Concurrency in JavaFX

http://docs.oracle.com/javafx/2/threads/jfxpub-threads.htm

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Concurrency in JavaFX

This article describes the capabilities provided by the javafx.concurrent package to create multithreaded applications. You learn how to keep your JavaFX application user interface (UI) responsive by delegating time-consuming task execution to background threads.

Why Use the javafx.concurrent Package?

The JavaFX scene graph, which represents the graphical user interface of a JavaFX application, is not thread-safe and can only be accessed and modified from the UI thread also known as the JavaFX Application thread. Implementing long-running tasks on the JavaFX Application thread inevitably makes an application UI unresponsive. A best practice is to do these tasks on one or more background threads and let the JavaFX Application thread process user events.

If you implement a background worker by creating a Runnable object and a new thread, at some point, you must communicate with the JavaFX Application thread, either with a result or with the progress of the background task, which is error prone. Instead, use the JavaFX APIs provided by the javafx.concurrent package, which takes care of multithreaded code that interacts with the UI and ensures that this interaction happens on the correct thread.

Overview of the javafx.concurrent Package

The Java platform provides a complete set of concurrency libraries available through the java.util.concurrent package. The javafx.concurrent package leverages the existing API by considering the JavaFX Application thread and other constraints faced by GUI developers.

The javafx.concurrent package consists of the Worker interface and two basic classes, Task and Service, both of which implement the Worker interface. The Worker interface provides APIs that are useful for a background worker to communicate with the UI. The Task class is a fully observable implementation of the java.util.concurrent.FutureTask class. The Task class enables developers to implement asynchronous tasks in JavaFX applications. The Service class executes tasks.

The WorkerStateEvent class specifies an event that occurs whenever the state of a Worker implementation changes. Both the Task and Service classes implement the EventTarget interface and thus support listening to the state events.

The Worker Interface

The Worker interface defines an object that performs some work on one or more background threads. The state of the Worker object is observable and usable from the JavaFX Application thread.

The lifecycle of the Worker object is defined as follows. When created, the Worker object is in the READY state. Upon being scheduled for work, the Worker object transitions to the SCHEDULED state. After that, when the Worker object is performing the work, its state becomes RUNNING. Note that even when the Worker object is immediately started without being scheduled, it first transitions to the SCHEDULED state and then to the RUNNING state. The state of a Worker object that completes successfully is SUCCEEDED, and the value property is set to the result of this Worker object. Otherwise, if any exceptions are thrown during the execution of the Worker object, its state becomes FAILED and the exception property is set to the type of the exception that occurred. At any time before the end of the Worker object the developer can interrupt it by invoking the cancel method, which puts the Worker object into the CANCELLED state.

The progress of the work being done by the Worker object can be obtained through three different properties such as totalWork, workDone, and progress.

For more information on the range of the parameter values, see the API documentation.

The Task Class

Tasks are used to implement the logic of work that needs to be done on a background thread. First, you need to extend the Task class. Your implementation of the Task class must override the call method to do the background work and return the result.

The call method is invoked on the background thread, therefore this method can only manipulate states that are safe to read and write from a background thread. For example, manipulating an active scene graph from the call method throws runtime exceptions. On the other hand, the Task class is designed to be used with JavaFX GUI applications, and it ensures that any changes to public properties, change notifications for errors or cancellation, event handlers, and states occur on the JavaFX Application thread. Inside the call method, you can use the updateProgress, updateMessage, updateTitle methods, which update the values of the corresponding properties on the JavaFX Application thread. However, if the task was canceled, a return value from the call method is ignored.

Note that the Task class fits into the Java concurrency libraries because it inherits from the java.utils.concurrent.FutureTask class, which implements the Runnable interface. For this reason, a Task object can be used within the Java concurrency Executor API and also can be passed to a thread as a parameter. You can call the Task object directly by using the FutureTask.run() method, which enables calling this task from another background thread. Having a good understanding of the Java concurrency API will help you understand concurrency in JavaFX.

A task can be started in one of the following ways:

• By starting a thread with the given task as a parameter:

  Thread th = new Thread(task);

  th.setDaemon(true);

  th.start();

• By using the ExecutorService API:

  ExecutorService.submit(task);

The Task class defines a one-time object that cannot be reused. If you need a reusable Worker object, use the Service class.

Cancelling the Task

There is no reliable way in Java to stop a thread in process. However, the task must stop processing whenever cancel is called on the task. The task is supposed to check periodically during its work whether it was cancelled by using the isCancelled method within the body of the call method. Example 1 shows a correct implementation of the Task class that checks for cancellation.

