Accessing JavaScript from Java
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These examples illustrate accessing JavaScript from Java applets using Netscape's "LiveConnect" JavaScript extensions. They only work in Netscape 3 and 4 and the Windows 95/NT versions of Internet Explorer 4. The process is discussed in detail in Chapter 19 of Core Web Programming, but the main examples from that chapter are summarized below. For information on the reverse process (accessing Java from JavaScript), see the Java from JavaScript thumbnail guide.

Building HTML dynamically using the javascript: URL

In Netscape 3 (and only Netscape 3), calling showDocument on a URL of the form "javascript:'[entire HTML document]'" will cause the browser to display that page. The idea is sketched below: 
String page = "'<HEAD><TITLE>...</TITLE></HEAD><BODY>...</BODY>'";
try {
  URL pageURL = new URL("javascript:" + page);
  getAppletContext().showDocument(pageURL);
} catch(MalformedURLException mue) {
  ...
}
To try an on-line example of this approach in action, see TestHTML.html, which uses TestHTML.java, an applet that gathers input via a LabeledTextField and then uses ShowHTML.java to display the results. The TestHTML.html file also uses TestHTMLTop.html and TestHTMLBottom.html as the contents of the top and bottom frame cells.

Using JSObject

The netscape.javascript.JSObject class allows applets on Netscape 3 and 4 to reliably access JavaScript objects, properties, and functions. Using it involves the following steps:
  1. Obtain and install the JSObject class. Get it by unzipping the java_30 or java_301 zip file or uncompressing the java40 JAR file that comes with the Netscape installation. Alternatively, just point your CLASSPATH at the archive file.
  2. Import it in your applet. Use:
    import netscape.javascript.JSObject;
  3. From the applet, obtain a JavaScript reference to the current window. For this, do something like:
    JSObject window =
      JSObject.getWindow(this); // this=applet
  4. Read the JavaScript properties of interest. Use getMember to access them. E.g.
    JSObject document =
      (JSObject)window.getMember("document");
    String cookies =
      (String)document.cookie;
    JSObject someForm =
      (JSObject)document.getMember("someFormName");
    JSObject someElement =
      (JSObject)someForm.getMember("someElementName");
    You can also use getSlot with an index to access elements of an array.
  5. Set the JavaScript properties of interest. Use the setMember method for this. E.g.
    document.setMember("bgColor", "red");
    someElement.setMember("value", "textfield value");
    Note that the second argument to setMember must be a Java Object, so primitive objects must be converted to their corresponding wrapper type before being passed.
  6. Call the JavaScript methods of interest. JavaScript methods are called using either the call method or by constructing a JavaScript expression containing the method calls and passing it to eval. The call method takes the method name and an array of arguments; eval takes a single string. For instance,
    String[] message = { "An alert message" };
    window.call("alert", message);
    window.eval("alert('An alert message')");
  7. Give the applet permission to access its Web page. This is done via the MAYSCRIPT attribute of the APPLET element. E.g.
    <APPLET CODE=... WIDTH=... HEIGHT=... MAYSCRIPT>
    ...
    </APPLET>
For a very simple example, see MatchColor.java, and applet that looks up the background color of the enclosing Web page (MatchColor.html in this case) via the JavaScript document.bgColor property.

For a more complex example, see Everest.html. Here, mouse movement and slider operations in the applet (Everest.java) result in changes in the CGI form entries in the enclosing Web page. The applet makes use of the LabeledCostSlider, CostSlider, and Slider classes.

More Info

Core Web Programming For more Java and JavaScript examples, see my Java Programming Resources page or the source code archive for Core Web Programming. For more info on Core Web Programming, see the reader reviews, the table of contents, and a sample chapter.