有朋友发现对于下面的test程序
import java.util.concurrent.TimeUnit;
import java.util.Random;
public class Test {
private final int TEST_STRING_LENGTH = 1024 * 64;
private String testString;
private byte[] bytes = new byte[TEST_STRING_LENGTH * 3];
private void generateRandomString() {
StringBuilder sb = new StringBuilder();
Random random = new Random();
for (int i = 0; i < TEST_STRING_LENGTH; i++) {
sb.append((char) (random.nextInt(26) + 'A'));
}
testString = sb.toString();
}
private static int encodeUTF8(String str, byte[] bytes) {
int offset = 0;
int len = str.length();
int sl = offset + len;
int dp = 0;
int dlASCII = dp + Math.min(len, bytes.length);
// ASCII only optimized loop
while (dp < dlASCII && str.charAt(offset) < '\u0080') {
bytes[dp++] = (byte) str.charAt(offset++);
}
return dp;
}
public static void main(String[] args) {
Test t = new Test();
t.generateRandomString();
for (int i = 0; i < 200000; i++) {
encodeUTF8(t.testString, t.bytes);
}
}
}
JDK-8268698之后这段代码慢了一倍：
Before JDK-8268698,
real 0m8.369s
user 0m8.386s
sys 0m0.019s
After JDK-8268698,
real 0m19.722s
user 0m19.748s
sys 0m0.013s
检查实现发现是str.charAt性能下降导致的。
class String{
public char charAt(int index) {
if (isLatin1()) {
return StringLatin1.charAt(value, index);
} else {
return StringUTF16.charAt(value, index);
}
}
}
final class StringLatin1 {
public static char charAt(byte[] value, int index) {
checkIndex(index, value.length, null);
return (char)(value[index] & 0xff);
}
}
