64 bits Sufficiently Unique Id Generator

SUID.java

/****************************************************************************
 * Copyright 2009-2015 Jean-Philippe Gravel, P. Eng. CSDP
 * 
 * Modified 6/8/15 - Osric Wilkinson - No point messing with random seeds.
 * Just use SecureRandom.
 * 
 * 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 org.formix.utils;

import java.util.Random;
import java.util.SecureRandom;

/**
 * Sufficiently Unique Identifier
 * 
 * @author formix
 *
 */
final class SUID {

	private static final int SHORT_MAX = 65536;
	private static Random rnd = new SecureRandom();
	private static int counter = -1;

	private SUID() {
	}

	/**
	 * Creates a unique 64 bits ID by aggregating the current time in
	 * milliseconds since epoch (Jan. 1, 1970) and using a 16 bits counter. The
	 * counter is initialized at a random number. This generator can create up
	 * to 65536 different id per millisecond.
	 * 
	 * @return a new id.
	 */
	public static synchronized long nextId() {
		long now = System.currentTimeMillis();
		if (counter == -1) {
			counter = rnd.nextInt(SHORT_MAX);
		}
		long id = (now << 16) | counter;
		counter = (counter + 1) % SHORT_MAX;
		return id;
	}
}