Java8流操作是否缓存？

问题描述

我在运行英特尔（R）Xeon（R）CPU E5-2680 0 @ 2.70GHz（2个CPU），~2.7GHz 的PC中运行以下示例代码

I ran below sample code in my PC running with Intel(R) Xeon(R) CPU E5-2680 0 @ 2.70GHz (2 CPUs), ~2.7GHz

    String format = "%7s run taken %6d micro seconds %5d findAny";

    // First run
    long start = System.nanoTime();
    int rand = IntStream.range(0, 100000).parallel().findAny().getAsInt();
    long end = System.nanoTime();
    System.out.println(String.format(format, "First", ((end - start) / 1000), rand));

    // Subsequent runs
    for (int i = 0; i < 25; i++) {
        start = System.nanoTime();
        rand = IntStream.range(0, 100000).parallel().findAny().getAsInt();
        end = System.nanoTime();
        System.out.println(String.format(format, "Subseq", ((end - start) / 1000), rand));
    }

其输出

  First run taken  92532 micro seconds 50000 findAny
 Subseq run taken     61 micro seconds 50000 findAny
 Subseq run taken     37 micro seconds 50000 findAny
 Subseq run taken     52 micro seconds 50000 findAny
 Subseq run taken     42 micro seconds 50000 findAny
 Subseq run taken     33 micro seconds 50000 findAny
 Subseq run taken     32 micro seconds 50000 findAny
 Subseq run taken     34 micro seconds 50000 findAny
 Subseq run taken     33 micro seconds 50000 findAny
 Subseq run taken     34 micro seconds 50000 findAny
 Subseq run taken     32 micro seconds 50000 findAny
 Subseq run taken     32 micro seconds 50000 findAny
 Subseq run taken     46 micro seconds 50000 findAny
 Subseq run taken     36 micro seconds 50000 findAny
 Subseq run taken     31 micro seconds 50000 findAny
 Subseq run taken     43 micro seconds 50000 findAny
 Subseq run taken     34 micro seconds 50000 findAny
 Subseq run taken     31 micro seconds 50000 findAny
 Subseq run taken     32 micro seconds 50000 findAny
 Subseq run taken     37 micro seconds 50000 findAny
 Subseq run taken     45 micro seconds 50000 findAny
 Subseq run taken     49 micro seconds 50000 findAny
 Subseq run taken     32 micro seconds 50000 findAny
 Subseq run taken     31 micro seconds 50000 findAny
 Subseq run taken     31 micro seconds 50000 findAny
 Subseq run taken     37 micro seconds 50000 findAny

我们可以看到第一次和后续运行之间的时间差异。

we could see the time taken difference between the first and subsequent runs.

1. 是否意味着流操作被缓存？是否为 Java8 中的流实现了内部缓存？


2. 有时 findAny 返回不同的值，但所花费的时间几乎等于后续运行，而不是像第一次运行那样

1. does it mean stream operations are cached? Is there any internal cache implemented for streams in Java8?
2. sometimes findAny returns different value but the time taken is almost equal to the subsequent runs not like the first run

见下文

  First run taken  84099 micro seconds 50000 findAny
 Subseq run taken    163 micro seconds 25000 findAny
 Subseq run taken     46 micro seconds 50000 findAny
 Subseq run taken     52 micro seconds 25000 findAny

推荐答案

不，为实现lambdas而生成的代码，以及加载的类是缓存的。

No, the code generated to implement the lambdas, and the classes loaded are cached.

Streams没有特殊缓存。

There is no special cache for Streams.

确实。没有关于结果的任何内容被缓存。第一次为加载代码支付罚金。

Indeed. Nothing about the result is cached. The first time you pay a penalty for loading the code.

BTW编码在运行至少10,000次之前并未真正优化。在计时之前，我会反复运行这个测试大约10秒钟。

BTW the coding isn't really optimised until it has been run at least 10,000 times. I would run this test repeatedly for around 10 seconds before timing it.

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