Exceptional behavior of parallel stream

[Paul Sandoz]

Hi Tagir,

After some discussion with Doug i logged an issue:

  https://bugs.openjdk.java.net/browse/JDK-8164690 <https://bugs.openjdk.java.net/browse/JDK-8164690>

but as of yet undecided whether we should do anything about it at the level of ForkJoinTask or specific to Stream (or even ConcurrentHashMap bulk ops).

We could mention, as an implementation detail (@implNote), on Stream and CHM about the current behaviour. I am reluctant to specify normatively.

Paul.

> On 17 Aug 2016, at 06:01, Tagir F. Valeev <amaembo at gmail.com> wrote:
> 
> Hello!
> 
> I found no information in Stream API documentation on how it behaves
> in case if exception occurs during the stream processing. While it's
> quite evident for sequential stream (stream processing is terminated
> and exception is propagated to the caller as is), the behavior for
> parallel streams differs from one might expect. Consider the following
> test:
> 
> String[] data = IntStream.range(0, 100).mapToObj(String::valueOf)
>                         .toArray(String[]::new);
> data[20] = "oops";
> try {
>    int sum = Arrays.stream(data).parallel().mapToInt(Integer::valueOf)
>                    .peek(System.out::println).sum();
>    System.out.println("Sum is "+sum);
> } catch (NumberFormatException e) {
>    System.out.println("Non-number appeared!");
> }
> 
> This parses integers stored in string array and sums them outputting
> every number to stdout once it processed. As data set contains
> non-number, the stream obviously fails with NumberFormatException. The
> typical output looks like this:
> 
> 62
> 63
> 12
> 31
> 87
> ...
> 28
> 92
> 29
> 8
> Non-number appeared!
> 9
> 30
> 
> So as you can see, the stream is not actually terminated when
> exception is thrown and caught: even after that some parallel tasks
> continue running, and you see more numbers printed after catch block
> is executed.
> 
> I consider such behavior as confusing and unexpected. Given the fact
> that stream may produce side-effects (e.g. if terminal op is forEach)
> this might lead to unforeseen consequences in user programs as
> left-over parallel stream tasks may continue mutate shared state after
> main stream thread exceptionally returns the control to the caller.
> 
> So I suggest that such behavior should be either fixed or explicitly
> documented. What do you think?
> 
> With best regards,
> Tagir Valeev.
>