It is automatically generated when you start the game in 1.5.2 or below, but for 1.6 and up, tlthe .jar is stored in the version itself.
Private Mod Note
Rollback Post to RevisionRollBack
DO NOT OPEN THAT SPOILER!!!
If you are reading this then you you totally ignored what I said about not opening that spoiler three spoilers ago.. Don't you remember? I said "DO NOT OPEN THAT SPOILER!!! But no, you were a rebel and did it anyway!