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JDC Tech Tips, February 19, 2002 (Java Media Framework, JSObject)

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  • Magno Cavalcante
    J D C T E C H T I P S TIPS, TECHNIQUES, AND SAMPLE CODE WELCOME to the Java Developer Connection(sm) (JDC) Tech Tips, February 19, 2002. This
    Message 1 of 1 , Feb 21, 2002
      J D C T E C H T I P S


      WELCOME to the Java Developer Connection(sm) (JDC) Tech Tips,
      February 19, 2002. This issue covers:

      * Playing Audio and Video With the Java(tm) Media
      * Using the JSObject Class in Applets

      These tips were developed using Java 2 SDK, Standard Edition,
      v 1.3 and Java Media Framework 2.1.1.

      This issue of the JDC Tech Tips is written by John Zukowski,
      president of JZ Ventures, Inc. (http://www.jzventures.com).

      You can view this issue of the Tech Tips on the Web at

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      You can use the Core Java 2 Platform libraries to display still
      images in GIF, JPEG, and PNG format. The libraries also provide
      limited support for displaying animation through the GIF89A image
      format. You can also play WAV, AU, MIDI, and AIFF-formatted audio
      files. This support might be sufficient for your programs, but
      if you need to work with other rich media formats, such as
      AVI files for video or MP3 files for audio, you need the Java
      Media Framework (JMF) API.

      The JMF API supports the playing, streaming, and capturing of
      audio and video. It provides a series of encoders and decoders to
      support different formats and offers a pluggable architecture for
      you to add support for additional formats.

      The latest version of the Java Media Framework software
      (JMF 2.1.1 a) is available for download from
      The JMF download comes in two flavors: a platform-specific
      version and a cross-platform version. Sun provides
      platform-specific versions for Solaris and Windows. (A Linux
      version is also available from Blackdown.)

      The Sun platform-specific versions include audio support through
      native libraries for the Java Sound API, the cross-platform
      version does not. After downloading the installation, you need to
      configure your machine to use the JMF libraries. With the
      platform-specific library versions, you need to add the jmf.jar
      and sound.jar files to your class path, and the JMF lib directory
      to your path. For the cross-platform version, you need to add
      a reference to the jmf.jar file into your class path.

      The Linux version comes in three flavors and supports both audio
      and video. For additional information and installation
      instructions see

      Playing multimedia files through the JMF libraries is simple. The
      key classes are Manager and Player. The Manager has a series of
      createPlayer() methods that each return a Player. After you
      create a Player, you tell the Player to start playing. For basic
      audio playing, all you need to do is:

      Player player = Manager.createPlayer(resource);

      The resource you pass into the createPlayer method can be one of
      three things: DataSource, MediaLocator, or URL. These represent
      different ways of specifying media: as a protocol handler
      (DataSource), through its content (MediaLocator), or by it
      location (URL). In most cases, working with the URL resources is
      simplest. For example, if you want to play a file that is on your
      local hard drive, you simply:

      1. Get the URL for the file through the toURL method of the File

      2. Pass the URL as the resource to createPlayer.

      3. Play the file using the play method:

      URL url = file.toURL();
      Player player = Manager.createPlayer(url);

      This process is not sufficient to play video. Calling play() for
      a video file is like watching your favorite video with the
      television set turned off. You might hear the audio but there's
      no picture. In order to get a picture you need to do a little
      extra work, such as register a ControllerListener.

      The Player is a type of Controller, and controllers let you
      register a ControllerListener. The ControllerListener contains
      a single method
      public void controllerUpdate(ControllerEvent event). You use the
      method to find out when various events happen with the media,
      such as when the end of a video file is reached, when an audio
      file has been loaded, or when the playing of media data has
      started and stopped.

      To respond to these events use the ControllerAdapter class.
      ControllerAdapter offers more than thirty different methods to
      respond to specific types of controller events. Each of these
      methods dispatches to a stub-method. Typically, you subclass the
      ControllerAdapter to override the specific method or methods of
      interest with your own event-handling logic. If you don't use
      this approach, you need to check for the correct type of event
      using instanceof in the controllerUpdate method.

      An event subtype that is particularly important is
      RealizeCompleteEvent. When this event happens, the
      ControllerAdapter delegates the handling to the realizeComplete
      method. Overriding this method permits you to get the visual
      component for the video player and the control panel
      component for audio and video playing. The control panel is
      where you can control audio volume, and start or stop video.
      Here is how you can get the different components and add them to
      your display.

