Tip 80: Resize applets within browser frames

Break the rules with a little bit of JavaScript

When an applet is embedded within a Web browser, the browser becomes the applet's frame. However, when the user resizes the browser frame, the applet does not adjust its size to fit within the new dimensions. This is because applets have hard-coded dimensions in their <APPLET> tags. Therefore, despite the fact that your applet may be using layout managers, it won't adjust its components accordingly. A little JavaScript easily remedies this problem.

A complete example

This approach uses JavaScript to handle the browser's

onResize

and

onLoad

events and forward new dimensions to the applet. The applet overrides the

setSize()

method to revalidate its component layout, and thus let its layout manager adjust itself to the new size. The HTML looks like this:

   <HTML>
   <HEAD>
   <TITLE>Resizable Applet Demo</TITLE>
   </HEAD>
   <BODY bgcolor=#C6C3C6 onResize="resize()" onLoad="resize()" 
   topmargin="0" leftmargin="0" marginwidth="0" marginheight="0">
   <SCRIPT LANGUAGE="JavaScript">
   function resize() {
    var w_newWidth,w_newHeight;
    var w_maxWidth=1600, w_maxHeight=1200;
    if (navigator.appName.indexOf("Microsoft") != -1)
    {
        w_newWidth=document.body.clientWidth;
        w_newHeight=document.body.clientHeight;
    }else{
        var netscapeScrollWidth=15;
        w_newWidth=window.innerWidth-netscapeScrollWidth;
        w_newHeight=window.innerHeight-netscapeScrollWidth;
    }
    if (w_newWidth>w_maxWidth)
        w_newWidth=w_maxWidth;
    if (w_newHeight>w_maxHeight)
        w_newHeight=w_maxHeight;
    document.myApplet.setSize(w_newWidth,w_newHeight);
    window.scroll(0,0);
        }
   window.onResize = resize;
   window.onLoad = resize;
   </SCRIPT>
   <APPLET NAME="myApplet" CODE="SizeApplet.class" WIDTH=1600 
           HEIGHT=1200>/APPLET>
   </BODY>
   </HTML>

Internet Explorer and Netscape Navigator implement JavaScript in slightly different ways. The onResize and onLoad parameters in the <BODY> tag specify the resize event handlers for Internet Explorer, while window.onResize = resize; and window.onLoad = resize; do so for Netscape Navigator. Both methods must be included in order to support the two major browsers. Whenever the browser frame is loaded or resized, the applet's resize() method is invoked.

Navigator and IE have different methods of accessing the browser window's dimensions. In Navigator, the window object is referenced; in IE, the document's body object is used. These objects also return slightly different values for the window dimensions. Thus, it is necessary to determine the browser in which the applet is running before sending the correct window dimensions. Netscape does not include the width of the scrollbars, and does not let you access this length, so 15 pixels is used as an offset in the netscapeScrollWidth variable.

The <APPLET> tag still specifies the applet's Width and Height parameters, but these dimensions now specify the applet's maximum bounds. The window's maximum width and height, w_maxWidth and w_maxHeight, are checked in the resize() method to ensure that the applet does not go beyond these bounds. The browser still takes up room for this dimension; thus, the scrollbars will not show all the information in the window. Signed JavaScript in Netscape has the ability to hide the scrollbars. The HTML page is also scrolled to the top left corner whenever it is resized to ensure that the applet is fully visible.

The Java code to support the dynamic resizing is trivial. The applet's setSize() method must be overridden to call the validate() method.

public void setSize(int width, int height)
{
   super.setSize(width,height);
   validate();
}

The example applet has an increment button and a label to illustrate the fact that the applet does not reinitialize when a resize event is processed. The full source of the applet is available in the Resources section.

Known limitations

Currently, JavaScript-to-Java communication is available in Windows versions of Netscape Navigator 4.0 and up and Internet Explorer 4.0 and up, and in Solaris and Linux versions of Navigator 4.0 and up. Support of this feature is being adopted in more Unix browsers, as the LiveConnect-SDK for Unix is available. It is not supported on the Macintosh platform.

Alternative approaches

Another approach to resizing the applet is to create the page dynamically each time the window is resized using the

document.write()

method. When the user resizes the window, the HTML page is reloaded, and the document is rewritten with a new

<APPLET>

tag that specifies the corresponding width and height boundaries. This approach is a little clumsy, because browsers can react prematurely when asked to reload the current document while resizing. They sometimes attempt to reload before the user is done resizing, thus providing an inaccurate size to the

<APPLET>

tag after resizing is complete.

To remove the gray/white space represented by the page's unused applet region, you could create a base applet that contains the real applet. This base applet's sole reasons for existence are, first, to react to the setSize() method and delegate the event to the second applet, which it contains; and second, to paint a background color or image that blends well with the HTML background. The base applet could also become reusable if the background color is passed as a <PARAM> from the HTML. This background color would be the same color used as the <BODY>'s background color.

A perfect match

JavaScript is usually thought of as an HTML designer's tool, rather than part of a Java developer's repertoire. However, when JavaScript and Java are combined, the two complement each other very well. This approach uses JavaScript-to-Java communication to solve the problem of forwarding the resize events to an embedded applet. As shown by this article's example, the combination of JavaScript and Java has a great deal of potential. And hopefully, as Java developers, we will learn more ways to utilize it.

Francis Lu is a senior consultant with Ernst and Young LLP. He has been developing and architecting Java applications for two years, and was recently on a team that developed KJava spotlets for the Palm KJava VM at JavaOne.

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