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java - 按正则表达式拆分与多个一个字符拆分性能

问题描述

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

为什么用正则表达式分割比用多个单个字符分割要慢，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. 我用的是 JMH 1.28

标签： javaperformancesplitjmh

解决方案

String.split实现具有按一个字符分割的优化快速路径。

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.
     */

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