      Component vc = player.getVisualComponent();
      if (vc != null) {
      contentPane.add(vc, BorderLayout.CENTER);
      Component cpc = player.getControlPanelComponent();
      if (cpc != null) {
      contentPane.add(cpc, BorderLayout.SOUTH);

      The following example puts together all the features discussed
      here. The program provides a button that brings up a JFileChooser
      to select what file you want to play. The program then acquires
      a player for the file, and starts playing the file. This triggers
      the acquisition of the visual components being added to the
      screen. All the JMF-related code is in the load method of the
      program. Beyond that, the remainder of the code just manages the

      You might wonder why the program stops the player before it
      starts another one. When you issue a Manager.createPlayer(url)
      call to create a new Player object, it is necessary to stop the
      prior one before starting the new one. If you don't stop the old
      player, you might hear an MP3 file playing while watching a video
      with its own sound track. If the video plays first, you need to
      remove the visual component for the video player, or else you see
      the "old" video clip, while listening to the new audio file.

      import javax.swing.*;
      import javax.media.*;
      import java.awt.*;
      import java.awt.event.*;
      import java.net.*;
      import java.io.*;

      public class PlayVideo extends JFrame {

      Player player;
      Component center;
      Component south;

      public PlayVideo() {
      JButton button = new JButton("Select File");
      ActionListener listener =
      new ActionListener() {
      public void actionPerformed(
      ActionEvent event) {
      JFileChooser chooser =
      new JFileChooser(".");
      int status =
      if (status ==
      JFileChooser.APPROVE_OPTION) {
      File file = chooser.getSelectedFile();
      try {
      } catch (Exception e) {
      System.err.println("Try again: " + e);

      public void load(final File file)
      throws Exception {
      URL url = file.toURL();
      final Container contentPane =
      if (player != null) {
      player = Manager.createPlayer(url);
      ControllerListener listener =
      new ControllerAdapter() {
      public void realizeComplete(
      RealizeCompleteEvent event) {
      Component vc =
      if (vc != null) {
      center = vc;
      } else {
      if (center != null) {
      Component cpc =
      if (cpc != null) {
      south = cpc;
      } else {
      if (south != null) {

      public static void main(String args[]) {
      PlayVideo pv = new PlayVideo();

      For more information about using JMF, see the Java Media
      Framework API Guide at

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      Applets are Java technology programs that run in the browser.
      Under most circumstances, an applet does its processing within
      the realm of the Java Runtime Environment (JRE). There are times
      however when it is necessary to jump out of the applet's secure
      runtime environment (the "sandbox") and communicate with the
      browser. The communication is done through the LiveConnect
      facility, and permits applets to work with JavaScript. The bulk
      of this communication is performed through the
      netscape.javascript.JSObject class.

      Originally provided as part of the Netscape browser environment,
      Microsoft eventually added this package to the runtime that
      ships with Internet Explorer. In addition, the package is now
      a standard part of the Java Runtime Environment installed with
      the Java Plug-in. That means that applets can use the package and
      expect to find the package when they execute. It isn't necessary
      to include the classes with a deliverable program.

      In order to develop applets that take advantage of this package,
      you need to add the necessary classes to your class path. The
      Java runtime environment already puts these classes in your
      runtime class path, by default. However the development
      environment doesn't. So you need to make available to your
      development environment the jaws.jar file (Windows) or
      javaplugin.jar file (Solaris) that ships with the Java Runtime.

      Assuming that you use the Java 2 Standard Edition (J2SE)
      version 1.3 Software Development Kit (SDK), you can place the
      jaws.jar file (Windows) in the jre\lib\ext directory or
      javaplugin.jar file (Solaris) in the jre/lib/ext directory under
      the installation directory for your SDK. Once copied, the
      appropriate classes (and others) will be available to your

      As previously mentioned, the key class for applet-to-JavaScript
      communications is the JSObject class. This class provides, among
      other things, the means to call JavaScript methods and evaluate
      raw JavaScript syntax. To demonstrate these capabilities, let's
      create an applet that prompts for raw HTML in its TextArea. Then
      let's open up a new browser window to display the HTML. Because
      the raw HTML is in the TextArea and not in a file, you can't just
      call the showDocument() method to open up a new URL.

