Task (JavaFX 17)

Module javafx.graphics
Package javafx.concurrent

## Class Task<V>

- java.lang.Object
- - java.util.concurrent.FutureTask<V>
  - - javafx.concurrent.Task<V>

- All Implemented Interfaces:
    `Runnable`, `Future<V>`, `RunnableFuture<V>`, `Worker<V>`, `EventTarget`

      public abstract class Task<V>
      extends FutureTask<V>
      implements Worker<V>, EventTarget

  A fully observable implementation of a `FutureTask`. `Task` exposes additional state and observable properties useful for programming asynchronous tasks in JavaFX, as defined in the `Worker` interface. An implementation of Task must override the `call()` method. This method is invoked on the background thread. Any state which is used in this method must be safe to read and write from a background thread. For example, manipulating a live scene graph from this method is unsafe and will result in runtime exceptions.

  Tasks are flexible and extremely useful for the encapsulation of "work". Because `Service` is designed to execute a Task, any Tasks defined by the application or library code can easily be used with a Service. Likewise, since Task extends from FutureTask, it is very easy and natural to use a Task with the java concurrency `Executor` API. Since a Task is Runnable, you can also call it directly (by invoking the `FutureTask.run()` method) from another background thread. This allows for composition of work, or pass it to a new Thread constructed and executed manually. Finally, since you can manually create a new Thread, passing it a Runnable, it is possible to use the following idiom:

               Thread th = new Thread(task);
               th.setDaemon(true);
               th.start();

  Note that this code sets the daemon flag of the Thread to true. If you want a background thread to prevent the VM from exiting after the last stage is closed, then you would want daemon to be false. However, if you want the background threads to simply terminate after all the stages are closed, then you must set daemon to true.

  Although `ExecutorService` defines several methods which take a Runnable, you should generally limit yourself to using the `execute` method inherited from `Executor`.

  As with FutureTask, a Task is a one-shot class and cannot be reused. See `Service` for a reusable `Worker`.

  Because the Task is designed for use with JavaFX GUI applications, it ensures that every change to its public properties, as well as change notifications for state, errors, and for event handlers, all occur on the main JavaFX application thread. Accessing these properties from a background thread (including the `call()` method) will result in runtime exceptions being raised. The only exception to this, is when initially configuring a Task, which may safely be done from any thread. However, once the Task has been initialized and started, it may only thereafter be used from the FX thread (except for those methods clearly marked as being appropriate for the subclass to invoke from the background thread).

  It is strongly encouraged that all Tasks be initialized with immutable state upon which the Task will operate. This should be done by providing a Task constructor which takes the parameters necessary for execution of the Task. Immutable state makes it easy and safe to use from any thread and ensures correctness in the presence of multiple threads.

  In Java there is no reliable way to "kill" a thread in process. However, when `cancel` is called on a Task, it is important that the Task stop processing. A "run-away" Task might continue processing and updating the message, text, and progress properties even after the Task has been cancelled! In Java, cancelling a Task is a cooperative endeavor. The user of the Task will request that it be cancelled, and the author of the Task must check whether is has been cancelled within the body of the `call` method. There are two ways this can be done. First, the Task author may check the isCancelled method, inherited from `FutureTask`, to see whether the Task has been cancelled. Second, if the Task implementation makes use of any blocking calls (such as NIO InterruptibleChannels or Thread.sleep) and the task is cancelled while in such a blocking call, an InterruptedException is thrown. Task implementations which have blocking calls should recognize that an interrupted thread may be the signal for a cancelled task and should double check the isCancelled method to ensure that the InterruptedException was thrown due to the cancellation of the Task.

  ## Examples

  The following set of examples demonstrate some of the most common uses of Tasks.

  ### A Simple Loop

  The first example is a simple loop that does nothing particularly useful, but demonstrates the fundamental aspects of writing a Task correctly. This example will simply loop and print to standard out on each loop iteration. When it completes, it returns the number of times it iterated.

           Task<Integer> task = new Task<Integer>() {
               @Override protected Integer call() throws Exception {
                   int iterations;
                   for (iterations = 0; iterations < 100000; iterations++) {
                       if (isCancelled()) {
                           break;
                       }
                       System.out.println("Iteration " + iterations);
                   }
                   return iterations;
               }
           };

  First, we define what type of value is returned from this Task. In this case, we want to return the number of times we iterated, so we will specify the Task to be of type Integer by using generics. Then, within the implementation of the `call` method, we iterate from 0 to 100000. On each iteration, we check to see whether this Task has been cancelled. If it has been, then we break out of the loop and return the number of times we iterated. Otherwise a message is printed to the console and the iteration count increased and we continue looping.

