[Chapter 14] And Then There Were Applets
Author: John Zukowski
Date: Thu Jul 31 14:53:49 1997

#

| Chapter 14 |

# 14. And Then There Were Applets

Contents:
What's a Java Applet?
AudioClip Interface
AppletContext Interface
AppletStub Interface
Audio in Applications

Although it is not part of the `java.awt` package, the `java.applet` package is closely related. The `java.applet` package provides support for running an applet in the context of a World Wide Web browser. It consists of one class (`Applet`) and three interfaces (`AppletContext`, `AudioClip`, and `AppletStub`). The `Applet` class supports the "applet life cycle" methods (`init()`, `start()`, `stop()`, `destroy()`) that you override to write an applet. `AudioClip` provides support for audio within applets. (Applications use the `sun.audio` package for audio support; `sun.audio` is also covered in this chapter.) The `AppletStub` and `AppletContext` interfaces provide a way for the applet to interact with its run-time environment. Many of the methods of `AppletStub` and `AppletContext` are duplicated in the `Applet` class.

## 14.1 What's a Java Applet?

Much of the initial excitement about Java centered around applets. Applets are small Java programs that can be embedded within HTML pages and downloaded and executed by a web browser. Because executing code from random Internet sites presents a security risk, Java goes to great lengths to ensure the integrity of the program executing and to prevent it from performing any unauthorized tasks.

An applet is a specific type of Java `Container`. The class hierarchy of an applet is shown in Figure 14.1.

#### Figure 14.1: Applet class hierarchy

When you are writing an applet, remember that you can use the features of its ancestors. In particular, remember to check the methods of the `Component`, `Container`, and `Panel` classes, which are inherited by the `Applet` class.

### Applet Methods

All the methods of `Applet`, except `setStub()`, either need to be overridden or are methods based on one of the `java.applet` interfaces. The system calls `setStub()` to set up the context of the interfaces. The browser implements the `AppletContext` and `AppletStub` interfaces. Constructor

public Applet ()

  The system calls the `Applet` constructor when the applet is loaded and before it calls `setStub()`, which sets up the applet's stub and context. When you subclass `Applet`, you usually do not provide a constructor. If you do provide a constructor, you do not have access to the `AppletStub` or `AppletContext` and, therefore, may not call any of their methods.

AppletStub setup

public final void setStub (AppletStub stub)

  The `setStub()` method of `Applet` is called by the browser when the applet is loaded into the system. It sets the `AppletStub` of the applet to `stub`. In turn, the `AppletStub` contains the applet's `AppletContext`.

Applet information methods

Several methods of `Applet` provide information that can be used while the applet is running.

public AppletContext getAppletContext ()

  The `getAppletContext()` method returns the current `AppletContext`. This is part of the applet's stub, which is set by the system when `setStub()` is called.

public URL getCodeBase ()

  The `getCodeBase()` method returns the complete URL of the .class file that contains the applet. This method can be used with the `getImage()` or the `getAudioClip()` methods, described later in this chapter, to load an image or audio file relative to the .class file location.

public URL getDocumentBase ()

  The `getDocumentBase()` method returns the complete URL of the .html file that loaded the applet. This can be used with the `getImage()` or `getAudioClip()` methods, described later in this chapter, to load an image or audio file relative to the .html file.

public String getParameter (String name)

  The `getParameter()` method allows you to get run-time parameters from within the `<APPLET>` tag of the .html file that loaded the applet. Parameters are defined by HTML `<PARAM>` tags, which have the form:

      <PARAM name="parameter" value="value>

  If the `name` parameter of `getParameter()` matches the `name` string of a `<PARAM>` tag, `getParameter()` returns the tag's `value` as a string. If `name` is not found within the `<PARAM>` tags of the `<APPLET>`, `getParameter()` returns `null`. The argument `name` is not case sensitive; that is, it matches parameter names regardless of case. Remember that `getParameter()` always returns a string, even though the parameter values might appear as integers or floating point numbers in the HTML file. In some situations, it makes sense to pass multiple values in a single parameter; if you do this, you have to parse the parameter string manually. Using a `StringTokenizer` will make the job easier.

