This wrapper (along with the Java 8-specific counterpart in src/main/java8) allows us to implement cleanup logic for objects that are garbage-collectable/no longer reachable.

Usage:

1. Create a subclass of CleanableState and attach it as a private field to the object you want to be cleaned up. You may call that field cleanableState.
2. Define all resources that need to be cleaned up in this subclass (instead of the observed object itself).
3. Make sure to not refer the observed object instance itself, as that will create a reference cycle and prevent proper cleanup.
4. Implement the doClean() method to perform all the necessary cleanup steps.
5. If the observed class implements close(), it's a good practice to have it just call cleanableState.runCleaner().

Implementation details:

• In Java 9 or later, Cleaner is used under the hood.
• In Java 8 or earlier, finalize() calls doClean() directly.