  Checking for isCancelled() in the loop body is critical, otherwise the developer may cancel the task, but the task will continue running and updating both the progress and returning the incorrect result from the end of the `call` method. A correct implementation of a Task will always check for cancellation.

  ### A Simple Loop With Progress Notification

  Similar to the previous example, except this time we will modify the progress of the Task in each iteration. Note that we have a choice to make in the case of cancellation. Do we want to set the progress back to -1 (indeterminate) when the Task is cancelled, or do we want to leave the progress where it was at? In this case, let's leave the progress alone and only update the message on cancellation, though updating the progress after cancellation is a perfectly valid choice.

           Task<Integer> task = new Task<Integer>() {
               @Override protected Integer call() throws Exception {
                   int iterations;
                   for (iterations = 0; iterations < 10000000; iterations++) {
                       if (isCancelled()) {
                           updateMessage("Cancelled");
                           break;
                       }
                       updateMessage("Iteration " + iterations);
                       updateProgress(iterations, 10000000);
                   }
                   return iterations;
               }
           };

  As before, within the for loop we check whether the Task has been cancelled. If it has been cancelled, we will update the Task's message to indicate that it has been cancelled, and then break as before. If the Task has not been cancelled, then we will update its message to indicate the current iteration and then update the progress to indicate the current progress.

  ### A Simple Loop With Progress Notification And Blocking Calls

  This example adds to the previous examples a blocking call. Because a blocking call may thrown an InterruptedException, and because an InterruptedException may occur as a result of the Task being cancelled, we need to be sure to handle the InterruptedException and check on the cancel state.

           Task<Integer> task = new Task<Integer>() {
               @Override protected Integer call() throws Exception {
                   int iterations;
                   for (iterations = 0; iterations < 1000; iterations++) {
                       if (isCancelled()) {
                           updateMessage("Cancelled");
                           break;
                       }
                       updateMessage("Iteration " + iterations);
                       updateProgress(iterations, 1000);

                       // Now block the thread for a short time, but be sure
                       // to check the interrupted exception for cancellation!
                       try {
                           Thread.sleep(100);
                       } catch (InterruptedException interrupted) {
                           if (isCancelled()) {
                               updateMessage("Cancelled");
                               break;
                           }
                       }
                   }
                   return iterations;
               }
           };

  Here we have added to the body of the loop a `Thread.sleep` call. Since this is a blocking call, I have to handle the potential InterruptedException. Within the catch block, I will check whether the Task has been cancelled, and if so, update the message accordingly and break out of the loop.

  ### A Task Which Takes Parameters

  Most Tasks require some parameters in order to do useful work. For example, a DeleteRecordTask needs the object or primary key to delete from the database. A ReadFileTask needs the URI of the file to be read. Because Tasks operate on a background thread, care must be taken to make sure the body of the `call` method does not read or modify any shared state. There are two techniques most useful for doing this: using final variables, and passing variables to a Task during construction.

  When using a Task as an anonymous class, the most natural way to pass parameters to the Task is by using final variables. In this example, we pass to the Task the total number of times the Task should iterate.

           final int totalIterations = 9000000;
           Task<Integer> task = new Task<Integer>() {
               @Override protected Integer call() throws Exception {
                   int iterations;
                   for (iterations = 0; iterations < totalIterations; iterations++) {
                       if (isCancelled()) {
                           updateMessage("Cancelled");
                           break;
                       }
                       updateMessage("Iteration " + iterations);
                       updateProgress(iterations, totalIterations);
                   }
                   return iterations;
               }
           };

  Since `totalIterations` is final, the `call` method can safely read it and refer to it from a background thread.

  When writing Task libraries (as opposed to specific-use implementations), we need to use a different technique. In this case, I will create an IteratingTask which performs the same work as above. This time, since the IteratingTask is defined in its own file, it will need to have parameters passed to it in its constructor. These parameters are assigned to final variables.

           public class IteratingTask extends Task<Integer> {
               private final int totalIterations;

               public IteratingTask(int totalIterations) {
                   this.totalIterations = totalIterations;
               }

               @Override protected Integer call() throws Exception {
                   int iterations = 0;
                   for (iterations = 0; iterations < totalIterations; iterations++) {
                       if (isCancelled()) {
                           updateMessage("Cancelled");
                           break;
                       }
                       updateMessage("Iteration " + iterations);
                       updateProgress(iterations, totalIterations);
                   }
                   return iterations;
               }
           }

  And then when used:

           IteratingTask task = new IteratingTask(8000000);

  In this way, parameters are passed to the IteratingTask in a safe manner, and again, are final. Thus, the `call` method can safely read this state from a background thread.

