为什么本机字符串getBytes方法比自定义实现的getBytesFast慢？

Question

在运行以下代码时，Java的原生方法getBytesFast() ()的执行似乎比自定义实现慢。可以使用Arrays.equals(str.getBytes(), getBytesFast(str))验证两个字节数组是否相等。

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

我正在寻找一个详细的文档说明为什么本机实现比自定义实现慢。

package com.test;

public class Performance {

    public static void main(String args[]) {

/*
* 提示：该行代码过长，系统自动注释不进行高亮。一键复制会移除系统注释 
* 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! 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;
    }
}

编辑：

卡尺基准结果

​

​

谢谢

Answer 1

String.getBytes考虑到系统的默认字符集。您的实现假设为ISO-8859-1。

String.getBytes最终会调用此方法。ce是CharsetEncoder。

byte[] encode(char[] ca, int off, int len) {
        int en = scale(len, ce.maxBytesPerChar());
        byte[] ba = new byte[en];
        if (len == 0)
            return ba;
        if (ce instanceof ArrayEncoder) {
            int blen = ((ArrayEncoder)ce).encode(ca, off, len, ba);
            return safeTrim(ba, blen, cs, isTrusted);
        } else {
            ce.reset();
            ByteBuffer bb = ByteBuffer.wrap(ba);
            CharBuffer cb = CharBuffer.wrap(ca, off, len);
            try {
                CoderResult cr = ce.encode(cb, bb, true);
                if (!cr.isUnderflow())
                    cr.throwException();
                cr = ce.flush(bb);
                if (!cr.isUnderflow())
                    cr.throwException();
            } catch (CharacterCodingException x) {
                // Substitution is always enabled,
                // so this shouldn't happen
                throw new Error(x);
            }
            return safeTrim(ba, bb.position(), cs, isTrusted);
        }
    }
}

private static int scale(int len, float expansionFactor) {
    // We need to perform double, not float, arithmetic; otherwise
    // we lose low order bits when len is larger than 2**24.
    return (int)(len * (double)expansionFactor);
}

private static char[] safeTrim(char[] ca, int len,
                               Charset cs, boolean isTrusted) {
    if (len == ca.length && (isTrusted || System.getSecurityManager() == null))
        return ca;
    else
        return Arrays.copyOf(ca, len);
}

使用CharsetEncoder所涉及的复杂性要大得多，这可能是您所看到的执行时间较慢的原因。

Answer 2

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，使用代码中的每个char到byte转换，您将丢弃一半的字符数据.。

关于字符和字节流的Java教程有一个很好的示例字符串，使用日语汉字：

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

对于该字符串中的每个字符，您的快速转换方法将丢弃信息的第一个字节(例如，65 in \u65e5)。你的链接还特别提到，String.getBytes()运行速度要慢几倍，“因为前者进行了正确的字节到字符转换，这涉及到每个字符的函数调用。”

如果您完全忽略字符编码，并将每个char中的高阶字节丢弃，那么您将得到一些加速。您只需记住，此方法只适用于特定的字符编码，并且有可能丢失数据。

Answer 3

这可能是因为String.getBytes()使用或委托一个字符集(JVM当前的默认字符集)，而您的“快速”实现只是硬编码的ISO-8859-1字符集。

(注:我没有核实你的结果，我只是在这里陈述我的观点。与微基准有关的评论在这里非常重要，而且肯定比我对你的问题的回答更有价值:)