java - 为什么原生 String getBytes 方法比自定义实现的 getBytesFast 慢？

Question

运行以下代码时，Java String 的本机方法getBytes()的执行似乎比自定义getBytesFast()实现慢。您可以使用来验证两个字节数组是否相等。Arrays.equals(str.getBytes(), getBytesFast(str))

getBytesFast 实现是此编程技巧文章 (1997) 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，说明为什么本机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! 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：

卡尺基准测试结果

谢谢

Answer 1

根据 [the documentation]，charJava 中的类型是 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次转换char都会丢弃一半的字符数据。byte

关于字符和字节流的Java 教程有一个很好的使用日文汉字的小示例字符串：

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

对于该字符串中的每个字符，您的快速转换方法将丢弃信息的第一个字节（例如65in \u65e5）。您的链接还特别提到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字符的高阶字节，char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。

Answer 2

String.getBytes考虑到系统的默认字符集。您的实施假定ISO-8859-1。

String.getBytes最终最终调用了这个方法。ce是一个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 a 涉及更大程度的复杂性CharsetEncoder，这可能导致您看到的执行时间较慢。

Answer 3

这可能是因为String.getBytes()使用或委托给一个字符集（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。

（注意：我没有验证您的结果，我只是在这里陈述我的假设。与微基准测试相关的评论在这里非常重要，而且绝对比我对您的问题的回答更有价值：）