  WARNING: Do not pass mutable state to a Task and then operate on it from a background thread. Doing so may introduce race conditions. In particular, suppose you had a SaveCustomerTask which took a Customer in its constructor. Although the SaveCustomerTask may have a final reference to the Customer, if the Customer object is mutable, then it is possible that both the SaveCustomerTask and some other application code will be reading or modifying the state of the Customer from different threads. Be very careful in such cases, that while a mutable object such as this Customer is being used from a background thread, that it is not being used also from another thread. In particular, if the background thread is reading data from the database and updating the Customer object, and the Customer object is bound to scene graph nodes (such as UI controls), then there could be a violation of threading rules! For such cases, modify the Customer object from the FX Application Thread rather than from the background thread.

           public class UpdateCustomerTask extends Task<Customer> {
               private final Customer customer;

               public UpdateCustomerTask(Customer customer) {
                   this.customer = customer;
               }

               @Override protected Customer call() throws Exception {
                   // pseudo-code:
                   //   query the database
                   //   read the values

                   // Now update the customer
                   Platform.runLater(new Runnable() {
                       @Override public void run() {
                           customer.setFirstName(rs.getString("FirstName"));
                           // etc
                       }
                   });

                   return customer;
               }
           }

  ### A Task Which Returns No Value

  Many, if not most, Tasks should return a value upon completion. For CRUD Tasks, one would expect that a "Create" Task would return the newly created object or primary key, a "Read" Task would return the read object, an "Update" task would return the number of records updated, and a "Delete" task would return the number of records deleted.

  However sometimes there just isn't anything truly useful to return. For example, I might have a Task which writes to a file. Task has built into it a mechanism for indicating whether it has succeeded or failed along with the number of bytes written (the progress), and thus there is nothing really for me to return. In such a case, you can use the Void type. This is a special type in the Java language which can only be assigned the value of `null`. You would use it as follows:

           final String filePath = "/foo.txt";
           final String contents = "Some contents";
           Task<Void> task = new Task<Void>() {
               @Override protected Void call() throws Exception {
                   File file = new File(filePath);
                   FileOutputStream out = new FileOutputStream(file);
                   // ... and other code to write the contents ...

                   // Return null at the end of a Task of type Void
                   return null;
               }
           };

  ### A Task Which Returns An ObservableList

  Because the ListView, TableView, and other UI controls and scene graph nodes make use of ObservableList, it is common to want to create and return an ObservableList from a Task. When you do not care to display intermediate values, the easiest way to correctly write such a Task is simply to construct an ObservableList within the `call` method, and then return it at the conclusion of the Task.

           Task<ObservableList<Rectangle>> task = new Task<ObservableList<Rectangle>>() {
               @Override protected ObservableList<Rectangle> call() throws Exception {
                   updateMessage("Creating Rectangles");
                   ObservableList<Rectangle> results = FXCollections.observableArrayList();
                   for (int i=0; i<100; i++) {
                       if (isCancelled()) break;
                       Rectangle r = new Rectangle(10, 10);
                       r.setX(10 * i);
                       results.add(r);
                       updateProgress(i, 100);
                   }
                   return results;
               }
           };

  In the above example, we are going to create 100 rectangles and return them from this task. An ObservableList is created within the `call` method, populated, and then returned.

  ### A Task Which Returns Partial Results

  Sometimes you want to create a Task which will return partial results. Perhaps you are building a complex scene graph and want to show the scene graph as it is being constructed. Or perhaps you are reading a large amount of data over the network and want to display the entries in a TableView as the data is arriving. In such cases, there is some shared state available both to the FX Application Thread and the background thread. Great care must be taken to never update shared state from any thread other than the FX Application Thread.

  The easiest way to do this is to take advantage of the `updateValue(Object)` method. This method may be called repeatedly from the background thread. Updates are coalesced to prevent saturation of the FX event queue. This means you can call it as frequently as you like from the background thread but only the most recent set is ultimately set.

           Task<Long> task = new Task<Long>() {
               @Override protected Long call() throws Exception {
                   long a = 0;
                   long b = 1;
                   for (long i = 0; i < Long.MAX_VALUE; i++) {
                       updateValue(a);
                       a += b;
                       b = a - b;
                   }
                   return a;
               }
           };

  Another way to do this is to expose a new property on the Task which will represent the partial result. Then make sure to use `Platform.runLater` when updating the partial result.

