JavaScript Support in Java 6 Update 10

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The minor release with major features, Java 6 Update 10 brought a host of improvements to the long-neglected Java applet. One of these improvements was better interoperability with JavaScript, which was delivered to the new Java plug-in in the form of LiveConnect technology. As Sun's entrant into the RIA race, JavaFX, comes closer to a 1.0 release, such interoperability is crucial to the success of this effort.

While it has always been possible (to a certain extent) to call between an applet and JavaScript running the same browser, LiveConnect vastly improves the technology by

  • Improving reliability. A number of bugs involving the stability of the browser when making JavaScript<-->Java calls have been fixed.
  • Improving performance. Dramatically better performance...always a good thing.
  • Making it more consistent across different browsers. Support for JavaScript interoperability was always a little better on Mozilla browsers, but now the functionality is more consistent across different browsers with the new plug-in.
  • Increasing the functionality. It's now possible to instantiate new Java objects from JavaScript, call methods on them, as well as reference third-party packages on all browsers.

Calling Java from JavaScript

The following code shows how to invoke methods on an applet through JavaScript:

Java:

package org.example;

public class MyApplet extends Applet {
     public String getMessage() {
          return "Hello World!";
     }
}

HTML/JavaScript:

<applet id="myApp" archive="helloworld.jar" code="org.example.MyApplet"/>

<script type="text/javascript">
     var msg = myApp.getMessage();
     alert(msg);
</script>

Public fields in the Java class can also be called from JavaScript, as well as static fields and methods. Due to a new synthetic keyword, Packages, that is now attached to every applet, it is now possible to instantiate/call methods on classes that are not applets. For example, if we add a new class in the org.example package,

package org.example;

public class MyNonApplet {
     public String getOtherMessage() {
          return "This is NOT a hello world!";
     }
}

we can now create an instance of this class from the JavaScript and do things with it. Using the example above, let's modify the script a little:

<script type="text/javascript">
     var other = new myApp.Packages.org.example.MyNonApplet();
     alert(other.getOtherMessage());
</script>

This opens up the intriguing possibility of accessing a rich set of Java APIs from client-side JavaScript. Take, for example, the ability to add SVG support in (ahem) the only major browser that does not support it*. With Apache Batik and the Java plug-in, full support of SVG in the browser--SVG JavaScript API and all--becomes a reality. (Batik 1.7, by the way, is one of the most compliant implementations of SVG. And SVG might just be a faster way into the web designer's heart than JavaFX, but I digress...)

Each time a call into an applet is made from JavaScript, it is handled by a worker thread that is associated with the applet (there is only one per applet).

Calling JavaScript from Java

All instances of JavaScript objects within Java code appear as instances of netscape.javascript.JSObject. There are essentially two ways of invoking JavaScript from Java:

  1. Calling the static JSObject.getWindow(Applet arg) method with the applet object passed in as an argument. This method returns a JSObject that represents the DOM Window. This JSObject can in turn be used to call() JavaScript functions or eval() JavaScript code. The API for JSObject is here.
  2. Add a method in your Java code that takes JSObject as a formal parameter. Communication can then be bootstrapped from the JavaScript side by calling this Java method and passing a JavaScript object as an argument. It's a little less convenient than option 1, but useful if some JavaScript code wants to delegate some work to the Java code.

Furthermore, you can actually modify the HTML DOM from the Java side using a common DOM API. For the purposes of compiling Java code written with references to JSObject, you would place the jre/lib/plugin.jar on your classpath. Note also that, since JavaScript is a single-threaded language, communication from Java to JavaScript is restricted to one thread at a time; so you should structure your code accordingly.

Other JVM Languages

Of course, the other interesting possibility is the idea of languages like Scala, JRuby, Groovy, Clojure, or Fan being able to run in the browser with support for JavaScript interaction. A truly sick mind (and you know who you are) might use the applet merely as a trojan horse in order to get his/her favorite language running in the browser. I'll leave it at that!

Sun's renewed interest in the applet--sparked in part by RIA and in no small way by the resurgence of alternative browsers--has opened a door many thought closed a long time ago. Interesting times, indeed.

* Even worse now that the Adobe SVG plug-in has been discontinued.

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