如果启动了任务的新实例,如何取消任务?

this answer Service Task Service.cancel()

ListView Service.setOnSucceeded Label

Task.call()

import javafx.application.Application;
import javafx.concurrent.Service;
import javafx.concurrent.Task;
import javafx.geometry.Insets;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.ListView;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class Main extends Application {

    private final ListView<String> listView = new ListView<>();
    private final Label label = new Label("Nothing selected");

    private final QueryService service = new QueryService();

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

    @Override
    public void start(Stage stage) throws Exception {
        service.setOnSucceeded(wse -> {
            label.setText(service.getValue());
            service.reset();
        });
        service.setOnFailed(wse -> {
            // you could also show an Alert to the user here
            service.getException().printStackTrace();
            service.reset();
        });

        // Simple UI
        VBox root = new VBox(5);
        root.setAlignment(Pos.CENTER);
        root.setPadding(new Insets(10));
        root.getChildren().addAll(listView, label);

        // Populate the ListView
        listView.getItems().addAll(
                "One", "Two", "Three", "Four", "Five"
        );

        listView.getSelectionModel().selectedItemProperty().addListener((observableValue, oldValue, newValue) -> {
            if (service.isRunning()) {
                service.cancel();
                service.reset();
            }
            service.setSelected(newValue);
            service.start();
        });

        stage.setScene(new Scene(root));
        stage.show();

    }

    private static class QueryService extends Service<String> {

        // Field representing a JavaFX property
        private String selected;

        private void setSelected(String selected) {
            this.selected = selected;
        }

        @Override
        protected Task<String> createTask() {
            return new Task<>() {

                // Task state should be immutable/encapsulated
                private final String selectedCopy = selected;

                @Override
                protected String call() throws Exception {
                    try {
                        long waitTime = (long) (Math.random() * 15_000);
                        System.out.println("Waiting " + waitTime);
                        Thread.sleep(waitTime);
                        return "You have selected item: " + selectedCopy;
                    } catch (InterruptedException ex) {
                        System.out.println("Task interrupted!");
                        throw ex;
                    }
                }

            };
        }

        @Override
        protected void succeeded() {
            System.out.println("Service succeeded.");
        }

        @Override
        protected void cancelled() {
            System.out.println("Service cancelled.");
        }

    }
}

Service.start() Executor executor property null

reset() onSucceeded onFailed READY state restart() start() cancel()

Runnable Callable Thread boolean FutureTask isCancelled() Future 出于某种原因(调用外部代码,而不是使用 任务 等)然后使用以下方法检查线程中断:

• Thread.interrupted()
  • 静态法
  • 只能检查电流 螺纹
  • 清除当前线程的中断状态
• Thread.isInterrupted()
  • 实例方法
  • 可以检查任何 螺纹 你提到过
  • 不清除中断状态

您可以通过以下方式获取对当前线程的引用: Thread.currentThread() .

在您的后台代码中,如果当前线程被中断,您将希望在适当的点检查, 布尔值 已设置标志,或 任务 已取消。如果有,那么您将执行任何必要的清理并停止执行(通过返回或抛出异常)。

此外,如果您的 螺纹 正在等待一些可中断的操作,例如阻塞IO,然后它将 InterruptedException 中断时。在你的MVCE中你使用 Thread.sleep 这是可中断的;这意味着当调用Cancel时,此方法将引发上述异常。

当我在上面说“清理”时,我指的是任何必要的清理。 在背景中 因为你还在后台。如果需要清除fx线程上的任何内容(例如更新UI),则可以使用 onCancelled 的属性 服务 .

在上面的代码中,您还会看到我使用了受保护的方法 succeeded() 和 cancelled() . 两个 任务 和 服务 提供这些方法(以及 Worker.State s)并且始终在fx线程上调用它们。但是,请阅读文档 ScheduledService 要求您调用其中一些方法的超级实现。

^二 如果使用 布尔值 标志确保对它的更新对其他线程可见。你可以这样做 volatile ,同步它,或使用 java.util.concurrent.atomic.AtomicBoolean .

演示类

package com.changehealthcare.pid;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class ThreadFun {
    public static ExecutorService threadExecutor = Executors.newFixedThreadPool(1);
    public static ServiceThread serviceThreads = new ServiceThread();
    public static Future futureService;

    public static void main(String[] args) throws Exception {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        String line = "";

           while (line.equalsIgnoreCase("quit") == false) {
               line = in.readLine();
               //do something
               if (futureService != null) {
                   System.out.println("try to cancel");
                   futureService.cancel(true);
               }
               futureService = threadExecutor.submit(serviceThreads);
           }

           in.close();

    }
}

ServiceThread类

package com.changehealthcare.pid;

public class ServiceThread implements Runnable {

    @Override
    public void run() {
        System.out.println("starting the service");

        boolean isResult = this.doStuff();

        if (isResult) {
            this.displayStuff();
        }
        System.out.println("done");
    }

    private boolean doStuff() {
        try {
            for (int i=0; i<=10; i++) {
                System.out.println(i);
                Thread.sleep(1000);
            }

            return true;
        }
        catch (InterruptedException e) {
            System.out.println("cancelled");
            return false;
        }
    }

    private void displayStuff() {
        System.out.println("display done");
    }
}

解释 演示应用程序充当system.in(键盘)的侦听器。如果按了任何字符(按Enter键),它将取消任何当前线程(如果存在),并提交一个新线程。

服务线程演示有一个run()方法,它调用dostuf()和displaystuff()。您可以为DisplayStuff注入UI部件。在dostuff()中,它捕获interupptedException,如果发生该异常,则不执行displayStuff()。

请检查一下,看它是否适合你的箱子。我们可以做更多的运动。

打电话 javafx.concurrent.Service.cancel() 将取消当前正在运行的任何 Task ,使其抛出 InterruptedException .