           Task<Long> task = new Task<Long>() {
               @Override protected Long call() throws Exception {
                   long a = 0;
                   long b = 1;
                   for (long i = 0; i < Long.MAX_VALUE; i++) {
                       final long v = a;
                       Platform.runLater(new Runnable() {
                           @Override public void run() {
                               updateValue(v);
                           }
                       }
                       a += b;
                       b = a - b;
                   }
                   return a;
               }
           };

  Suppose instead of updating a single value, you want to populate an ObservableList with results as they are obtained. One approach is to expose a new property on the Task which will represent the partial result. Then make sure to use `Platform.runLater` when adding new items to the partial result.

           public class PartialResultsTask extends Task<ObservableList<Rectangle>> {
               // Uses Java 7 diamond operator
               private ReadOnlyObjectWrapper<ObservableList<Rectangle>> partialResults =
                       new ReadOnlyObjectWrapper<>(this, "partialResults",
                               FXCollections.observableArrayList(new ArrayList<Rectangle>()));

               public final ObservableList<Rectangle> getPartialResults() { return partialResults.get(); }
               public final ReadOnlyObjectProperty<ObservableList<Rectangle>> partialResultsProperty() {
                   return partialResults.getReadOnlyProperty();
               }

               @Override protected ObservableList<Rectangle> call() throws Exception {
                   updateMessage("Creating Rectangles...");
                   for (int i=0; i<100; i++) {
                       if (isCancelled()) break;
                       final Rectangle r = new Rectangle(10, 10);
                       r.setX(10 * i);
                       Platform.runLater(new Runnable() {
                           @Override public void run() {
                               partialResults.get().add(r);
                           }
                       });
                       updateProgress(i, 100);
                   }
                   return partialResults.get();
               }
           }

  ### A Task Which Modifies The Scene Graph

  Generally, Tasks should not interact directly with the UI. Doing so creates a tight coupling between a specific Task implementation and a specific part of your UI. However, when you do want to create such a coupling, you must ensure that you use `Platform.runLater` so that any modifications of the scene graph occur on the FX Application Thread.

           final Group group = new Group();
           Task<Void> task = new Task<Void>() {
               @Override protected Void call() throws Exception {
                   for (int i=0; i<100; i++) {
                       if (isCancelled()) break;
                       final Rectangle r = new Rectangle(10, 10);
                       r.setX(10 * i);
                       Platform.runLater(new Runnable() {
                           @Override public void run() {
                               group.getChildren().add(r);
                           }
                       });
                   }
                   return null;
               }
           };

  ### Reacting To State Changes Generically

  Sometimes you may want to write a Task which updates its progress, message, text, or in some other way reacts whenever a state change happens on the Task. For example, you may want to change the status message on the Task on Failure, Success, Running, or Cancelled state changes.

           Task<Integer> task = new Task<Integer>() {
               @Override protected Integer call() throws Exception {
                   int iterations = 0;
                   for (iterations = 0; iterations < 100000; iterations++) {
                       if (isCancelled()) {
                           break;
                       }
                       System.out.println("Iteration " + iterations);
                   }
                   return iterations;
               }

               @Override protected void succeeded() {
                   super.succeeded();
                   updateMessage("Done!");
               }

               @Override protected void cancelled() {
                   super.cancelled();
                   updateMessage("Cancelled!");
               }

               @Override protected void failed() {
                   super.failed();
                   updateMessage("Failed!");
               }
           };

  Since:
    JavaFX 2.0

- - ### Property Summary

    Properties
    Type | Property | Description
    --- | --- | ---
    `ObjectProperty<EventHandler<WorkerStateEvent>>` | `onCancelled` | The onCancelled event handler is called whenever the Task state transitions to the CANCELLED state.
    `ObjectProperty<EventHandler<WorkerStateEvent>>` | `onFailed` | The onFailed event handler is called whenever the Task state transitions to the FAILED state.
    `ObjectProperty<EventHandler<WorkerStateEvent>>` | `onRunning` | The onRunning event handler is called whenever the Task state transitions to the RUNNING state.
    `ObjectProperty<EventHandler<WorkerStateEvent>>` | `onScheduled` | The onSchedule event handler is called whenever the Task state transitions to the SCHEDULED state.
    `ObjectProperty<EventHandler<WorkerStateEvent>>` | `onSucceeded` | The onSucceeded event handler is called whenever the Task state transitions to the SUCCEEDED state.

    - ### Properties declared in interface javafx.concurrent.Worker

      `exception, message, progress, running, state, title, totalWork, value, workDone`

  - ### Nested Class Summary

    - ### Nested classes/interfaces declared in interface javafx.concurrent.Worker

      `Worker.State`

  - ### Constructor Summary

    Constructors
    Constructor | Description
    --- | ---
    `Task()` | Creates a new Task.

