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Trail: Writing Applets
Lesson: Taking Advantage of the Applet API

Using the <APPLET> Tag

This section tells you most of what you need to know to use the <APPLET> tag. It starts by showing you the tag's simplest form. It then discusses some of the most common additions to that simple form: the <PARAM> tag, alternate HTML code and text, the CODEBASE attribute, and the ARCHIVE attribute. For a detailed description of the <APPLET> tag, refer to The <APPLET> Tag.

You should already have seen the simplest form of the <APPLET> tag: 

<APPLET CODE=AppletSubclass.class WIDTH=anInt HEIGHT=anInt>
</APPLET>
This tag tells the browser to load the applet whose Applet subclass is named AppletSubclass, displaying it in an area of the specified width and height.

Specifying Parameters

Some applets let the user customize the applet's configuration with parameters, as described in Defining and Using Applet Parameters. For example, AppletButton (an applet used throughout this tutorial to provide a button that brings up a window) allows the user to set the button's text by specifying the value of a parameter named BUTTONTEXT.

The user specifies the value of a parameter using a <PARAM> tag. The <PARAM> tags should appear just after the <APPLET> tag for the applet they affect: 

<APPLET CODE=AppletSubclass.class WIDTH=anInt HEIGHT=anInt>
<PARAM NAME=parameter1Name VALUE=aValue>
<PARAM NAME=parameter2Name VALUE=anotherValue>
</APPLET>

Here's an example of the <PARAM> tag in use. 

<APPLET CODE="Animator.class" WIDTH=460 HEIGHT=160>
<PARAM NAME="imageSource" VALUE="images/Beans">
<PARAM NAME="backgroundColor" VALUE="0xc0c0c0">
<PARAM NAME="endImage" VALUE=10>
<PARAM NAME="soundSource" VALUE="audio">
<PARAM NAME="soundtrack" VALUE="spacemusic.au">
<PARAM NAME="sounds"
    VALUE="1.au|2.au|3.au|4.au|5.au|6.au|7.au|8au|9.au|0.au">
<PARAM NAME="pause" VALUE=200>
. . .
</APPLET>

Specifying Alternate HTML Code and Text

Note the ellipsis points (". . .") in the previous HTML example. What did the example leave out? It omitted alternate HTML code -- HTML code interpreted only by browsers that don't understand the <APPLET> tag. Alternate HTML code is any text that appears between the <APPLET> and </APPLET> tags, after any <PARAM> tags. Java-enabled browsers ignore alternate HTML code.

To specify alternate text to Java-enabled browsers and other browsers that understand the <APPLET> tag, use the ALT attribute. If the browser can't display an applet for some reason, it can display the applet's ALT text.

We use alternate HTML code throughout the online version of this tutorial to tell readers about the applets they're missing. Often, the alternate HTML code includes one or more pictures of the applet. Here's the complete HTML code for the Animator example shown previously: 

<APPLET CODE="Animator.class" WIDTH=460 HEIGHT=160
 ALT="If you could run this applet, you'd see some animation">
<PARAM NAME="imageSource" VALUE="images/Beans">
<PARAM NAME="backgroundColor" VALUE="0xc0c0c0">
<PARAM NAME="endImage" VALUE=10>
<PARAM NAME="soundSource" VALUE="audio">
<PARAM NAME="soundtrack" VALUE="spacemusic.au">
<PARAM NAME="sounds"
       VALUE="1.au|2.au|3.au|4.au|5.au|6.au|7.au|8au|9.au|0.au">
<PARAM NAME="pause" VALUE=200>
Your browser is completely ignoring the &lt;APPLET&gt; tag!
</APPLET>
An applet that doesn't understand the <APPLET> tag ignores everything in the previous HTML code except the line that starts with "Your". A browser that does understand the <APPLET> tag ignores everything on that line. If the applet-savvy browser can't run the applet, it might display the ALT text.

Specifying the Applet Directory

By default, a browser looks for an applet's class and archive files in the same directory as the HTML file that has the <APPLET> tag. (If the applet's class is in a package, then the browser uses the package name to construct a directory path underneath the HTML file's directory.) Sometimes, however, it's useful to put the applet's files somewhere else. You can use the CODEBASE attribute to tell the browser in which directory the applet's files are located: 
<APPLET CODE=AppletSubclass.class CODEBASE=aURL
        WIDTH=anInt HEIGHT=anInt>
</APPLET>
If aURL is a relative URL, then it's interpreted relative to the HTML document's location. By making aURL an absolute URL, you can load an applet from just about anywhere -- even from another HTTP server.

This tutorial uses CODEBASE="someDirectory/" frequently, since we group the examples for each lesson in subdirectories. For example, here's the <APPLET> tag that includes the Simple applet in The Life Cycle of an Applet(in the Writing Applets trail): 

<APPLET CODE=Simple.class CODEBASE="example/"
        WIDTH=500 HEIGHT=20>
</APPLET>
The following figure shows the location of the class file, relative to the HTML file, when CODEBASE is set to "example/".

[PENDING: A figure that shows this: 

      CODEBASE = "example/"

        someDirectory
              |
    +---------+---------+
    |                   |
HTML File            example
                        |
                   Class file

The next figure shows where the applet class can be if you specify an absolute URL for the value of CODEBASE.

[PENDING: A figure that shows this: 

CODEBASE = "http://someServer/.../someOtherDirectory/"

someDirectory      someOtherDirectory
      |         (might be on another server)
      |                   |
  HTML File          Class file

Combining an Applet's Files into a Single File

If your applet has more than one file, you should consider providing an archive file that bundles the applet's files into a single file. Whether archive files make sense for your applet depends on several factors, including your applet's size, performance considerations, and the environment you expect your users to have.

Archive files reduce your applet's total download time. Much of the time saved comes from reducing the number of HTTP connections that the browser must make. Each HTTP connection can take several seconds to start. This means that for a multifile applet, connection time can dwarf transfer time. You can further reduce transfer time by compressing the files in your archive file.

If you specify one or more archive files, then the browser looks for the archive files in the same directory that it would search for the applet class file. The browser then looks for the applet's class files in the archive files. If a file isn't in the archive, then the browser generally tries to load it just as it would if the archive file weren't present.

The standard Java archive format, called JAR, was introduced in JDK 1.1 and is based on the ZIP file format. You specify JAR files using the ARCHIVE attribute of the <APPLET> tag. You can specify multiple archive files by separating them with commas: 

<APPLET CODE="AppletSubclass.class" ARCHIVE="file1, file2"
	WIDTH=anInt HEIGHT=anInt>
</APPLET>
Unfortunately, not all browsers understand the same archive format or use the same HTML code to specify the applet archive. Watch this page for the latest information about browser support for archives. To learn how to create a JAR file, see Platform-specific Details: Creating a JAR File.

Other <APPLET> Tag Attributes

This section didn't discuss every attribute of the <APPLET> tag. Other attributes -- which might seem familiar, since the <IMG> HTML tag uses them -- include ALIGN, VSPACE, and HSPACE. The <APPLET> tag also allows you to load a serialized (saved) applet by specifying the OBJECT attribute instead of specifying a class file with CODE. Finally, you can name an applet using the NAME attribute. For a detailed description of the <APPLET> tag, see The <APPLET> Tag.

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