取消当前正在运行的任何任务(如果有)。状态将设置为 CANCELLED .
_{~ Service (JavaFX 8) - cancel ~}

如果需要额外的清理,可以提供 Task.onCancelled()

class SingleTaskRunner {

    private Task<?> lastTask = null; 

    void runTask(Task<?> task) {
        registerTask(task);
        new Thread(task).start();
    }

    private synchronized void registerTask(Task<?> task) {
        if (lastTask != null) {
            lastTask.cancel(true);
        }
        lastTask = task;
    }
}

Nghia Do's answer

registerTask cancel Task

ExecutorService new Thread(task).start() Thread creation is expensive

1. Thread.sleep() Future.cancel() Future.cancel() calls Thread.interrupt()

3. onSucceeded Task.call()

public class TaskRace extends Application {

    private final ListView<String> listView = new ListView<>();
    private final Label label = new Label("Nothing selected");
    private final SingleTaskRunner runner = new SingleTaskRunner();

    private final long startMillis = System.currentTimeMillis();

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

    @Override
    public void start(Stage stage) {

        // Simple UI
        VBox root = new VBox(5);
        root.setAlignment(Pos.CENTER);
        root.setPadding(new Insets(10));
        root.getChildren().addAll(listView, label);

        // Populate the ListView
        listView.getItems().addAll(
                "One", "Two", "Three", "Four", "Five"
        );


        // Add listener to the ListView to start the task whenever an item is selected
        listView.getSelectionModel().selectedItemProperty().addListener((observableValue, oldValue, newValue) -> {

            if (newValue != null) {
                // Create the background task
                MyTask task = new MyTask();

                // Update the label when the task is completed
                task.setOnSucceeded(event -> {
                    label.setText(task.getValue());
                    println("Assigned " + task.selectedItem);
                });
                task.setOnCancelled(event -> println("Cancelled " + task.selectedItem));

                runner.runTask(task);
            }

        });

        stage.setScene(new Scene(root));
        stage.show();

    }

    private void println(String string) {
        System.out.format("%5.2fs: %s%n", 0.001 * (System.currentTimeMillis() - startMillis), string);
    }

    private class MyTask extends Task<String> {

        final String selectedItem = listView.getSelectionModel().getSelectedItem();

        @Override
        protected String call() {
            int ms = new Random().nextInt(10000);
            println(String.format("Will return %s in %.2fs", selectedItem, 0.001 * ms));

            // Do long-running task (takes random time)
            long limitMillis = System.currentTimeMillis() + ms;
            while (System.currentTimeMillis() < limitMillis) {
            }

            println("Returned " + selectedItem);
            return "You have selected item: " + selectedItem;
        }
    }
}

my answer Slaw's answer

Slaw's solution

volatile

private volatile String lastSelectedItem = null;

lastSelectedItem = task.selectedItem;

if (task.selectedItem.equals(lastSelectedItem))

public class TaskRaceWithoutCancelation extends Application {

    private final ListView<String> listView = new ListView<>();
    private final Label label = new Label("Nothing selected");

    private final long startMillis = System.currentTimeMillis();

    private volatile String lastSelectedItem = null;

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

    @Override
    public void start(Stage stage) {

        // Simple UI
        VBox root = new VBox(5);
        root.setAlignment(Pos.CENTER);
        root.setPadding(new Insets(10));
        root.getChildren().addAll(listView, label);

        // Populate the ListView
        listView.getItems().addAll(
                "One", "Two", "Three", "Four", "Five"
        );


        // Add listener to the ListView to start the task whenever an item is selected
        listView.getSelectionModel().selectedItemProperty().addListener((observableValue, oldValue, newValue) -> {

            if (newValue != null) {
                // Create the background task
                MyTask task = new MyTask();
                lastSelectedItem = task.selectedItem;

                // Update the label when the task is completed
                task.setOnSucceeded(event -> {
                    if (task.selectedItem.equals(lastSelectedItem)) {
                        label.setText(task.getValue());
                        println("Assigned " + task.selectedItem);
                    }
                });

                new Thread(task).start();
            }

        });

        stage.setScene(new Scene(root));
        stage.show();

    }

    private void println(String string) {
        System.out.format("%5.2fs: %s%n", 0.001 * (System.currentTimeMillis() - startMillis), string);
    }

    private class MyTask extends Task<String> {

        final String selectedItem = listView.getSelectionModel().getSelectedItem();

        @Override
        protected String call() {
            int ms = new Random().nextInt(10000);
            println(String.format("Will return %s in %.2fs", selectedItem, 0.001 * ms));

            // Do long-running task (takes random time)
            long limitMillis = System.currentTimeMillis() + ms;
            while (System.currentTimeMillis() < limitMillis) {
            }

            println("Returned " + selectedItem);
            return "You have selected item: " + selectedItem;
        }
    }
}