The old way we retrieved the classpath wasn't ever supported by Java, it was an implementation detail on the JVM that got removed in Java 9, even though Java 9 was "backwards compatible", it broke code that wasn't officially supported.

- Used local variable type inference
- Made use of final consistent
- JavaApp already had a constructor that takes just the sourcecode and manages getting the classpath
- Used Java 8+ lambda syntax over new Runnable() { ..run } syntax
- Made variables more understandable. They were lacking context before that took a little too long to figure out on the first read, i.e:
    - is input the filepath or the data that'll go in the file?
    - is output the filepath or the data that'll go in the file?
    - is source the filepath or the data that'll go in the file?
the answer was different for each of these but nothing about their names or type signified this, so I changed them. 
Also "ret" isn't a good name.