  Enabling your applets to accept parameters allows them to be customized at run-time by the HTML author, without providing the source code. This provides greater flexibility on the Web without requiring any recoding. Example 14.1 shows how an applet reads parameters from an HTML file. It contains three parts: the HTML file that loads the applet, the applet source code, and the output from the applet.

  #### Example 14.1: Getting Parameters from an HTML File

      <APPLET CODE=ParamApplet WIDTH=100 HEIGHT=100>
      <PARAM NAME=one VALUE=1.0>
      <PARAM name=TWO value=TOO>
      </APPLET>
      public class ParamApplet extends java.applet.Applet {
          public void init () {
              String param;
              float one;
              String two;
              if ((param = getParameter ("ONE")) == null) {
                  one = -1.0f;  // Not present
              } else {
                  one = Float.valueOf (param).longValue();
              }
              if ((param = getParameter ("two")) == null) {
                  two = "two";
              } else {
                  two = param.toUpperCase();
              }
              System.out.println ("One: " + one);
              System.out.println ("Two: " + two);
          }
      }
      One: 1
      Two: TOO

public String getAppletInfo ()

  The `getAppletInfo()` method lets an applet provide a short descriptive string to the browser. This method is frequently overridden to return a string showing the applet's author and copyright information. How (or whether) to display this information is up to the browser. With appletviewer, this information is displayed when the user selects the Info choice under the Applet menu. Neither Netscape Navigator nor Internet Explorer currently display this information.

public String[][] getParameterInfo ()

  The `getParameterInfo()` method lets an applet provide a two-dimensional array of strings describing the parameters it reads from `<PARAM>` tags. It returns an array of three strings for each parameter. In each array, the first `String` represents the parameter name, the second describes the data type, and the third is a brief description or range of values. Like `getAppletInfo()`, how (or whether) to display this information is up to the browser. With appletviewer, this information is displayed when the user selects the Info choice under the Applet menu. Neither Netscape Navigator nor Internet Explorer currently display this information. The following code shows how an applet might use `getParameterInfo()` and `getAppletInfo()`:

      public String getAppletInfo() {
          String whoami = "By John Zukowski (c) 1997";
          return whoami;
      }
      public String[][] getParameterInfo() {
          String[][] strings = {
              {"parameter1",     "String",      "Background Color name"},
              {"parameter2",     "URL",         "Image File"},
              {"parameter3",     "1-10",        "Number in Series"}
          };
          return strings;
      }

public void showStatus (String message)

  The `showStatus()` method displays `message` on the browser's status line, if it has one. Again, how to display this string is up to the browser, and the browser can overwrite it whenever it wants. You should only use `showStatus()` for messages that the user can afford to miss.

public boolean isActive ()

  The `isActive()` method returns the current state of the applet. While an applet is initializing, it is not active, and calls to `isActive()` return `false`. The system marks the applet active just prior to calling `start()`; after this point, calls to `isActive()` return `true`.

public Locale getLocale ()

  The `getLocale()` method retrieves the current `Locale` of the applet, if it has one. Using a `Locale` allows you to write programs that can adapt themselves to different languages and different regional variants. If no `Locale` has been set, `getLocale()` returns the default `Locale`. The default `Locale` has a user language of English and no region. To change the default `Locale`, set the system properties `user.language` and `user.region`, or call `Locale.setDefault()` (`setDefault()` verifies access rights with the security manager).[1]

  > [1] For more on the `Locale` class, see Java Fundamental Classes Reference, by Mark Grand, from O'Reilly & Associates.