  - ### Method Summary

    All Methods Instance Methods Abstract Methods Concrete Methods
    Modifier and Type | Method | Description
    --- | --- | ---
    `<T extends Event> void` | `addEventFilter​(EventType<T> eventType, EventHandler<? super T> eventFilter)` | Registers an event filter to this task.
    `<T extends Event> void` | `addEventHandler​(EventType<T> eventType, EventHandler<? super T> eventHandler)` | Registers an event handler to this task.
    `protected abstract V` | `call()` | Invoked when the Task is executed, the call method must be overridden and implemented by subclasses.
    `protected void` | `cancelled()` | A protected convenience method for subclasses, called whenever the state of the Task has transitioned to the CANCELLED state.
    `protected void` | `failed()` | A protected convenience method for subclasses, called whenever the state of the Task has transitioned to the FAILED state.
    `void` | `fireEvent​(Event event)` | Fires the specified event.
    `EventHandler<WorkerStateEvent>` | `getOnCancelled()` | The onCancelled event handler is called whenever the Task state transitions to the CANCELLED state.
    `EventHandler<WorkerStateEvent>` | `getOnFailed()` | The onFailed event handler is called whenever the Task state transitions to the FAILED state.
    `EventHandler<WorkerStateEvent>` | `getOnRunning()` | The onRunning event handler is called whenever the Task state transitions to the RUNNING state.
    `EventHandler<WorkerStateEvent>` | `getOnScheduled()` | The onSchedule event handler is called whenever the Task state transitions to the SCHEDULED state.
    `EventHandler<WorkerStateEvent>` | `getOnSucceeded()` | The onSucceeded event handler is called whenever the Task state transitions to the SUCCEEDED state.
    `ObjectProperty<EventHandler<WorkerStateEvent>>` | `onCancelledProperty()` | The onCancelled event handler is called whenever the Task state transitions to the CANCELLED state.
    `ObjectProperty<EventHandler<WorkerStateEvent>>` | `onFailedProperty()` | The onFailed event handler is called whenever the Task state transitions to the FAILED state.
    `ObjectProperty<EventHandler<WorkerStateEvent>>` | `onRunningProperty()` | The onRunning event handler is called whenever the Task state transitions to the RUNNING state.
    `ObjectProperty<EventHandler<WorkerStateEvent>>` | `onScheduledProperty()` | The onSchedule event handler is called whenever the Task state transitions to the SCHEDULED state.
    `ObjectProperty<EventHandler<WorkerStateEvent>>` | `onSucceededProperty()` | The onSucceeded event handler is called whenever the Task state transitions to the SUCCEEDED state.
    `<T extends Event> void` | `removeEventFilter​(EventType<T> eventType, EventHandler<? super T> eventFilter)` | Unregisters a previously registered event filter from this task.
    `<T extends Event> void` | `removeEventHandler​(EventType<T> eventType, EventHandler<? super T> eventHandler)` | Unregisters a previously registered event handler from this task.
    `protected void` | `running()` | A protected convenience method for subclasses, called whenever the state of the Task has transitioned to the RUNNING state.
    `protected void` | `scheduled()` | A protected convenience method for subclasses, called whenever the state of the Task has transitioned to the SCHEDULED state.
    `protected <T extends Event> void` | `setEventHandler​(EventType<T> eventType, EventHandler<? super T> eventHandler)` | Sets the handler to use for this event type.
    `void` | `setOnCancelled​(EventHandler<WorkerStateEvent> value)` | The onCancelled event handler is called whenever the Task state transitions to the CANCELLED state.
    `void` | `setOnFailed​(EventHandler<WorkerStateEvent> value)` | The onFailed event handler is called whenever the Task state transitions to the FAILED state.
    `void` | `setOnRunning​(EventHandler<WorkerStateEvent> value)` | The onRunning event handler is called whenever the Task state transitions to the RUNNING state.
    `void` | `setOnScheduled​(EventHandler<WorkerStateEvent> value)` | The onSchedule event handler is called whenever the Task state transitions to the SCHEDULED state.
    `void` | `setOnSucceeded​(EventHandler<WorkerStateEvent> value)` | The onSucceeded event handler is called whenever the Task state transitions to the SUCCEEDED state.
    `protected void` | `succeeded()` | A protected convenience method for subclasses, called whenever the state of the Task has transitioned to the SUCCEEDED state.
    `protected void` | `updateMessage​(String message)` | Updates the `message` property.
    `protected void` | `updateProgress​(double workDone, double max)` | Updates the `workDone`, `totalWork`, and `progress` properties.
    `protected void` | `updateProgress​(long workDone, long max)` | Updates the `workDone`, `totalWork`, and `progress` properties.
    `protected void` | `updateTitle​(String title)` | Updates the `title` property.
    `protected void` | `updateValue​(V value)` | Updates the `value` property.

