java-8 - Spliterator:线程安全与否？

Question

我正在查看 Spliterator 的文档，根据它，Spliterator 不是线程安全的:

Despite their obvious utility in parallel algorithms, spliterators are not expected to be thread-safe; instead, implementations of parallel algorithms using spliterators should ensure that the spliterator is only used by one thread at a time. This is generally easy to attain via serial thread-confinement, which often is a natural consequence of typical parallel algorithms that work by recursive decomposition.

但是，在其进一步的文档中，陈述了与上述声明相矛盾的声明:

Structural interference of a source can be managed in the following ways (in approximate order of decreasing desirability):

The source manages concurrent modifications.For example, a key set of a java.util.concurrent.ConcurrentHashMap is a concurrent source. A Spliterator created from the source reports a characteristic of CONCURRENT.

那么这是否意味着从线程安全集合生成的 Spliterator 将是线程安全的？对吗？

Answer 1

不，报告 CONCURRENT 特性的 Spliterator 将具有线程安全的 source，这意味着即使在源同时被修改。但是 Spliterator 本身可能仍然具有不能同时操作的状态。

请注意，您的引用源于对“如何管理源的结构干扰”的描述，而不是关于拆分器的一般行为。

这也在 documentation of the CONCURRENT characteristic itself 中提供:

Characteristic value signifying that the element source may be safely concurrently modified (allowing additions, replacements, and/or removals) by multiple threads without external synchronization. If so, the Spliterator is expected to have a documented policy concerning the impact of modifications during traversal.

没有别的。

因此，这些特征的后果小得惊人。报告 CONCURRENT 或 IMMUTABLE 的 Spliterator 永远不会抛出 ConcurrentModificationException，仅此而已。在所有其他方面，Stream API 无法识别这些特征之间的差异，因为 Stream API 从不执行任何源操作，事实上，它不会实际上知道来源(除了间接通过 Spliterator)，所以它无法进行此类操作，也无法检测是否发生了并发修改。