按正则表达式拆分与多个单字符拆分性能

我使用此基准比较了通过正则表达式和多个单字符拆分来拆分字符串

import org.openjdk.jmh.annotations.*;

import java.io.IOException;
import java.util.concurrent.TimeUnit;

public class Test {
    static String start = "1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.1, 2, 3, 4, 5, 6, 7, 8. 9. 10. 11. 12.";

    public static void main(String[] args) throws IOException {
        org.openjdk.jmh.Main.main(args);
    }

    @Fork(value = 1, warmups = 0)
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Warmup(iterations = 0)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public static void splitByRegex() {
        String test = start;

        test = String.join("_", test.split("[1,.]"));
    }

    @Fork(value = 1, warmups = 0)
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @Warmup(iterations = 0)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public static void multipleSplitByOneChar() {
        String test = start;

        test = String.join("_", test.split("\\."));
        test = String.join("_", test.split(","));
        test = String.join("_", test.split("1"));
    }
}

并得到了这些结果

Benchmark                    Mode  Cnt      Score     Error  Units
Test.multipleSplitByOneChar  avgt    5  10493,118 ± 572,528  ns/op
Test.splitByRegex            avgt    5  15519,418 ± 913,220  ns/op

为什么通过正则表达式拆分比通过多个单个字符拆分慢，即使它们产生相同的结果？

笔记：

1. 我在 JDK 14.0.2 上运行代码
2. 我使用了 JMH 1.28

回答

public String[] split(String regex, int limit) {
/* fastpath if the regex is a
* (1) one-char String and this character is not one of the
*     RegEx's meta characters ".$|()[{^?*+\\", or
* (2) two-char String and the first char is the backslash and
*     the second is not the ascii digit or ascii letter.
*/