java - 为什么 native String getBytes 方法比自定义实现的 getBytesFast 方法慢？

Question

当运行以下代码时，Java String 的本地方法 getBytes() 的执行似乎比自定义的 getBytesFast()慢/strong> 实现。您可以使用 Arrays.equals(str.getBytes(), getBytesFast(str)) 来验证两个字节数组是否相等。

getBytesFast 实现是本编程技巧文章(1997 年)中包含的实现的修改版本:http://java.sun.com/developer/technicalArticles/Programming/Performance/

我正在寻找一个有据可查的答案，说明为什么 native 实现比自定义实现慢。

package com.test;

public class Performance {

    public static void main(String args[]) {

        final String str = "This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! 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This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! 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        long startTime_1 = System.nanoTime();
        str.getBytes();
        System.out.println(System.nanoTime() - startTime_1);

        long startTime_2 = System.nanoTime();
        getBytesFast(str);
        System.out.println(System.nanoTime() - startTime_2);
    }

    private static byte[] getBytesFast(String str) {
        final char buffer[] = new char[str.length()];
        final int length = str.length();
        str.getChars(0, length, buffer, 0);
        final byte b[] = new byte[length];
        for (int j = 0; j < length; j++)
            b[j] = (byte) buffer[j];
        return b;
    }
}

已编辑:

Caliper 基准测试结果

谢谢

Answer 1

根据[文档]，Java 中的char 类型是16 位Unicode 字符，而byte 类型是8 位有符号整数。这意味着在您的代码中进行的每个 char-to-byte 转换都您将丢弃一半的字符数据。

Java tutorial on Character and Byte Streams有一个使用日文汉字的不错的小示例字符串:

String jaString = new String("\u65e5\u672c\u8a9e\u6587\u5b57\u5217");

对于该字符串中的每个字符，您的快速转换方法会丢弃信息的第一个字节(例如 \u65e5 中的 65)。 Your link还特别提到 String.getBytes() 运行速度慢了好几倍，“因为前者确实正确地进行了字节到字符的转换，这涉及到每个字符的函数调用。”

如果您完全忽略字符编码并丢弃每个 char 的高位字节，那么您将获得一点加速。您只需要记住，此方法仅适用于某些字符编码，并且有可能丢失数据。