Applet life cycle

The browser calls four methods of the `Applet` class to execute the applet. These methods constitute the applet's life cycle. The default versions don't do anything; you must override at least one of them to create a useful applet.

public void init ()

  The `init()` method is called once when the applet is first loaded. It should be used for tasks that need to be done only once. `init()` is often used to load images or sound files, set up the screen, get parameters out of the HTML file, and create objects the applet will need later. You should not do anything that might "hang" or wait indefinitely. In a sense, `init()` does things that might otherwise be done in an applet's constructor.

public void start ()

  The `start()` method is called every time the browser displays the web page containing the applet. `start()` usually does the "work" of the applet. It often starts threads, plays sound files, or does computation. `start()` may also be called when the browser is de-iconified.

public void stop ()

  The `stop()` method is called whenever the browser leaves the web page containing the applet. It should stop or suspend anything that the applet is doing. For example, it should suspend any threads that have been created and stop playing any sound files.`stop()` may also be called when the browser is iconified.

public void destroy ()

  The `destroy()` method is called when the browser determines that it no longer needs to keep the applet around--in practice, when the browser decides to remove the applet from its cache or the browser exits. After this point, if the browser needs to display the applet again, it will reload the applet and call the applet's `init()` method. `destroy()` gives the applet a final opportunity to release any resources it is using (for example, close any open sockets). Most applets don't need to implement `destroy()`. It is always a good idea to release resources as soon as they aren't needed, rather than waiting for `destroy()`. There are no guarantees about when `destroy()` will be called; if your browser has a sufficiently large cache, the applet may stay around for a very long time.

Applet-sizing methods

public void resize(int width, int height)

  The `resize()` method changes the size of the applet space to `width` x `height`. The browser must support changing the applet space or else the sizing does not change. Netscape Navigator does not allow an applet to change its size; the applet is sized to the region allocated by the `<APPLET>` tag, period.

  Because `Applet` is a subclass of `Component`, it inherits the Java 1.1 method `setSize()`, which has the same function.

public void resize (Dimension dim)

  This `resize()` method calls the previous version of `resize()` with a width of `dim.width` and a height of `dim.height`.

Images

We have discussed `Image` objects extensively in Chapter 2, Simple Graphics, and Chapter 12, Image Processing, and used them in many of our examples. When writing an applet, you can use the `getImage()` method directly. In applications, you must go through `Toolkit` (which the following methods call) to get images.

public Image getImage (URL url)

  The `getImage()` method loads the image file located at `url`. `url` must be a complete and valid URL. The method returns a system-specific object that subclasses `Image` and returns immediately. The `Image` is not loaded until needed, either by `prepareImage()`, `MediaTracker`, or `drawImage()`.

public Image getImage (URL url, String filename)

  The `getImage()` method loads the image file located at `url` in `filename`. The applet locates the file relative to the specified URL; that is, if the URL ends with a filename, the applet removes the filename and appends the `filename` argument to produce a new URL. `getImage()` returns a system-specific object that subclasses `Image` and returns immediately. The `Image` is not loaded until needed, either by `prepareImage()`, `MediaTracker`, or `drawImage()`.

  In most cases, the `url` argument is a call to `getDocumentBase()` or `getCodeBase()`; most often, image files are located in the same directory as the HTML file, the applet's Java class file, or their own subdirectory.

Audio

Every Java platform is guaranteed to understand Sun's AU file format, which contains a single channel of 8000 Hz µLaw encoded audio data.[2] Java applets do not require any helper applications to play audio; they use the browser's audio capabilities. You can use an independent application, like Sun's audiotool, to control the volume. Of course, the user's workstation or PC needs audio hardware, but these days, it's hard to buy a computer that isn't equipped for audio.

> [2] The AU format is explained in the Audio File Format FAQ (version 3.10) located at ftp://ftp.cwi.nl/pub/audio/index.html in files AudioFormats.part1 and AudioFormats.part2.

