JarStubGenerator is a project aimed at modifying jar files so the actual implementation stays hidden from the user and the jar can be published without needing to worry about leaking a not free-to-use jar.
In the space of Minecraft plugin development, some developers create plugins without APIs, and we need to use the jar to access their methods. This is not a safe thing to do as it can violate the license of the plugin.
Download the jar from the GitHub releases page, and run it with:
java -jar JarStubGenerator.jar <options>
-p/-path - the path to the folder or the file containing the jar file(s)
-o/-out - the path to the output folder, defaults to path
-s/-suffix - the suffix of the generated jar
-keep-fields - keep all the fields of classes
-f/-field-whitelist - a list of field names separated by ; e.g. yourField;yourField2