the JAR file contains Java executable code in compressed format, while the JAD file is a text file that provides information about the application. Not all WAP devices require JAD files, but most do, so it's as well to provide them.
Mobile devices that support Java applications (known as midlets) generally provide a number of ways for the user to get applications onto the device. For WAP devices, an interesting approach is to make the applications available for download using WAP protocols. In WAP jargon this is called over the air (OTA) installation. Devices that don't support OTA will use a cable, or bluetooth, or infra-red communication. These installation methods are not relevant to the present discussion, as they are proprietary and not covered by WAP standards. I will concentrate here on making Java applications available OTA.
WAP phones that support OTA installation normally provide the user with at least one of the following methods of download.
The user follows a link in the browser to a JAD or JAR file (JAD is more widely supported). The device downloads the JAD, locates the corresponding JAR, downloads the JAR, and installs it. Normally the user gets asked at some point if it is OK to install.
The user tells the phone to install an application, and provides a URL. The URL links to a WML page that contains a list of hyperlinks to JAD or JAR files. The device extracts the hyperlink text from the page, and presents a menu of application choices. When the user selects an application, the device downloads the corresponding JAD and JAR file, and installs the application.
