You can try the following:

 URLDisplayer.getDefault().showURL(URL url);

See: http://www.netbeans.org/project/www/download/dev/javadoc/org-openide-awt/org/openide/awt/HtmlBrowser.URLDisplayer.html

In NetBeans platform, this opens in internal Swing web browser.

In NetBeans IDE, this opens in system's default web browser.

The module responsible for opening systems's default web browser is org-netbeans-modules-extbrowser.jar If u install this module in netbeans platform, then you can open system's default browser from the platform also.