try-with-resources and null resource

[Rémi Forax]

On 01/22/2011 06:10 PM, Peter Levart wrote:
> Perhaps the check for null should be done at cleanup time and do nothing if resource is null. A
> null value can be considered as a "valid" resource and the cleanup should not throw NPE if this
> is the case...
>
> Peter

The same argument was rejected for enhanced for loop.

I believe that throwing a NPE is more consistent with rest of the language,
unboxing, for(:) or switch on enum/string throws a NPE if the value is null.

Rémi

>
> On Saturday, January 22, 2011 02:09:31 pm 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.
>> I don't like the idea that a user mistake blow up in a generated code,
>> try-with-resources should protect itself.
>>
>> http://www.google.com/search?q=Throw+early+catch+late
>>
>>>> If the nullcheck is done before entering in the try block,
>>>> the exception will be:
>>>> Exception in thread "main" java.lang.NullPointerException
>>>>
>>>>       at TryWithresourceNPE.main(TryWithresourceNPE.java:5)
>>>>
>>>> which is in my opinion much better.
>>>>
>>>> Rémi
>>>> PS: the nullcheck can be done easily by calling getClass() on the
>>>> expression
>>>>
>>>> 4. try(AutoCloseable c = null) {
>>>> 5.     // nothing
>>>> 6. }
>>>>
>>>> will be translated to
>>>>
>>>> aconst_null
>>>> dup
>>>> invokevirtual java/lang/Object getClass ()Ljava/lang/Class;
>>>> astore local_slot_of_c
>>> Yes, javac internal uses that idiom for null checks and the JVM knows
>>> how to optimize this well :-)
>>>
>>> -Joe
>> Rémi