Questions about Stream/Iterable/Files - and possibly the compiler

[Fabrizio Giudici]

On Fri, 06 Nov 2015 14:39:29 +0100, Paul Sandoz <paul.sandoz at oracle.com>  
wrote:

>
>> On 6 Nov 2015, at 14:19, Fabrizio Giudici  
>> <fabrizio.giudici at tidalwave.it> wrote:
>>
>>
>>> I logged an issue:
>>>
>>>  https://bugs.openjdk.java.net/browse/JDK-8141608  
>>> <https://bugs.openjdk.java.net/browse/JDK-8141608>
>>
>> Thanks to Remi and Paul for the complete explanation. Concerning  
>> JDK-8141608, I lile Peter Levart's comment about making a specific  
>> Collector.
>
> There is a problem with that approach. At the moment the Collector does  
> not get to control whether the stream is executed in parallel or  
> sequentially.

Sure, I understand. I think it would be ok if the Collector has as a  
pre-requisite that it can't be used with a parallel Stream. I assume that  
if the stream is not parallel, the runtime won't ever try to execute the  
Collector in parallel... right? That's what I see from the tests I've  
executed so far.

I've tried to write such a FileCollector, with a check that throws an  
exception if the accumulator Supplier function is called more than once,  
with code such as:

     private final AtomicBoolean parallelChecker = new AtomicBoolean();

     @Override
     public Supplier<PrintWriter> supplier()
       {
         return this::oneShotPrintWriterSupplier;
       }

     private PrintWriter oneShotPrintWriterSupplier()
       {
         if (parallelChecker.getAndSet(true))
           {
             fail();
           }

         return pw;
       }

It seems to properly detect when it's called with a parallel Stream. Do  
you think that it's safe enough?
Thanks.


-- 
Fabrizio Giudici - Java Architect @ Tidalwave s.a.s.
"We make Java work. Everywhere."
http://tidalwave.it/fabrizio/blog - fabrizio.giudici at tidalwave.it