      The first thing you need to do to communicate with JavaScript is
      get a JSObject instance. The class has no constructor. Instead,
      you call the getWindow method of the class, passing in
      a parameter of the applet instance:

      JSObject topWindow = JSObject.getWindow(this);

      Calling methods on the JSObject object is done using the call
      method. This method works by passing in the text name of the
      method to invoke, and an Object array of the arguments.

      public Object call(String name, Object args[])

      To pass in arguments, you create the array, and then fill it,
      based upon how many arguments are necessary:

      Object args[] = new args[x]
      args[0] = ...;
      args[1] = ...;
      args[x] = ...;

      For instance, to open a window you need to call the open method
      of the window object. Because getWindow returns the window
      associated with the applet, you already have the window object.
      The open method accepts three arguments:

      * The URL of what to open
      * The name of the window
      * Any attributes as a comma-separated list

      The attributes describe how the window will look. Some of the
      more standard ones are:

      * width (number)
      * height (number)
      * location (boolean)
      * menubar (boolean)
      * scrollbars (boolean)
      * status (boolean)
      * toolbar (boolean)

      The boolean values can be yes, no, 1, or 0.

      For instance, if you want a 300x300 window with no location,
      menubar, status bar, or toolbar, the attribute string would look
      like this:


      Using that attribute string, you can open an unnamed window (with
      no content) with a three element array where only the third
      element is set:

      Object args[] = {"","","width=..."};

      Then, pass those arguments and the method name on to the call
      method of the window received from the prior getWindow call.

      topWindow.call("open", args);

      To actually fill the window with the contents from the TextArea,
      you must call the write method on the named "document" member
      within the window.

      Getting the document member involves calling the getMember method
      and passing it the name of the member you want, in this case,

      JSObject document = (

      You then need to call the write method of document to change the
      content of the window. Here, you simply construct another
      argument array, this time with only one element for the text:

      String htmlText = ...;

      args = new Object[] {htmlText};

      Then, call the write method:

      document.call("write", args);

      That's essentially all there is to communicating with the
      JSObject object. Another thing you might want to do with JSObject
      is call the eval method. Given a string, the eval method treats
      the string as a JavaScript expression and evaluates it.

      Here is an applet that puts together all the pieces that were
      covered in this tip:

      import java.applet.*;
      import java.awt.*;
      import java.awt.event.*;
      import netscape.javascript.*;

      public class JSPopup extends Applet {
      public void init() {
      final TextArea ta = new TextArea();
      Button button = new Button("Launch");
      ActionListener listener =
      new ActionListener() {
      public void actionPerformed(ActionEvent e) {
      String htmlText = ta.getText();
      JSObject topWindow =
      Object args[] = new Object[3];
      args[2] = "width=300,height=300," +
      JSObject popupWindow =
      (JSObject)topWindow.call("open", args);
      JSObject document = (JSObject)
      args = new Object[] {htmlText};
      document.call("write", args);
      setLayout(new BorderLayout());
      add(ta, BorderLayout.CENTER);
      add(button, BorderLayout.SOUTH);

      When using applet-to-JavaScript communications, you must set the
      MAYSCRIPT parameter. This tells the security environment to allow
      the two systems to communicate.

      <applet code=JSPopup width=300 height=300>

      Compile the applet and then run it in your browser. Then enter
      some HMTL content in the text area:

      <H1>Hello, World</H1>

      Then click the Launch button in the applet. You should see a new
      window displayed. If you don't see a new window, try using a
      different browser (a number of bugs have been reported
      regarding Java-JavaScript communications with 1.3.1 Plugin and

      You could also use this technique to display the response from
      applet-to-servlet communications where the applet has to manually
      send a POST request to the servlet.

      Even without Java Plugin installed, you can still use the
      netscape.javascript.JSObject class with the native virtual
      machine in Internet Explorer 4.x - 6.x and Netscape Communicator
      4.7x. The JavaScript communications support in a native virtual
      machine differs from that provided with the Java Plug-in. The
      basics described in this tip are the same. However, the native
      virtual machines rely on an older version of the package. You can
      find documentation for the older package at

      . . . . . . . . . . . . . . . . . . . . . .

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      Copyright 2002 Sun Microsystems, Inc. All rights reserved.
      901 San Antonio Road, Palo Alto, California 94303 USA.

      This document is protected by copyright. For more information, see:


      JDC Tech Tips
      February 19, 2002

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