    - ### Methods declared in class java.util.concurrent.FutureTask

      `cancel, done, get, get, isCancelled, isDone, run, runAndReset, set, setException, toString`

    - ### Methods declared in class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

    - ### Methods declared in interface javafx.event.EventTarget

      `buildEventDispatchChain`

    - ### Methods declared in interface javafx.concurrent.Worker

      `cancel, exceptionProperty, getException, getMessage, getProgress, getState, getTitle, getTotalWork, getValue, getWorkDone, isRunning, messageProperty, progressProperty, runningProperty, stateProperty, titleProperty, totalWorkProperty, valueProperty, workDoneProperty`

- - ### Property Detail

    - #### onScheduled

          public final ObjectProperty<EventHandler<WorkerStateEvent>> onScheduledProperty

      The onSchedule event handler is called whenever the Task state transitions to the SCHEDULED state.
      Since:
        JavaFX 2.1
      See Also:
        `getOnScheduled()`, `setOnScheduled(EventHandler)`

    - #### onRunning

          public final ObjectProperty<EventHandler<WorkerStateEvent>> onRunningProperty

      The onRunning event handler is called whenever the Task state transitions to the RUNNING state.
      Since:
        JavaFX 2.1
      See Also:
        `getOnRunning()`, `setOnRunning(EventHandler)`

    - #### onSucceeded

          public final ObjectProperty<EventHandler<WorkerStateEvent>> onSucceededProperty

      The onSucceeded event handler is called whenever the Task state transitions to the SUCCEEDED state.
      Since:
        JavaFX 2.1
      See Also:
        `getOnSucceeded()`, `setOnSucceeded(EventHandler)`

    - #### onCancelled

          public final ObjectProperty<EventHandler<WorkerStateEvent>> onCancelledProperty

      The onCancelled event handler is called whenever the Task state transitions to the CANCELLED state.
      Since:
        JavaFX 2.1
      See Also:
        `getOnCancelled()`, `setOnCancelled(EventHandler)`

    - #### onFailed

          public final ObjectProperty<EventHandler<WorkerStateEvent>> onFailedProperty

      The onFailed event handler is called whenever the Task state transitions to the FAILED state.
      Since:
        JavaFX 2.1
      See Also:
        `getOnFailed()`, `setOnFailed(EventHandler)`

  - ### Constructor Detail

    - #### Task

          public Task()

      Creates a new Task.

  - ### Method Detail

    - #### call

          protected abstract V call()
                             throws Exception

      Invoked when the Task is executed, the call method must be overridden and implemented by subclasses. The call method actually performs the background thread logic. Only the updateProgress, updateMessage, updateValue and updateTitle methods of Task may be called from code within this method. Any other interaction with the Task from the background thread will result in runtime exceptions.
      Returns:
        The result of the background work, if any.
      Throws:
        `Exception` - an unhandled exception which occurred during the background operation

    - #### onScheduledProperty

          public final ObjectProperty<EventHandler<WorkerStateEvent>> onScheduledProperty()

      The onSchedule event handler is called whenever the Task state transitions to the SCHEDULED state.
      Since:
        JavaFX 2.1
      See Also:
        `getOnScheduled()`, `setOnScheduled(EventHandler)`

    - #### getOnScheduled

          public final EventHandler<WorkerStateEvent> getOnScheduled()

      The onSchedule event handler is called whenever the Task state transitions to the SCHEDULED state.
      Returns:
        the onScheduled event handler, if any
      Since:
        JavaFX 2.1

    - #### setOnScheduled

          public final void setOnScheduled​(EventHandler<WorkerStateEvent> value)

      The onSchedule event handler is called whenever the Task state transitions to the SCHEDULED state.
      Parameters:
        `value` - the event handler, can be null to clear it
      Since:
        JavaFX 2.1

    - #### scheduled

          protected void scheduled()

      A protected convenience method for subclasses, called whenever the state of the Task has transitioned to the SCHEDULED state. This method is invoked on the FX Application Thread after the Task has been fully transitioned to the new state.
      Since:
        JavaFX 2.1

    - #### onRunningProperty

          public final ObjectProperty<EventHandler<WorkerStateEvent>> onRunningProperty()

      The onRunning event handler is called whenever the Task state transitions to the RUNNING state.
      Since:
        JavaFX 2.1
      See Also:
        `getOnRunning()`, `setOnRunning(EventHandler)`

    - #### getOnRunning

          public final EventHandler<WorkerStateEvent> getOnRunning()

      The onRunning event handler is called whenever the Task state transitions to the RUNNING state.
      Returns:
        the onRunning event handler, if any
      Since:
        JavaFX 2.1

    - #### setOnRunning

          public final void setOnRunning​(EventHandler<WorkerStateEvent> value)

      The onRunning event handler is called whenever the Task state transitions to the RUNNING state.
      Parameters:
        `value` - the event handler, can be null to clear it
      Since:
        JavaFX 2.1

    - #### running

          protected void running()

      A protected convenience method for subclasses, called whenever the state of the Task has transitioned to the RUNNING state. This method is invoked on the FX Application Thread after the Task has been fully transitioned to the new state.
      Since:
        JavaFX 2.1

    - #### onSucceededProperty

          public final ObjectProperty<EventHandler<WorkerStateEvent>> onSucceededProperty()

