UUIDUtils工具类-根据时间生成UUID

/*
* Copyright (c) 2011-2013 GoPivotal, Inc. All Rights Reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*       http://www.apache.org/licenses/LICENSE-2.0 *
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package reactor.util;

import java.math.BigInteger;
import java.security.SecureRandom;
import java.util.Random;
import java.util.UUID;
import java.util.concurrent.locks.ReentrantLock;

/**
* Helper for creating random and Type 1 (time-based) UUIDs.
*
* @author Jon Brisbin
*/
public abstract class UUIDUtils {

private static boolean IS_THREADLOCALRANDOM_AVAILABLE = false;
private static       Random random;
private static final long   leastSigBits;
private static final ReentrantLock lock = new ReentrantLock();
private static long lastTime;

static {
try {
IS_THREADLOCALRANDOM_AVAILABLE = null != UUIDUtils.class.getClassLoader().loadClass(
"java.util.concurrent.ThreadLocalRandom"
);
} catch(ClassNotFoundException e) {
}

byte[] seed = new SecureRandom().generateSeed(8);
leastSigBits = new BigInteger(seed).longValue();
if(!IS_THREADLOCALRANDOM_AVAILABLE) {
random = new Random(leastSigBits);
}
}

private UUIDUtils() {
}

/**
* Create a new random UUID.
*
* @return the new UUID
*/
public static UUID random() {
byte[] randomBytes = new byte[16];
if(IS_THREADLOCALRANDOM_AVAILABLE) {
java.util.concurrent.ThreadLocalRandom.current().nextBytes(randomBytes);
} else {
random.nextBytes(randomBytes);
}

long mostSigBits = 0;
for(int i = 0; i < 8; i++) {
mostSigBits = (mostSigBits << 8) | (randomBytes[i] & 0xff);
}
long leastSigBits = 0;
for(int i = 8; i < 16; i++) {
leastSigBits = (leastSigBits << 8) | (randomBytes[i] & 0xff);
}

return new UUID(mostSigBits, leastSigBits);
}

/**
* Create a new time-based UUID.
*
* @return the new UUID
*/
public static UUID create() {
long timeMillis = (System.currentTimeMillis() * 10000) + 0x01B21DD213814000L;

lock.lock();
try {
if(timeMillis > lastTime) {
lastTime = timeMillis;
} else {
timeMillis = ++lastTime;
}
} finally {
lock.unlock();
}

// time low
long mostSigBits = timeMillis << 32;

// time mid
mostSigBits |= (timeMillis & 0xFFFF00000000L) >> 16;

// time hi and version
mostSigBits |= 0x1000 | ((timeMillis >> 48) & 0x0FFF); // version 1

return new UUID(mostSigBits, leastSigBits);
}

}