try-with-resources and null resource

[Peter Levart]

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
 
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