Java8 stream operations are cached? [duplicate]

问题

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));
    }

its output

  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

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

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

See below

  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

回答1:

does it mean stream operations are cached?

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

Is there any internal cache implemented for streams in Java8?

There is no special cache for Streams.

sometimes findAny returns different value but the time taken is almost equal to the subsequent runs not like the first run

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

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.

来源：https://stackoverflow.com/questions/40084636/java8-stream-operations-are-cached

标签

java

caching

java-8

java-stream