RFR: 8017513: Support for closeable streams

[Zhong Yu]

On Thu, Jul 11, 2013 at 4:08 PM, Peter Levart <peter.levart at gmail.com> wrote:
> Hi Paul,
>
> I think the MayHoldCloseableResource extends AutoClosable is correct and
> AutoClosable extends MayHoldCloseableResource would be wrong.
>
> And exactly because of "Liskov":
>
> MayHoldCloseableResource contract says: "If you know it holds a resource,
> call close(), otherwise you need not call close(), but it's not wrong to
> call it anyway - you know whether it holds resource by looking at
> @HoldsResource annotation"
>
> AutoClosable contract says: "It holds a resource, you should call close()"
>
>
> Now imagine code that was written for the AutoClosable contract. Would it
> work if you pass it an instance of MayHoldCloseableResource? Allways.
>
> Now imagine generic code that was written for MayHoldCloseableResource
> contract and which uses the lookup of @HoldsResource at runtime to decide

How do you lookup an annotation on an _instance_ at runtime? And why
do we even care? Just call close() regardless.

And we can revert the parent/child relation, because the "otherwise
specified" clause is a panacea.


Zhong Yu


> whether to call close() or not. Would it work if you pass it an instance of
> AutoClosable? Never (since AutoClosable says nothing about any annotation).
>
> So I argue that MayHoldCloseableResource should be a subtype of AutoClosable
> and not the other way around.
>
> (I have not said anything about whether the MayHoldCloseableResource is
> actually needed or not.)
>
>
> Regards, Peter
>
>
>
> On 07/11/2013 10:22 PM, Paul Benedict wrote:
>>
>> Paul S.'s said the "negative" of using AutoCloseable is "it is no longer
>> clear whether a stream should be closed or not" (6/20). That's true
>> because
>> the semantics of AutoCloseable indicates you have a resource that requires
>> closing.
>>
>> However, the choice to make MayHoldCloseableResource a sub-interface of
>> AutoClosable should be resisted. It's an inverted design. The Liskov
>> *substitution
>> principle *says that sub-interfaces can't loosen the contracts of their
>> superinterface. If anything, AutoCloseable should be subclass of this new
>> interface, which MIGHT hold a resource that requires closing. The current
>> choice is just plainly backwards.
>>
>> For the above reason stated, and for the fact the interface adds no new
>> functionality, it's superfluous. If the interface relationship can't be
>> inverted, then chuck it -- it does nothing anyway. At the least, keep the
>> annotation.
>>
>> Paul
>>
>>
>> On Thu, Jul 11, 2013 at 3:01 PM, Henry Jen <henry.jen at oracle.com> wrote:
>>
>>> On 07/11/2013 01:13 AM, Florian Weimer wrote:
>>>>
>>>> On 07/10/2013 11:30 PM, Henry Jen wrote:
>>>>
>>>>> A new interface, java.util.MayHoldCloseableResource, indicates an
>>>>> implementation may or may not hold a resource need to be closed.
>>>>
>>>> Why doesn't close() throw Exception?
>>>
>>> Because there is really much a developer can do on that situation. The
>>> API simply make it not throw a *checked* exception.
>>>
>>> See EG discussion on this topic,
>>>
>>>
>>>
>>> http://mail.openjdk.java.net/pipermail/lambda-libs-spec-experts/2013-June/002081.html
>>>
>>>>> Annotation {@link HoldsResource} may be used to guide users/static
>>>>> analysis tools that a MHCR instance that definitely hold a Closeable
>>>>> resource.
>>>>
>>>> All this looks a bit odd to me.  I suppose the idea is that you don't
>>>> want to give up the last reference to a closeable resource without
>>>> calling close()—and not leak references which out-live the call to
>>>> close().  This is definitely not a property of the type of the resource,
>>>> so I don't see why the MayHoldCloseableResource interface is needed (or
>>>> can confer relevant information).  The HoldsResource annotation could be
>>>> useful, but based on the current documentation, it's not clear if it is
>>>> actually intended to express the data flow property.
>>>>
>>> I would suggest you look at EG discussion on this topic. The MHCR is
>>> different from AutoCloseable on the chances of holding critical
>>> resources.
>>>
>>> Perhaps that suggests the javadoc is not clear enough, I would like to
>>> know what is important and missing.
>>>
>>> Cheers,
>>> Henry
>>>
>>>
>>>
>