Trail: Deployment
Lesson: Java Applets
Section: Doing More With Applets

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Manipulating DOM of Applet's Web Page

Every web page is composed of a series of nested objects. These objects make up the Document Object Model (DOM). A Java applet can traverse and modify objects of its parent web page using the Common DOM API.

Consider an example of a Java applet that dumps the contents of its parent web page.

In order to traverse and manipulate the DOM tree, you must first obtain a reference to the Document object for the web page. You can do so by using the getDocument method in the com.sun.java.browser.plugin2.DOM class. Here is a code snippet that retrieves a reference to a Document object in the DOMDump applet's start method. See inline comments in the code.

public void start() {
    try {
        // use reflection to get document
        Class c =
          Class.forName("com.sun.java.browser.plugin2.DOM");
        Method m = c.getMethod("getDocument",
          new Class[] { java.applet.Applet.class });
        
        // cast object returned as HTMLDocument;
        // then traverse or modify DOM
        HTMLDocument doc = (HTMLDocument) m.invoke(null,
            new Object[] { this });
        HTMLBodyElement body =
            (HTMLBodyElement) doc.getBody();
        dump(body, INDENT);
    } catch (Exception e) {
        System.out.println("New Java Plug-In not available");
        // In this case, you could fallback to the old
        // bootstrapping mechanism available in the
        // com.sun.java.browser.plugin.dom package
    }
}

Now that you have a reference to the Document object, you can traverse and modify the DOM tree using the Common DOM API. The DOMDump applet traverses the DOM tree and writes its contents to the Java Console log.

private void dump(Node root, String prefix) {
    if (root instanceof Element) {
        System.out.println(prefix +
            ((Element) root).getTagName() + 
            " / " + root.getClass().getName());
    } else if (root instanceof CharacterData) {
        String data =
            ((CharacterData) root).getData().trim();
        if (!data.equals("")) {
            System.out.println(prefix +
                "CharacterData: " + data);
        }
    } else {
        System.out.println(prefix +
            root.getClass().getName());
    }
    NamedNodeMap attrs = root.getAttributes();
    if (attrs != null) {
        int len = attrs.getLength();
        for (int i = 0; i < len; i++) {
            Node attr = attrs.item(i);
            System.out.print(prefix + HALF_INDENT +
                "attribute " + i + ": " +
                attr.getNodeName());
            if (attr instanceof Attr) {
                System.out.print(" = " +
                    ((Attr) attr).getValue());
            }
            System.out.println();
        }
    }

    if (root.hasChildNodes()) {
        NodeList children = root.getChildNodes();
        if (children != null) {
            int len = children.getLength();
            for (int i = 0; i < len; i++) {
                dump(children.item(i), prefix +
                    INDENT);
            }
        }
    }
}

Open AppletPage.html in a browser to view the DOMDump applet running. Check the Java Console log for a dump of the DOM tree of the web page.

Note: If you don't see the applet running, you need to install at least the Java SE Development Kit (JDK) 6 update 10 release.

Note: If you don't see the example running, you might need to enable the JavaScript interpreter in your browser so that the Deployment Toolkit script can function properly.

Download the source code for the DOM Dump example to experiment further.

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