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

[David Holmes]

PS.

create(): can also throw SecurityException if not allowed to 
create/start threads.

David

On 10/12/2015 4:40 AM, Roger Riggs wrote:
> 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
>>>>
>>
>