RFR: 8017513: Support for closeable streams

Peter Levart peter.levart at gmail.com
Thu Jul 11 21:08:46 UTC 2013

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 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

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