try-with-resources and null resource

[Joe Darcy]

Rémi Forax wrote:
> On 01/24/2011 08:00 PM, Joe Darcy wrote:
>> 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 have no problem if the error come from the close() itself,
> the stacktrace will be explicit in that case.
>
> But here it comes because the spec allows try(...) to be used with null,
> oh no wait! the generated finally block doesn't allow it.

I would assign the responsibility differently: because the programmer 
uses a null expression to initialize a variable, a NullPointerException 
is generated when the variable is used.  If the null-initialized 
variable is used inside the statement, there is an additional suppressed 
exception because of the guarantee to implicitly call close on the value 
when the block is exited.

>
> If you take a look to how try/finally are coded now, you will find a 
> nullcheck
> before the try or in the finally block.
>
>>
>>> 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."
>
> You don't have to explain something which is not surprising.

New features should be expected to require nonzero explanation simply by 
virtue of being new.

-Joe