Upcoming change in semantics to try-with-resources on a null resource

[Joe Darcy]

Rémi Forax wrote:
> On 02/17/2011 10:52 PM, Joe Darcy wrote:
>   
>> On 2/16/2011 3:06 PM, Joe Darcy wrote:
>>     
>>> Hello.
>>>
>>> After due consideration the JSR 334 expert group has decided the
>>> semantics of the try-with-resources statement on a null resource
>>> should be changed as follows: the compiler-generated calls to close a
>>> resource will only occur if the resource is non-null.
>>>
>>> Concretely, the semantics of the desugaring of the finally block are
>>> changed from
>>>
>>> finally {
>>>   if (#primaryException != null) {
>>>     try {
>>>       #resource.close();
>>>     } catch(Throwable #suppressedException) {
>>>       #primaryException.addSuppressed(#suppressedException);
>>>     }
>>>   } else {
>>>     #resource.close();
>>>   }
>>> }
>>>
>>> to
>>>
>>> finally {
>>>   if (#primaryException != null) {
>>>     try {
>>>       if(#resource != null)
>>>         #resource.close();
>>>     } catch(Throwable #suppressedException) {
>>>       #primaryException.addSuppressed(#suppressedException);
>>>     }
>>>   } else {
>>>       if(#resource != null)
>>>         #resource.close();
>>>   }
>>> }
>>>
>>>       
>> FYI, inspired by a comment left on my blog [1], javac will hoist the
>> null check of the resource, resulting in the semantically equivalent but
>> shorter code
>>
>> finally {
>>     if (#resource != null)
>>       if (#primaryException != null) {
>>         try {
>>           // if(#resource != null)
>>             #resource.close();
>>         } catch(Throwable #suppressedException) {
>>           #primaryException.addSuppressed(#suppressedException);
>>         }
>>       } else {
>>         // if(#resource != null)
>>           #resource.close();
>>       }
>>     }
>> }
>>
>> -Joe
>>
>> [1]
>> http://blogs.sun.com/darcy/entry/project_coin_null_try_with#comment-1297968215000
>>     
>
> There is another cheap optimization.
> Only generate the nullcheck if the resource is not initialized with a 
> new XXX.
>    try(XXX resource = new XXX()) {
>       ...
>    }
>
> Allocating the resource in the try expression is really a common code.
>
> Rémi
>
>   

Hi Rémi.

Thanks for the suggestion; I've recored it in 7020499 "Project Coin: 
improvements to try-with-resources desugaring."

I don't plan to use that optimization in the initial push of the new 
semantics, but we'll certainly consider it for future refinements.

Thanks,

-Joe