Thread.sleep（）的意外行为

Question

I have constructed a simple program to play a song.

import javafx.scene.media.Media;
import javafx.scene.media.MediaPlayer;
import java.io.File;
import javax.swing.*;
import java.awt.*;
import javafx.embed.swing.JFXPanel;
import javafx.scene.Scene;
import javafx.application.Platform;
import javafx.scene.Group; 

public class Fade {
    private void initAndShowGUI() {
        // This method is invoked on Swing thread
        JFrame frame = new JFrame("FX");
        final JFXPanel fxPanel = new JFXPanel();
        frame.add(fxPanel);
        frame.setVisible(true);

        Platform.runLater(new Runnable() {
            @Override
            public void run() {
                initFX(fxPanel);
            }
        });
    }

    private void initFX(JFXPanel fxPanel) {
    // This method is invoked on JavaFX thread
        Group root = new Group(); 
        Scene scene = new Scene(root, 500, 500, true);
        fxPanel.setScene(scene);

        try {
            String bip = new File("./Sound/Welcome To The Show - L1-5.mp3").toURI().toString();
            Media hit = new Media(bip);
            MediaPlayer mediaPlayer = new MediaPlayer(hit);
            mediaPlayer.play();

            Thread.sleep(10000);
            System.out.println("Fade");
        } catch (Exception e) {
            System.out.println("Exception");
            System.out.println(e);
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new Fade().initAndShowGUI();
            }
        });
    }
}

However when this code is run the Thread.sleep(10000) runs before the song starts playing instead of the song starting playing then 10 seconds later fade is printed out to the console. How may I fix this?

Answer 1

You are blocking the JavaFX Application Thread, so it cannot do its normal work (which presumably includes processing the media player's instruction to start) until your method completes. You should never block this thread, as it is guaranteed to make the UI unresponsive.

The simplest way to implement a pause in JavaFX is to use a PauseTransition :

MediaPlayer mediaPlayer = new MediaPlayer(hit);
mediaPlayer.play();

// never do this:
//Thread.sleep(10000);

PauseTransition pause = new PauseTransition(Duration.millis(10000));
pause.setOnFinished(e -> {
    System.out.println("Fade");
});
pause.play();

(As an aside, I assume your real application has some reason to mix swing and JavaFX. Otherwise you should make this a pure JavaFX app.)

Answer 2

Operating on a MediaPlayer is asynchronous. This explains why "Fade" is printed before the actual music begins. From the documentation :

The operation of a MediaPlayer is inherently asynchronous. A player is not prepared to respond to commands quasi-immediately until its status has transitioned to MediaPlayer.Status.READY , which in effect generally occurs when media pre-roll completes. Some requests made of a player prior to its status being READY will however take effect when that status is entered. These include invoking play() without an intervening invocation of pause() or stop() before the READY transition, as well as setting any of the autoPlay , balance , mute , rate , startTime , stopTime , and volume properties.

You can control if the player is actually playing by polling its status . From play() (emphasis mine):

Starts playing the media. If previously paused, then playback resumes where it was paused. If playback was stopped, playback starts from the startTime. When playing actually starts the status will be set to MediaPlayer.Status.PLAYING .

To register a callback when the player starts playing, you can hook into the onPlayingProperty by adding a change listener or directly setting an event handler by calling setOnPlaying(runnable)