try-with-resources and null resource

[Joe Darcy]

On 1/22/2011 5:09 AM, Rémi Forax wrote:
> On 01/21/2011 08:38 PM, Joe Darcy wrote:
>> Rémi Forax wrote:
>>> I think try-with-resources should do a null check before entering in 
>>> the try block.
>>>
>>> 4. try(AutoCloseable c = null) {
>>> 5.     // nothing
>>> 6. }
>>>
>>> $ java TryWithresourceNPE
>>> Exception in thread "main" java.lang.NullPointerException
>>>      at TryWithresourceNPE.main(TryWithresourceNPE.java:6)
>>>
>>> I got a NPE from the ends of the try block, I think it will be 
>>> better to detect the case
>>> before entering in the try block like foreach or switch does.
>>>
>>> And with this code:
>>>
>>> 5. try(InputStream i = null) {
>>> 6.       i.available();
>>> 7. }
>>>
>>> I got the exception below which is not understandable if you don't know
>>> how try-with-resources is translated.
>>>
>>> $ java TryWithresourceNPE
>>> Exception in thread "main" java.lang.NullPointerException
>>>      at TryWithresourceNPE.main(TryWithresourceNPE.java:6)
>>>      Suppressed: java.lang.NullPointerException
>>>          at TryWithresourceNPE.main(TryWithresourceNPE.java:7)
>>
>> I'm not too concerned about that stacktrace since the primary 
>> exception points to the right location.
>
> But the suppressed exception occurs in a generated code that the user 
> don't write.

But that is the whole point of try-with-resources, to allow users to not 
have to write the messy and error-prone clean-up code.

> I don't like the idea that a user mistake blow up in a generated code,
> try-with-resources should protect itself.

I think this stack trace can easily be explained to programmers by 
saying "the suppressed exception comes from the close call you did have 
to write yourself."

-Joe