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

[Roger Riggs]

Hi,

The example is revised to caution about inner classes and lambdas.

[1]http://cr.openjdk.java.net/~rriggs/webrev-cleaner-8138696/
[2]http://cr.openjdk.java.net/~rriggs/cleaner-doc/index.html

Thanks, Roger

On 12/9/2015 11:04 AM, Peter Levart wrote:
> 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
>>>
>