaryStage.show();

    // create and start threads

    Runnable lightGreen = new LightGreen();
    Runnable lightYellow = new LightYellow();
    Runnable lightRed = new LightRed();

    Thread threadGreen = new Thread(lightGreen);
    Thread threadYellow = new Thread(lightYellow);
    Thread threadRed = new Thread(lightRed);

    while(true){
        threadGreen  = new Thread(lightGreen);
        threadGreen.start();
        try{
            threadGreen.join();
        }
        catch(Exception e){

        }
        threadYellow = new Thread(lightYellow);
        threadYellow.start();
        try{
            threadYellow.join();
        }
        catch(Exception e){

        }
        threadRed =  new Thread(lightRed);
        threadRed.start();
        try{
            threadRed.join();
        }
        catch(Exception e){

        }
    }
}

thread classes - 

class LightGreen implements Runnable {

    @Override
    public void run() {

        lock.lock();
        System.out.println("GREEn");
        circleGreen.setFill( Color.GREEN );
        circleYellow.setFill( Color.WHITE );
        circleRed.setFill( Color.WHITE );
        lock.unlock();

        try {
            Thread.sleep(3000L);
        } catch (InterruptedException ex) {
        }      
    }
}

class LightYellow implements Runnable {

    @Override
    public void run() {

        lock.lock();
        System.out.println("YELLOW");
        circleGreen.setFill( Color.WHITE );
        circleYellow.setFill( Color.YELLOW );
        circleRed.setFill( Color.WHITE );
        lock.unlock();
        try {
            Thread.sleep(3000L);
        } catch (InterruptedException ex) {
        }

    }
}

class LightRed implements Runnable {

    @Override
    public void run() {

        lock.lock();
        System.out.println("RED");
        circleGreen.setFill( Color.WHITE );
        circleYellow.setFill( Color.WHITE );
        circleRed.setFill( Color.RED );
        lock.unlock();
        try {
            Thread.sleep(3000L);
        } catch (InterruptedException ex) {
        }    
    }
}

How can i resolve this? I have been told Platform.runlater can be used but not sure how to use it. I need to use multithreading for this task.

Upvotes: 0

Views: 831

Answers (1)

c0der
c0der

Reputation: 18792

Here is a quick demo of how to use JavaFx PauseTransition to update gui:

public class TrafficLight extends Application{

    private static final double RADIUS = 50;
    private static final double PAUSE = 1;
    private Circle  circleRed,  circleYellow,  circleGreen;
    private Color[] colors = {Color.RED, Color.YELLOW, Color.GREEN};
    private int onColor = 0;

    @Override
    public void start(Stage primaryStage) {

        circleRed = new Circle(RADIUS);
        circleRed.setFill(Color.WHITE);
        circleGreen = new Circle(RADIUS);
        circleGreen.setFill(Color.WHITE);
        circleYellow = new Circle(RADIUS);
        circleYellow.setFill(Color.WHITE);

        TilePane light = new TilePane(circleGreen, circleYellow, circleRed);
        light.setPadding( new Insets(20, 20, 20, 20) );
        Scene scene = new Scene(light, RADIUS*8, RADIUS*3);

        primaryStage.setTitle("Traffic Light Simulator");
        primaryStage.setScene(scene);
        primaryStage.show();
        update();
    }

    private void update() {

        PauseTransition pause = new PauseTransition(Duration.seconds(PAUSE));
        pause.setOnFinished(event ->{
            circleRed.setFill((onColor == 0) ?  colors[onColor] :Color.WHITE );
            circleYellow.setFill((onColor == 1) ? colors[onColor] :Color.WHITE);
            circleGreen.setFill((onColor == 2) ? colors[onColor] :Color.WHITE);
            onColor = ((onColor +1) >= colors.length) ? 0 : onColor+1;

            pause.play();
        });

        pause.play();
    }

    public static void main(String[] args) {
        launch(args);
    }
}

Edit
If you need to do it with threads, although I think it is not the optimal tool, try this technique: 

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.layout.TilePane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Circle;
import javafx.stage.Stage;

public class TrafficLight extends Application{

    private static final double RADIUS = 50;

    private Circle  circleRed,  circleYellow,  circleGreen;
    private Color[] colors = {Color.RED, Color.YELLOW, Color.GREEN};
    private int onColor = 0;
    private static int threadNumber = 0, invokeThreadNumber =0;
    private static final Object myLock = new Object();

    @Override
    public void start(Stage primaryStage) {

        circleRed = new Circle(RADIUS);
        circleRed.setFill(Color.WHITE);
        circleGreen = new Circle(RADIUS);
        circleGreen.setFill(Color.WHITE);
        circleYellow = new Circle(RADIUS);
        circleYellow.setFill(Color.WHITE);

        TilePane light = new TilePane(circleGreen, circleYellow, circleRed);
        light.setPadding( new Insets(20, 20, 20, 20) );
        Scene scene = new Scene(light, RADIUS*8, RADIUS*3);

        primaryStage.setTitle("Traffic Light Simulator");
        primaryStage.setScene(scene);
        primaryStage.show();
        control();
    }

    private void control() {
        //invoke 3 synchronized control threads 
        new Thread( new ColorControl()).start();
        new Thread( new ColorControl()).start();
        new Thread( new ColorControl()).start();
    }

    private void update() {
        circleRed.setFill((onColor == 0) ?  colors[onColor] :Color.WHITE );
        circleYellow.setFill((onColor == 1) ? colors[onColor] :Color.WHITE);
        circleGreen.setFill((onColor == 2) ? colors[onColor] :Color.WHITE);
        onColor = ((onColor +1) >= colors.length) ? 0 : onColor+1;
    }

    class ColorControl implements Runnable {

        private int threadID;
        private static final long PAUSE = 1000;
        private int MAX_THREADS = 3;
        private boolean isStopped = false;

        ColorControl() {
            threadID = threadNumber ++;
        }

        void reset() {
            threadNumber = 0; invokeThreadNumber =0;
        }

        @Override
        public void run() {
            synchronized (myLock) {

                while (! isStopped  ) {
                    while (threadID != invokeThreadNumber) {
                        try {
                            myLock.wait();
                        } catch (InterruptedException e) {}
                    }
                    //do work here
                    update();

                    try {
                        Thread.sleep(PAUSE);
                    } catch (InterruptedException ex) { ex.printStackTrace();}
                    invokeThreadNumber++;
                    myLock.notifyAll();
                    if( invokeThreadNumber >= MAX_THREADS ) {
                        reset();
                    }
                }
            }
        }
    }

    public static void main(String[] args) {
        launch(args);
    }
}

Upvotes: 1

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