Javascript [Electronic resources] : The Definitive Guide (4th Edition) نسخه متنی

14.9 Applets

The applets[] array of
the
Document object
contains objects
that represent the applets embedded in the document with the

<applet> or
<object> tag. An applet is a portable,
secure
Java
program that is loaded over the Internet and executed by the web
browser; both Netscape and Internet Explorer support Java
(although IE 6 no longer includes Java support by default).

As
of Netscape 3 and Internet Explorer 3, both browsers allow JavaScript
to invoke public methods and read and write the public properties of
Java applets. (As we'll see in Chapter 22,
Netscape also supports much richer bidirectional interactions between
JavaScript and Java.) All applets have a few standard public methods
that they inherit from their superclasses, but the most interesting
methods and properties vary on a case-by-case basis. If you are the
author of the applet that you want to control from JavaScript, you
already know what public methods and properties it defines. If you
are not the author, you should consult the applet's
documentation to determine what you can do with it.

Here's how you might embed a Java applet in a web page with the
<applet> tag and then invoke the
start( ) and stop( ) methods of
that applet from JavaScript event handlers:

<applet name="animation" code="Animation.class" width="500" height="200">
</applet>
<form>
<input type="button" value="Start" onclick="document.animation.start(  );">
<input type="button" value="Stop" onclick="document.animation.stop(  );">
</form>

All applets define start( ) and stop(
) methods. In this hypothetical example, the methods cause
an animation to start and stop; by defining the HTML form,
we've given the user control over starting and stopping the
applet. Note that we've used the name
attribute of the <applet>
tag, so we can refer to the applet by name, rather than as a numbered
element of the applets[] array.

This example does not fully demonstrate the power of JavaScript to
script Java applets: the Java methods invoked from the JavaScript
event handlers are passed no arguments and return no values. In fact,
JavaScript can pass numbers, strings, and boolean values as arguments
to Java methods and can accept numbers, strings, and boolean return
values from those functions. (As we'll see in Chapter 22, Netscape can also pass and return JavaScript
and Java objects to and from Java methods.) The automatic conversion
of data between JavaScript and Java allows for rich interactions
between the two programming environments. For example, an applet
might implement a method that returns a string of JavaScript code.
JavaScript could then use the eval( ) method to
evaluate that code.

Applets can also implement methods that don't operate on the
applet itself, but instead simply serve as conduits between
JavaScript and the Java environment. For instance, an applet might
define a method that invokes the System.getProperty(
) method for a given string argument. This applet would
allow JavaScript to look up the value of Java system properties and
determine, for example, the version of Java that is supported by the
browser.