按正则表达式拆分与多个一个字符拆分性能

Question

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

为什么按正则表达式拆分比按多个单独的字符拆分慢，即使它们产生相同的结果？

注：

我运行 JDK 14.0.2
我用的是JMH 1.28

Answer 1

String.split 实现优化了 fast path 以一个字符拆分。

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.
     */

按正则表达式拆分与多个一个字符拆分性能

Split by regex vs multiple one char splits performance

java

performance

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jmh