运行以下代码时,执行Java String的本机方法 getBytes()似乎比自定义 getBytesFast()实现的慢。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实现是此编程技巧文章(1997)中包括的实现的修改版本:http://java.sun.com/developer/technicalArticles/Programming/Performance/
我正在寻找一个有据可查的答案,以说明为什么本机实现比自定义实现要慢。
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! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This 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! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This 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! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test! This is a performance test!";
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
参考方案
String.getBytes
考虑了系统的默认字符集。您的实现采用ISO-8859-1。String.getBytes
最终最终调用此方法。 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
涉及更大的复杂度,这可能导致您看到的执行时间变慢。
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