The Java Media Framework API is rumored to provide support for additional audio formats, like Microsoft's .wav files or Macintosh/SGI .aiff audio files. At present, if you want your Java program to play audio files in other formats, you must first convert the audio file to the .au format, using a utility like SOX (Sound Exchange).[3] Once converted, your Java program can play the resulting .au file normally. (If you are interested in more information about audio, look in the alt.binaries.sounds.d newsgroup.)

> [3] SOX is available at http://www.spies.com/Sox. The current version of SOX is 10; version 11 is in gamma release. The UNIX source is located in sox10.tar.gz , while the DOS executable is sox10dos.zip.

The `Applet` class provides two ways to play audio clips. The first mechanism provides a method to load and play an audio file once:

public void play (URL url)

  The `play()` method downloads and plays the audio file located at `url`. `url` must be a complete and valid URL. If `url` is invalid, no sound is played. Some environments throw an exception if the URL is invalid, but not all. Calling `play()` within an applet's `destroy()` method usually has no effect; the applet and its resources will probably be deallocated before `play()` has time to download the audio file.

public void play (URL url, String filename)

  This version of `play()` downloads and plays the audio file located at `url` in the file `filename`. The applet locates the file relative to the specified URL; that is, if the URL ends with a filename, the applet removes the filename and appends the `filename` argument to produce a new URL. If the resulting URL is invalid, no sound is played. Some environments throw an exception if the URL is invalid, but not all.

  In most cases, the `url` argument is a call to `getDocumentBase()` or `getCodeBase()`; most often, sound files are located in the same directory as the HTML file or the applet's Java class file. For some reason, you cannot have a double dot (`..`) in the URL of an audio file; you can in the URL of an image file. Putting a double dot in the URL of an audio file raises a security exception in an applet causing `play()` to fail.

  The following applet plays an audio file located relative to the HTML file from which the applet was loaded:

      import java.net.*;
      import java.applet.*;
      public class audioTest extends Applet {
          public void init () {
              System.out.println ("Before");
              play (getDocumentBase(), "audio/flintstones.au");
              System.out.println ("After");
          }
      }

The second way to play audio files splits the process into two steps: you get an `AudioClip` object and then play it as necessary. This procedure eliminates a significant drawback to `play()`: if you call `play()` repeatedly, it reloads the audio file each time, making the applet much slower.

public AudioClip getAudioClip (URL url)

  The `getAudioClip()` method loads the audio file located at `url`. `url` must be a complete and valid URL. Upon success, `getAudioClip()` returns an instance of a class that implements the `AudioClip` interface. You can then call methods in the `AudioClip` interface (see AudioClip Interface) to play the clip. If an error occurs during loading (e.g., because the file was not found or the URL was invalid), `getAudioClip()` returns `null`.

  `getAudioClip()` sounds similar to `getImage()`, and it is. However, Java currently loads audio clips synchronously; it does not start a separate thread as it does for images. You may want to create a helper class that loads audio clips in a separate thread.

  The actual class of the `AudioClip` object depends on the platform you are using; you shouldn't need to know it. If you are curious, the appletviewer uses the class `sun.applet.AppletAudioClip`; Netscape Navigator uses the class `netscape.applet.AppletAudioClip`.

public AudioClip getAudioClip (URL url , String filename)

  This version of the `getAudioClip()` method loads the audio file located at `url` in the file `filename`. The applet locates the file relative to the specified URL; that is, if the URL ends with a filename, the applet removes the filename and appends the `filename` argument to produce a new URL. If the resulting URL is invalid, the file is not loaded. Upon success, `getAudioClip()` returns an instance of a class that implements the `AudioClip` interface. You can then call methods in the `AudioClip` interface (see AudioClip Interface) to play the clip. If an error occurs during loading (e.g., because the file was not found or the URL was invalid), `getAudioClip()` returns `null`.

  In most cases, the `url` argument is a call to `getDocumentBase()` or `getCodeBase()`; most often, sound files are located in the same directory as the HTML file or the applet's Java class file.

AWTError |  | AudioClip Interface