Ошибка IllegalThreadStateException при попытке запуска потока во второй раз

Question

Простой секундомер, но когда пытаюсь запустить во второй раз поток который подсчитывает время вылазит IllegalThreadStateException:

Exception in thread "JavaFX Application Thread" java.lang.IllegalThreadStateException
    at java.base/java.lang.Thread.start(Thread.java:792)
    at com.company.Stopwatch.lambda$start$0(Stopwatch.java:30)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:86)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:49)
    at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
    at javafx.graphics/javafx.scene.Node.fireEvent(Node.java:8889)
    at javafx.controls/javafx.scene.control.Button.fire(Button.java:203)
    at javafx.controls/com.sun.javafx.scene.control.behavior.ButtonBehavior.mouseReleased(ButtonBehavior.java:208)
    at javafx.controls/com.sun.javafx.scene.control.inputmap.InputMap.handle(InputMap.java:274)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:247)
    at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
    at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
    at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
    at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
    at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
    at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
    at javafx.graphics/javafx.scene.Scene$MouseHandler.process(Scene.java:3856)
    at javafx.graphics/javafx.scene.Scene.processMouseEvent(Scene.java:1851)
    at javafx.graphics/javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2584)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:409)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler$MouseEventNotification.run(GlassViewEventHandler.java:299)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.lambda$handleMouseEvent$2(GlassViewEventHandler.java:447)
    at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:412)
    at javafx.graphics/com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:446)
    at javafx.graphics/com.sun.glass.ui.View.handleMouseEvent(View.java:556)
    at javafx.graphics/com.sun.glass.ui.View.notifyMouse(View.java:942)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at javafx.graphics/com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
    at java.base/java.lang.Thread.run(Thread.java:832)

вот код программы:

package com.company;

import javafx.application.Application;
import javafx.application.Platform;
import javafx.geometry.Insets;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class Stopwatch extends Application {

    public static Label timeAmount = new Label("00:00:00"); // вывод времени
    private Button start = new Button("Start");
    private Button stop = new Button("Stop");

    private StackPane root;
    public static Scene stopwatchScene;

    private StopwatchThread stopwatch = new StopwatchThread("stopwatch");

    public static void main(String[] args){ Application.launch(args); }
    public void start(Stage stage){

        timeAmount.setStyle("-fx-font: 50 arial");

        start.setOnAction(event -> {
            stopwatch.start(); // запускаем новый класс

            start.setDisable(true);
            stop.setDisable(false);
        });

        stop.setOnAction(event -> {
            stopwatch.stop();

            start.setDisable(false);
            stop.setDisable(true);
        });
        stop.setDisable(true);

        StackPane.setAlignment(start, Pos.BOTTOM_CENTER);
        StackPane.setAlignment(stop, Pos.BOTTOM_CENTER);

        StackPane.setMargin(start, new Insets(0, 0, 0 , 50));
        StackPane.setMargin(stop, new Insets(0, 50, 0 , 0));

        root = new StackPane(timeAmount, start, stop); //распологаме элементы
        stopwatchScene = new Scene(root);

        stage.setScene(stopwatchScene);
        stage.setTitle("Stopwatch");
        stage.show();
        stage.setMinWidth(250);
        stage.setMinHeight(160);
        stage.setMaxHeight(150);
        stage.setResizable(false);
    }

    private static class StopwatchThread extends Thread { // класс с секундомером

        private byte sec = 0;
        private byte min = 0;
        private byte hours = 0;

        StopwatchThread(String name){super(name);}

        public void run(){

            while(true) { // костыль для подсчёта секунд
                if (sec == 59) {
                    if (min == 59) {
                        min = 0;
                        hours++;
                    } else {
                        sec = 0;
                        min++;
                    }
                } else {
                    sec++;
                }

                Runnable setTimeLeft = new Runnable() {
                    @Override
                    public void run() {
                        timeAmount.setText(hours + ":" + min + ":" + sec); // выводим время
                    }
                };
                Platform.runLater(setTimeLeft);

                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

Answer 1

В целом, я бы всё сделал по другому, но если побыстрому исправлять твой код, то

        private StopwatchThread stopwatch;

...

        start.setOnAction(event -> {
            stopwatch = new StopwatchThread("stopwatch"); // новый поток
            stopwatch.start(); 
            start.setDisable(true);
            stop.setDisable(false);
        });

        stop.setOnAction(event -> {
            stopwatch.setRunning(false);
            start.setDisable(false);
            stop.setDisable(true);
        });
...

 private static class StopwatchThread extends Thread {
        private boolean running = false;
...
        public synchronized void setRunning(boolean running) {
          this.running = running; 
          notify();
        }
...
        public synchronized void run(){
            running = true;
            while(running) { 
...    
                try {
                    wait(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
...

Ещё работа с таймером неправильная, ни sleep, ни wait не гарантируют точность задержки. На длинной дистанции расхождение будет слишком большим.

Как сделал бы я

Переменная volatile long startMs.
При нажатии на Start инициализируется startMs = System.currentMillis();
При нажатии на Stop сбрасывается startMs = 0
Какой-нибудь стандартный андроидный таймер, который сработывает каждые 500мс и проверяет startMs. Если 0, то таймер не запущен. Если не 0, то

Duration elapsed = Duration.ofMillis(System.currentMillis() - startMs);
int hours = elapsed.toHours(); // 0 .. infinity
int minutes = elapsed.toMinutesPart(); // 0..59
int seconds = elapsed.toSecondsPart(); // 0..59

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Ошибка IllegalThreadStateException при попытке запуска потока во второй раз

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