RFR 9: 8138696 : java.lang.ref.Cleaner - an easy to use alternative to finalization

[Peter Levart]

Hi Chris,

On 12/09/2015 04:03 PM, Chris Hegarty wrote:
> Peter,
>
> On 09/12/15 07:05, Peter Levart wrote:
>> Hi,
>>
>> I think the only way to try to prevent such things is with a good
>> example in javadoc that "screams" of possible miss-usages.
>>
>>
>> public static class CleanerExample implements AutoCloseable {
>>
>>          private static final Cleaner cleaner = ...; // preferably a
>> shared cleaner
>>
>>          private final PrivateNativeResource pnr;
>>
>>          private final Cleaner.Cleanable cleanable;
>>
>>          public CleanerExample(args, ...) {
>>
>>              // prepare captured state as local vars...
>>              PrivateNativeResource _pnr = ...;
>>
>>              this.cleanable = cleaner.register(this, () -> {
>>                  // DON'T capture any instance fields with lambda since
>> that would
>>                  // capture 'this' and prevent it from becoming
>
> I assume that the WARNING should include anonymous inner classes too
> ( which I expect are quite common, though less now with lambda ) ?
>
> Is "leaking" 'this' in a constructor a potential issue with respect
> to the visibility of pnr? As well as causing red-squiggly lines in
> the IDE ;-)

'this' only leaks to the 'referent' field of PhantomReference where by 
definition is not accessible.

'this' can become phantom-reachable before CleanerExample constructor 
ends. But this is harmless, because the code that may execute at that 
time does not access the object any more, so the object may be safely 
collected.

Cleanup action can run at any time after registration even before 
CleanerExample constructor ends. But this is harmless too, because it 
only accesses PrivateNativeResource which is fully constructed before 
registration of cleanup action.

I see no issues apart from IDE(s) not seeing no issues.

Regards, Peter

>
> -Chris.
>
>
>> phantom-reachable!!!
>>                  _pnr.close();
>>              });
>>
>>              this.pnr = _pnr;
>>          }
>>
>>          public void close() {
>>              cleanable.clean();
>>          }
>>
>>
>> Regards, Peter
>>