Example 1

import javafx.concurrent.Task;

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;
    }
};

If the task implementation has blocking calls such as Thread.sleep and the task is cancelled while in a blocking call, an InterruptedException is thrown. For these tasks, an interrupted thread may be the signal for a cancelled task. Therefore, tasks that have blocking calls must double-check the isCancelled method to ensure that the InterruptedException was thrown due to the cancellation of the task as shown in Example 2.

Example 2

import javafx.concurrent.Task;

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

Showing the Progress of a Background Task

A typical use case in multithreaded applications is showing the progress of a background task. Suppose you have a background task that counts from one to one million and a progress bar, and you must update the progress on this progress bar as the counter runs in the background. Example 3 shows how to update a progress bar.

Example 3

import javafx.concurrent.Task;

Task task = new Task<Void>() {
    @Override public Void call() {
        static final int max = 1000000;
        for (int i=1; i<=max; i++) {
            if (isCancelled()) {
               break;
            }
            updateProgress(i, max);
        }
        return null;
    }
};
ProgressBar bar = new ProgressBar();
bar.progressProperty().bind(task.progressProperty());
new Thread(task).start();

First, you create the task by overriding the call method where you implement the logic of the work to be done and invoke the updateProgress method, which updates the progress, totalWork, and workDone properties of the task. This is important because you can now use the progressProperty method to retrieve the progress of the task and bind the progress of the bar to the progress of the task.

The Service Class

The Service class is designed to execute a Task object on one or several background threads. The Service class methods and states must only be accessed on the JavaFX Application thread. The purpose of this class is to help the developer to implement the correct interaction between the background threads and the JavaFX Application thread.

You have the following control over the Service object: you can start, cancel and restart it as you need. To start the Service object, use the Service.start() method.

Using the Service class, you can observe the state of the background work and optionally cancel it. Later, you can reset the service and restart it. Thus, the service can be defined declaratively and restarted on demand.

When implementing the subclasses of the Service class, be sure to expose the input parameters to the Task object as properties of the subclass.

The service can be executed in one of the following ways:

• By an Executor object, if it is specified for the given service

• By a daemon thread, if no executor is specified

• By a custom executor such as a ThreadPoolExecutor

Example 4 shows an implementation of the Service class which reads the first line from any URL and returns it as a string.

Example 4

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;
import javafx.application.Application;
import javafx.beans.property.SimpleStringProperty;
import javafx.beans.property.StringProperty;
import javafx.concurrent.Service;
import javafx.concurrent.Task;
import javafx.concurrent.WorkerStateEvent;
import javafx.event.EventHandler;
import javafx.stage.Stage;

public class FirstLineServiceApp extends Application {

    @Override
    public void start(Stage stage) throws Exception {
        FirstLineService service = new FirstLineService();
        service.setUrl("http://google.com");
        service.setOnSucceeded(new EventHandler<WorkerStateEvent>() {

            @Override
            public void handle(WorkerStateEvent t) {
                System.out.println("done:" + t.getSource().getValue());
            }
        });
        service.start();
    }

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

    private static class FirstLineService extends Service<String> {
        private StringProperty url = new SimpleStringProperty();

        public final void setUrl(String value) {
            url.set(value);
        }

        public final String getUrl() {
            return url.get();
        }

        public final StringProperty urlProperty() {
           return url;
        }

        protected Task<String> createTask() {
            final String _url = getUrl();
            return new Task<String>() {
                protected String call() 
                    throws IOException, MalformedURLException {
                        String result = null;
                        BufferedReader in = null;
                        try {
                            URL u = new URL(_url);
                            in = new BufferedReader(
                                new InputStreamReader(u.openStream()));
                            result = in.readLine();
                        } finally {
                            if (in != null) {
                                in.close();
                            }
                        }
                        return result;
                }
            };
        }
    }
}

The WorkerStateEvent Class and State Transitions

Whenever the state of the Worker implementation changes, an appropriate event, defined by the WorkerStateEvent class, occurs. For example, when the Task object transitions to the SUCCEEDED state, the WORKER_STATE_SUCCEEDED event occurs, the onSucceeded event handler is called, after which the protected convenience method succeeded is invoked on the JavaFX Application thread.

There are several protected convenience methods such as cancelled, failed, running, scheduled, and succeeded, which are invoked when the Worker implementation transitions to the corresponding state. These methods can be overridden by subclasses of the Task and Service classes when the state is changed to implement the logic of your application. Example 5 shows a Task implementation that updates the status message on the task's success, cancellation, and failure.

Example 5

import javafx.concurrent.Task;

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!");
    }
};

Conclusion

In this article, you learned the basic capabilities provided by the javafx.concurrent package and became familiar with several examples of the Task and Service classes implementation. For more examples of how to create the Task implementation correctly, see the API documentation for the Task class.