The whole reflection thing is a red herring. As is the performance thing.
The reason not to call close() has nothing to do with the expensive of
calling close; it is about not mucking up your code for no reason.
Consider:
list.stream()
.filter(s -> s.startsWith("foo"))
.map(String::toUpperCase)
.collect(toList());
vs
List<String result;
try ( Stream s = list.stream().filter(...).map(...) ) {
result = s.collect(toList());
}
It would be utterly criminal if someone were to restructure the above
code into the below code because some IDE inspection complained about
"must call close or use TWR."
On 7/11/2013 5:17 PM, roger riggs wrote:
Hi,
Wouldn't the close() implementation be nil in most cases if there was no
resource.
That kind of method runs very quickly and I would expect the compiler to
inline nothing.
It would be quicker to just call close() than to engage reflection to
determine
if it really did and then decide to call it. Or am I missing some point
about
generating code or in some cases not needing/wanting to close it?
Roger
On 7/11/2013 5:08 PM, Peter Levart 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 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....@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
