From Forum Nokia Wiki

Some phones won't be able to download jar files above some size from the internet unless a JAD file is used to get information about the jar file first.

To create a jad file from jar file:

1. Extract the jar file.
   If you don't know about a program to extract it, you can always rename it to zip and then extract it.
2. Locate the META-INF/MANIFEST.MF file and copy it to where the jar file is located.
3. Rename the copied MANIFEST.MF to the same name as the jar file but with .jad extension.
4. Open the copied file in text editor and edit the file as follows:
   1. Delete the lines, if they exist, that begin with:
      1. MIDlet-Jar-URL:
      2. MIDlet-Jar-Size:
   2. Add the following lines:
      1. MIDlet-Jar-URL: followed by a space character and the full jar filename.
      2. MIDlet-Jar-Size: followed by a space character and size (in bytes) of the jar file.
   3. Make sure the following lines exist, if they're not add each of them followed by space and its corresponding value:
      1. MIDlet-Name:
      2. MIDlet-Version:
      3. MIDlet-Vendor:
   4. Fix the following lines:
      1. It can cause problems if you have a non numeric text in the MIDlet-Version: line (. (dot) is permitted), if it's so you can just remove the non-numeric characters from the version value.
      2. Sometimes line's value will wrap to the next line. You should fix this, so that the whole value of that line will be on the same line. Happens a lot on MIDlet-1: (or 2,3.. instead of 1) lines.
      3. To place the Java application into the Games folder by default you can add or edit if it exists the following line:
         Nokia-MIDlet-Category: Game
         More info here: How to install an application on the Game phone's menu
   5. It is safe to delete all lines that DO NOT begin with the following:
      1. MIDlet-
      2. MicroEdition-
      3. Nokia-MIDlet-Category
   6. If the application's certificate is causing problems on installation, the lines that begin with the following can be removed:
      1. MIDlet-Certificate-
      2. MIDlet-Jar-RSA-

Links

Online JAD generator