RFR: 8301684: Fix test code to not get finalizer deprecation warnings

[Coleen Phillimore]

On Fri, 10 Mar 2023 08:23:45 GMT, Afshin Zafari <duke at openjdk.org> wrote:

> To replace the finalizer use in the code, the `Cleaner` approach is used as stated in [Oracle doc on deprecated finalize() method](https://docs.oracle.com/javase/10/docs/api/java/lang/Object.html#finalize()).
> Briefly:
> 1.  An instance of `Cleaner` (`java.lang.ref`) is created.
> 2. Using the `Cleaner`, an object is registered with a `Runnable` callback that is notified when the object is no longer reachable (GC'ed).
> 3. Write code in the callback to do other cleanings.
> 4. Or, use the `Cleanable` object which is returned from registration above and call its `clean` method to explicitly clean the object.
> 
> 
> Cleaner c = new Cleaner();
> Cleanable cleanable = c.register(an_obj, a_runnable);
> ...
>   //JVM notifies by calling the 
>     a_runnable.run() {...}
> ... 
> // possible to explicit cleaning 
> cleanable.clean(an_obj); 
> 
> ### Tests
> local: vmTestbase/nsk/monitoring/stress/classload
> mach5: tier 1-5

I don't know how cleaners work so here are some superficial comments.

test/hotspot/jtreg/vmTestbase/nsk/share/ClassUnloader.java line 102:

> 100:      * Here it will be used for tracking the local instance of 'CustomClassLoader'.
> 101:      */
> 102:     private static class CustomClassLoaderCleaner implements Runnable{

nit: can you add a space before open { in these lines?
Also use // comments ?

test/hotspot/jtreg/vmTestbase/nsk/share/ClassUnloader.java line 318:

> 316: 
> 317:         // Since customClassLoader object is set to null, we should also null the cleaning
> 318:         // objbects to let them be created when new customClassLoader is created.

typo: objbects

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PR: https://git.openjdk.org/jdk/pull/12968