The uploaded JAR file is analyzed to verify if it is an actual Java game/application you can use on your mobile. Based on this JAR file the corresponding JAD file is generated.
The JAD file is only needed if you use the Over-The-Air (OTA) method to download the Java game/application onto your Web/WAP enabled phone.
There are two ways to install a Java application on your mobile phone:
The direct method.
Download the Java application from the web onto your PC, then upload onto your phone using an USB cable, InfraRed (IR) or Bluetooth connection.
This method only requires the JAR file. The JAD file is NOT needed.
The Over-The-Air (OTA) method.
The Java application is downloaded onto your mobile by using the WAP or Web browser built into your phone.
This method requires 2 files:
The .JAR (= Java ARchive) file which contains the actual Java game/application.
The size of this file can be several KBytes or even MBytes.
The .JAD (= Java Application Description) file which is a small text file, containing a few lines of information about the Java game/application. Information such as the JAR file size and the URL where to find the JAR file. The size of this file is usually less then 1KByte.
To install the Java application on your phone:
Open the built-in WAP or Web browser and first download the JAD file.
Based on the information in the JAD file your phone applies several checks, one of which is to verify if it has enough storage space to install the JAR file.
When all checks are performed the JAR file is downloaded onto your phone using the "MIDlet-Jar-URL" attribute specified in the JAD file.