try-with-resources and null resource

[Reinier Zwitserloot]

I don't understand why one couldn't explain t-w-r by showing "if (resource
!= null) resource.close();" in the finally block.

It is then your opinion that this code is abusing the t-w-r construct,
because Class.getResourceAsStream can return null?

try (InputStream introTextStream =
getClass().getResourceAsStream("intro.txt")) {
    ...
}

If so, we've clearly progressed into the "Everyone has their own 'feeling',
none of them agree with each other, and making language decisions based on
these feelings is a silly thing to do" phase. Let's check how often the
above occurs. I'm betting it'll be "loads of times". Also contrast what
you'd have to write if the above is not okay:

InputStream introTextStream1 = getClass().getResourceAsStream();
if (introTextStream1 != null) {
    try (InputStream introTextStream2 = introTextStream1) {
        ...
    }
}

I assume I don't have to resort to 'feeling' to claim that the above is just
bad.

 --Reinier Zwitserloot



On Fri, Jan 28, 2011 at 7:38 AM, David Holmes <David.Holmes at oracle.com>wrote:

> Tim Peierls said the following on 01/28/11 00:47:
> > I don't know about C#, but I like the formulation that can be sloppily
> > summarized as "t-w-r calls close on non-null resources".
>
> So to summarize there are three potential candidates for:
>
> try (Resource r = getResource() {
>   // stuff here
> }
>
> I'm ignoring the suppressed exception aspects for simplicity:
>
> a) Simple transformation into try-finally (current implemented semantics)
>
>   Resource r = getResource();
>   try {
>     // stuff here
>   }
>   finally {
>     r.close();
>   }
>
> b) Transformation into try-finally with explicit null-check on entry
>
>   Resource r = getResource();
>   Objects.requireNonNull(r); // heh heh - couldn't resist!
>   try {
>     // stuff here
>   }
>   finally {
>     r.close();
>   }
>
> c) Simple transformation into try-finally but only close a non-null
> resource
>
>   Resource r = getResource();
>   try {
>     // stuff here
>   }
>   finally {
>     if (r != null)
>       r.close();
>   }
>
>
> Option (a) is, perhaps, the most consistent with preserving the
> behaviour of current code that would be modified to use the new
> construct. This construct will be taught by analogy as it is the only
> effective way to explain things "try-with-resources behaves as-if you
> had written ....". That said I think existing code would have an
> explicit null-check to avoid the try-finally in the null case. And I
> think it is potentially risky to execute the try block only to have NPE
> thrown later.
>
> Option (c) seems appealing in some sense because there's no NPE's at all
> if you don't use "r", but I think that appeal is superficial. To me the
> fundamental question to ask is: what is try-with-resources for? And I
> believe the answer to that is to initialize, use and then clean-up an
> AutoCloseable object. So to me using try-with-resource on a resource
> that might be null is simply a mis-use of try-with-resources: if it can
> be null you can't autoclose it, so trying to is a programming error and
> programming errors should be detected and reported as early as possible.
> So to me option (b) is the preferred semantics for this new construct.
>
> David Holmes
>
>