Java 9 Cleaner Correct Usage
The docs do explicitly mention:
The choice of a new cleaner or sharing an existing cleaner is determined by the use case.
Also:
Each cleaner operates independently, managing the pending [...]
which implies multiple Cleaner instances are allowed within an application.
Again, since the factory method Cleaner::create is provided and is documented as
Returns a new Cleaner.
I do not see why only one Cleaner per application should be used, although the comment definitely states otherwise.
By surfing the web for a minute I found a few examples ( e.g this article ), and all use a static Cleaner for each AutoCloseable sub-class.
private final static Cleaner cleaner = Cleaner.create();
Why is it preferable to have one shared Cleaner (static) within a library?
A cleaner has an associated thread. Threads are limited native resources. So the goal is to limit the amount of Threads created by not creating more cleaners than necessary.
Does anybody have a good example about how to use Cleaner instead of overriding finalize()?
You posted the reference example. You need to ask more specific questions if that is insufficient.