      The onSucceeded event handler is called whenever the Task state transitions to the SUCCEEDED state.
      Since:
        JavaFX 2.1
      See Also:
        `getOnSucceeded()`, `setOnSucceeded(EventHandler)`

    - #### getOnSucceeded

          public final EventHandler<WorkerStateEvent> getOnSucceeded()

      The onSucceeded event handler is called whenever the Task state transitions to the SUCCEEDED state.
      Returns:
        the onSucceeded event handler, if any
      Since:
        JavaFX 2.1

    - #### setOnSucceeded

          public final void setOnSucceeded​(EventHandler<WorkerStateEvent> value)

      The onSucceeded event handler is called whenever the Task state transitions to the SUCCEEDED state.
      Parameters:
        `value` - the event handler, can be null to clear it
      Since:
        JavaFX 2.1

    - #### succeeded

          protected void succeeded()

      A protected convenience method for subclasses, called whenever the state of the Task has transitioned to the SUCCEEDED state. This method is invoked on the FX Application Thread after the Task has been fully transitioned to the new state.
      Since:
        JavaFX 2.1

    - #### onCancelledProperty

          public final ObjectProperty<EventHandler<WorkerStateEvent>> onCancelledProperty()

      The onCancelled event handler is called whenever the Task state transitions to the CANCELLED state.
      Since:
        JavaFX 2.1
      See Also:
        `getOnCancelled()`, `setOnCancelled(EventHandler)`

    - #### getOnCancelled

          public final EventHandler<WorkerStateEvent> getOnCancelled()

      The onCancelled event handler is called whenever the Task state transitions to the CANCELLED state.
      Returns:
        the onCancelled event handler, if any
      Since:
        JavaFX 2.1

    - #### setOnCancelled

          public final void setOnCancelled​(EventHandler<WorkerStateEvent> value)

      The onCancelled event handler is called whenever the Task state transitions to the CANCELLED state.
      Parameters:
        `value` - the event handler, can be null to clear it
      Since:
        JavaFX 2.1

    - #### cancelled

          protected void cancelled()

      A protected convenience method for subclasses, called whenever the state of the Task has transitioned to the CANCELLED state. This method is invoked on the FX Application Thread after the Task has been fully transitioned to the new state.
      Since:
        JavaFX 2.1

    - #### onFailedProperty

          public final ObjectProperty<EventHandler<WorkerStateEvent>> onFailedProperty()

      The onFailed event handler is called whenever the Task state transitions to the FAILED state.
      Since:
        JavaFX 2.1
      See Also:
        `getOnFailed()`, `setOnFailed(EventHandler)`

    - #### getOnFailed

          public final EventHandler<WorkerStateEvent> getOnFailed()

      The onFailed event handler is called whenever the Task state transitions to the FAILED state.
      Returns:
        the onFailed event handler, if any
      Since:
        JavaFX 2.1

    - #### setOnFailed

          public final void setOnFailed​(EventHandler<WorkerStateEvent> value)

      The onFailed event handler is called whenever the Task state transitions to the FAILED state.
      Parameters:
        `value` - the event handler, can be null to clear it
      Since:
        JavaFX 2.1

    - #### failed

          protected void failed()

      A protected convenience method for subclasses, called whenever the state of the Task has transitioned to the FAILED state. This method is invoked on the FX Application Thread after the Task has been fully transitioned to the new state.
      Since:
        JavaFX 2.1

    - #### updateProgress

          protected void updateProgress​(long workDone,
                                        long max)

      Updates the `workDone`, `totalWork`, and `progress` properties. Calls to updateProgress are coalesced and run later on the FX application thread, and calls to updateProgress, even from the FX Application thread, may not necessarily result in immediate updates to these properties, and intermediate workDone values may be coalesced to save on event notifications. `max` becomes the new value for `totalWork`.

      This method is safe to be called from any thread.

      Parameters:
        `workDone` - A value from Long.MIN_VALUE up to max. If the value is greater than max, then it will be clamped at max. If the value passed is negative then the resulting percent done will be -1 (thus, indeterminate).
        `max` - A value from Long.MIN_VALUE to Long.MAX_VALUE.
      See Also:
        `updateProgress(double, double)`

    - #### updateProgress

          protected void updateProgress​(double workDone,
                                        double max)

      Updates the `workDone`, `totalWork`, and `progress` properties. Calls to updateProgress are coalesced and run later on the FX application thread, and calls to updateProgress, even from the FX Application thread, may not necessarily result in immediate updates to these properties, and intermediate workDone values may be coalesced to save on event notifications. `max` becomes the new value for `totalWork`.

      This method is safe to be called from any thread.

