I've been thinking about multirelease jars. First step is source layout.
Here's a strawman modeled after the jar layout.
We add a versions directory to the existing src directory. Inside the
versions directory one can have directories named 9, 10, 11, etc. for each
major Java version that has custom code. Each numbered directory is a full
src folder that contains main, test, etc.
Can anyone poke holes in this? Is there a reason why this wouldn't suffice?