这是 Files.lines() 中的错误，还是我误解了并行流? [英] Is this a bug in Files.lines(), or am I misunderstanding something about parallel streams?

问题描述

环境:Ubuntu x86_64 (14.10)、Oracle JDK 1.8u25

我尝试使用 Files.lines() 但我想 .skip()第一行(它是一个带有标题的 CSV 文件).因此，我尝试这样做:

试试 (最终流流 = Files.lines(thePath, StandardCharsets.UTF_8).skip(1L).parallel();){//等等}

但随后一列未能解析为 int...

所以我尝试了一些简单的代码.文件问题很简单:

$ cat info.csvstartDate;treeDepth;nrMatchers;nrLines;nrChars;nrCodePoints;nrNodes1422758875023;34;54;151;4375;4375;27486$

代码同样简单:

public static void main(final String... args){最终路径 path = Paths.get("/home/fge/tmp/dd/info.csv");Files.lines(path, StandardCharsets.UTF_8).skip(1L).parallel().forEach(System.out::println);}

我系统地得到以下结果(好吧，我只运行了大约 20 次):

startDate;treeDepth;nrMatchers;nrLines;nrChars;nrCodePoints;nrNodes

我在这里遗漏了什么?

<小时>

编辑问题或误解似乎比这更根深蒂固(以下两个示例是由 FreeNode 的 ##java 上的一个人编写的):

public static void main(final String... args){new BufferedReader(new StringReader("Hello
World")).lines().skip(1L).parallel().forEach(System.out::println);最终迭代器迭代器= Arrays.asList("Hello", "World").iterator();最终 Spliterator分离器= Spliterators.spliteratorUnknownSize(iter, Spliterator.ORDERED);最终流秒= StreamSupport.stream(spliterator, true);s.skip(1L).forEach(System.out::println);}

打印:

你好你好

呃.

@Holger 建议对于任何 ORDERED 而不是 SIZED 的流会发生这种情况:

Stream.of("Hello", "World").filter(x -> 真).平行线().跳过(1L).forEach(System.out::println);

此外，它源于已经发生的所有讨论，即问题(如果是一个?)与 .forEach()(如 @SotiriosDelimanois 首先指出).

解决方案

由于问题的当前状态与此处先前的陈述完全相反，应该注意的是，现在有一个 Brian Goetz 的明确声明 关于无序特征经过 skip 操作的反向传播被认为是一个错误.还说明现在考虑根本没有终端操作的有序性的反向传播.

还有一个相关的bug报告，JDK-8129120，状态是已在 Java 9 中修复"并且它反向移植到 Java 8，更新 60>

我用 jdk1.8.0_60 做了一些测试，现在的实现似乎确实展示了更直观的行为.

Environment: Ubuntu x86_64 (14.10), Oracle JDK 1.8u25

I try and use a parallel stream of Files.lines() but I want to .skip() the first line (it's a CSV file with a header). Therefore I try and do this:

try (
    final Stream<String> stream = Files.lines(thePath, StandardCharsets.UTF_8)
        .skip(1L).parallel();
) {
    // etc
}

But then one column failed to parse to an int...

So I tried some simple code. The file is question is dead simple:

$ cat info.csv 
startDate;treeDepth;nrMatchers;nrLines;nrChars;nrCodePoints;nrNodes
1422758875023;34;54;151;4375;4375;27486
$

And the code is equally simple:

public static void main(final String... args)
{
    final Path path = Paths.get("/home/fge/tmp/dd/info.csv");
    Files.lines(path, StandardCharsets.UTF_8).skip(1L).parallel()
        .forEach(System.out::println);
}

And I systematically get the following result (OK, I have only run it something around 20 times):

startDate;treeDepth;nrMatchers;nrLines;nrChars;nrCodePoints;nrNodes

What am I missing here?

---

EDIT It seems like the problem, or misunderstanding, is much more rooted than that (the two examples below were cooked up by a fellow on FreeNode's ##java):

public static void main(final String... args)
{
    new BufferedReader(new StringReader("Hello
World")).lines()
        .skip(1L).parallel()
        .forEach(System.out::println);

    final Iterator<String> iter
        = Arrays.asList("Hello", "World").iterator();
    final Spliterator<String> spliterator
        = Spliterators.spliteratorUnknownSize(iter, Spliterator.ORDERED);
    final Stream<String> s
        = StreamSupport.stream(spliterator, true);

    s.skip(1L).forEach(System.out::println);
}

This prints:

Hello
Hello

Uh.

@Holger suggested that this happens for any stream which is ORDERED and not SIZED with this other sample:

Stream.of("Hello", "World")
    .filter(x -> true)
    .parallel()
    .skip(1L)
    .forEach(System.out::println);

Also, it stems from all the discussion which already took place that the problem (if it is one?) is with .forEach() (as @SotiriosDelimanolis first pointed out).

解决方案

Since the current state of the issue is quite the opposite of the earlier statements made here, it should be noted, that there is now an explicit statement by Brian Goetz about the back-propagation of the unordered characteristic past a skip operation is considered a bug. It’s also stated that it is now considered to have no back-propagation of the ordered-ness of a terminal operation at all.

There is also a related bug report, JDK-8129120 whose status is "fixed in Java 9" and it’s backported to Java 8, update 60

I did some tests with jdk1.8.0_60 and it seems that the implementation now indeed exhibits the more intuitive behavior.

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