try-with-resources and null resource (original) (raw)

Reinier Zwitserloot reinier at zwitserloot.com
Thu Jan 27 23:37:43 PST 2011

Using t-w-r with a null resource would do something other than throw an NPE in the body if the body contains an if-based null check, for example.

--Reinier Zwitserloot

On Fri, Jan 28, 2011 at 8:18 AM, David Holmes <David.Holmes at oracle.com>wrote:

Reinier Zwitserloot said the following on 01/28/11 16:54:

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

One could of course do this. But most usages will involve accessing resource within the try block, which would result in NPE anyway. 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")) { ... } getResourceAsStream is a problematic example as it should not have been defined to return null. But that aside ... yes, that is my opinion. If the stream here may not exist I have to deal with that fact in my code. But you need to expand your example to show what the try block actually attempts to do with the stream that might be null. Your expectations in the null case will naturally influence what you expect the construct to do. 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". The question to answer is: what do those "loads of times" do if they encounter a null? 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. What's the bad part? Having to declare a secondary variable? Certainly not ideal, but hardly fatal - we have to do that to capture locals as finals for inner class access. Also the secondary variable is a consequence of a different decision to limit the expression to be a local variable declaration. I wasn't involved in any discussions of that and I think it unfortunate that you can't use: final InputStream istream = getClass().getResourceAsStream(); if (istream != null) { try (istream) { ... } } provided istream is a reference to a final, or effectively final, value. To me a null reference is a bad input for try-with-resources and bad input should be rejected. David Holmes --Reinier Zwitserloot

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

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