When the Worker.State FAILED occurs in a Task?

Question

How to detect that a Worker.State FAILED occurs in a Task? or to say when the Worker.State FAILED occurs in a Task? Is it when an exception happens in the call() method of Task?
Consider the code below:
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URI;
import java.net.URISyntaxException;
import java.net.URL;
import java.net.URLConnection;
import javafx.concurrent.Task;
* Background task to fetch the all classes documentation page from a URL
public class FetchDocListTask extends Task<String> {
  private final String docsDirUrl;
  public FetchDocListTask(String docsDirUrl) {
  this.docsDirUrl = docsDirUrl;
  @Override
  protected String call() throws Exception {
  System.out.println("---- FetchDocListTask  docsUrl = " + docsDirUrl);
  StringBuilder builder = new StringBuilder();
  try {
  URI uri = new URI(docsDirUrl + "allclasses-frame.html");
  URL url = uri.toURL();
  URLConnection urlConnection = url.openConnection();
  urlConnection.setConnectTimeout(5000); // set timeout to 5 secs
  InputStream in = urlConnection.getInputStream();
  BufferedReader reader = new BufferedReader(
  new InputStreamReader(in));
  String line;
  while ((line = reader.readLine()) != null) {
  builder.append(line);
  builder.append('\n');
  reader.close();
  } catch (URISyntaxException e) {
  e.printStackTrace();
  return builder.toString();
When the State.FAILED occurs? In fact, I want to write a code to simply detect whether the computer is connected to the internet. Hope for help~

Answer 1

Yes. If the call() method exits due to an unhandled exception, the state property changes to FAILED. If a return statement is successfully executed, the state property changes to SUCCEEDED. There are no other possibilities.
You can test with something like:
import javafx.application.Application;
    import javafx.concurrent.Task;
    import javafx.concurrent.WorkerStateEvent;
    import javafx.event.EventHandler;
    import javafx.scene.Scene;
    import javafx.scene.control.TextArea;
    import javafx.scene.layout.BorderPane;
    import javafx.stage.Stage;
    public class TaskStateTest extends Application {
        public static void main(String[] args) { launch(args); }
        @Override
        public void start(final Stage primaryStage) {
            Task<Void> exceptionHandlingTask = new Task<Void>() {
                @Override
                protected Void call() throws Exception {
                    try {
                        throw new Exception("Boom");
                    } catch (Exception exc) {
                        System.out.println(exc.getMessage() + " handled");
                    return null;
            Task<Void> exceptionThrowingTask = new Task<Void>() {
                @Override
                protected Void call() throws Exception {
                    throw new Exception("Boom");
        //            return null;
            final BorderPane root = new BorderPane();
            final TextArea textArea = new TextArea();
            root.setCenter(textArea);
            primaryStage.setScene(new Scene(root, 600, 400));
            primaryStage.show();
            registerHandlers(exceptionHandlingTask, "exceptionHandlingTask", textArea);
            registerHandlers(exceptionThrowingTask, "exceptionThrowingTask", textArea);
            Thread t1 = new Thread(exceptionHandlingTask);
            Thread t2 = new Thread(exceptionThrowingTask);
            t1.start();
            t2.start();
        private void registerHandlers(final Task<Void> task, final String msg, final TextArea textArea) {
            task.setOnFailed(new EventHandler<WorkerStateEvent>() {
                @Override
                public void handle(WorkerStateEvent event) {
                    textArea.appendText(msg + " failed\n");
            task.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
                @Override
                public void handle(WorkerStateEvent event) {
                    textArea.appendText(msg +  " succeeded\n");