try-with-resources and null resource

[Rémi Forax]

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