It's funny because copying "MySQL Connector / J" to Google got the source code as the first result.
But if you are looking for the source code for a free application, the most convenient way I found is to search for the related package in Ubuntu (or Debian) and download the related source code.
It works best if you use them, you can just use aptitude search / apt-get source combo to download it. But if you use a different operating system, you can just do a Google search, for example site:packages.ubuntu.com mysql connector/j
Rosh oxymoron
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