      Parameters:
        `workDone` - A value from Double.MIN_VALUE up to max. If the value is greater than max, then it will be clamped at max. If the value passed is negative, or Infinity, or NaN, then the resulting percentDone will be -1 (thus, indeterminate).
        `max` - A value from Double.MIN_VALUE to Double.MAX_VALUE. Infinity and NaN are treated as -1.
      Since:
        JavaFX 2.2

    - #### updateMessage

          protected void updateMessage​(String message)

      Updates the `message` property. Calls to updateMessage are coalesced and run later on the FX application thread, so calls to updateMessage, even from the FX Application thread, may not necessarily result in immediate updates to this property, and intermediate message values may be coalesced to save on event notifications.

      This method is safe to be called from any thread.

      Parameters:
        `message` - the new message

    - #### updateTitle

          protected void updateTitle​(String title)

      Updates the `title` property. Calls to updateTitle are coalesced and run later on the FX application thread, so calls to updateTitle, even from the FX Application thread, may not necessarily result in immediate updates to this property, and intermediate title values may be coalesced to save on event notifications.

      This method is safe to be called from any thread.

      Parameters:
        `title` - the new title

    - #### updateValue

          protected void updateValue​(V value)

      Updates the `value` property. Calls to updateValue are coalesced and run later on the FX application thread, so calls to updateValue, even from the FX Application thread, may not necessarily result in immediate updates to this property, and intermediate values may be coalesced to save on event notifications.

      This method is safe to be called from any thread.

      Parameters:
        `value` - the new value
      Since:
        JavaFX 8.0

    - #### addEventHandler

          public final <T extends Event> void addEventHandler​(EventType<T> eventType,
                                                              EventHandler<? super T> eventHandler)

      Registers an event handler to this task. Any event filters are first processed, then the specified onFoo event handlers, and finally any event handlers registered by this method. As with other events in the scene graph, if an event is consumed, it will not continue dispatching.
      Type Parameters:
        `T` - the specific event class of the handler
      Parameters:
        `eventType` - the type of the events to receive by the handler
        `eventHandler` - the handler to register
      Throws:
        `NullPointerException` - if the event type or handler is null
      Since:
        JavaFX 2.1

    - #### removeEventHandler

          public final <T extends Event> void removeEventHandler​(EventType<T> eventType,
                                                                 EventHandler<? super T> eventHandler)

      Unregisters a previously registered event handler from this task. One handler might have been registered for different event types, so the caller needs to specify the particular event type from which to unregister the handler.
      Type Parameters:
        `T` - the specific event class of the handler
      Parameters:
        `eventType` - the event type from which to unregister
        `eventHandler` - the handler to unregister
      Throws:
        `NullPointerException` - if the event type or handler is null
      Since:
        JavaFX 2.1

    - #### addEventFilter

          public final <T extends Event> void addEventFilter​(EventType<T> eventType,
                                                             EventHandler<? super T> eventFilter)

      Registers an event filter to this task. Registered event filters get an event before any associated event handlers.
      Type Parameters:
        `T` - the specific event class of the filter
      Parameters:
        `eventType` - the type of the events to receive by the filter
        `eventFilter` - the filter to register
      Throws:
        `NullPointerException` - if the event type or filter is null
      Since:
        JavaFX 2.1

    - #### removeEventFilter

          public final <T extends Event> void removeEventFilter​(EventType<T> eventType,
                                                                EventHandler<? super T> eventFilter)

      Unregisters a previously registered event filter from this task. One filter might have been registered for different event types, so the caller needs to specify the particular event type from which to unregister the filter.
      Type Parameters:
        `T` - the specific event class of the filter
      Parameters:
        `eventType` - the event type from which to unregister
        `eventFilter` - the filter to unregister
      Throws:
        `NullPointerException` - if the event type or filter is null
      Since:
        JavaFX 2.1

    - #### setEventHandler

          protected final <T extends Event> void setEventHandler​(EventType<T> eventType,
                                                                 EventHandler<? super T> eventHandler)

      Sets the handler to use for this event type. There can only be one such handler specified at a time. This handler is guaranteed to be called first. This is used for registering the user-defined onFoo event handlers.
      Type Parameters:
        `T` - the specific event class of the handler
      Parameters:
        `eventType` - the event type to associate with the given eventHandler
        `eventHandler` - the handler to register, or null to unregister
      Throws:
        `NullPointerException` - if the event type is null
      Since:
        JavaFX 2.1

    - #### fireEvent

          public final void fireEvent​(Event event)

      Fires the specified event. Any event filter encountered will be notified and can consume the event. If not consumed by the filters, the event handlers on this task are notified. If these don't consume the event either, then all event handlers are called and can consume the event.

      This method must be called on the FX user thread.

      Parameters:
        `event` - the event to fire
      Since:
        JavaFX 2.1