Generates COMB UUID/GUID in Java

CombUuidCreator.java

package your.package.name;

import java.security.SecureRandom;
import java.util.Random;
import java.util.UUID;

/**
 * A UUID generator that creates COMB UUIDs.
 * 
 * The COMB UUIDs combine the creation time and random bytes.
 * 
 * The PREFIX or SUFFIX has 6 bytes and corresponds to the milliseconds since
 * 1970-01-01T00:00:00Z (Unix epoch).
 * 
 * For RFC-4122 compliance, it uses the version number 4.
 * 
 * Read: The Cost of GUIDs as Primary Keys
 * http://www.informit.com/articles/article.aspx?p=25862
 * 
 * Tags: uuid guid comb comb-guid comb-uuid uuid-generator guid-generator comb-generator generator comb time random
 *
 * @author: Fabio Lima 2020
 */
public abstract class CombUuidCreator {

	private static final int RANDOM_VERSION = 4;

	/**
	 * Returns a prefix COMB UUID.
	 * 
	 * It uses a thread local {@link SecureRandom}.
	 * 
	 * @return a random-based UUID
	 */
	public static UUID getPrefixComb() {
		return getPrefixComb(SecureRandomLazyHolder.INSTANCE.get());
	}

	/**
	 * Returns a prefix COMB UUID.
	 * 
	 * It uses any instance of {@link Random}.
	 * 
	 * @return a random-based UUID
	 */
	public static UUID getPrefixComb(Random random) {
		return getCombGuid(random, /* prefix = */true);
	}

	/**
	 * Returns a suffix COMB UUID.
	 * 
	 * It uses a thread local {@link SecureRandom}.
	 * 
	 * @return a random-based UUID
	 */
	public static UUID getSuffixComb() {
		return getSuffixComb(SecureRandomLazyHolder.INSTANCE.get());
	}

	/**
	 * Returns a suffix COMB UUID.
	 * 
	 * It uses any instance of {@link Random}.
	 * 
	 * @return a random-based UUID
	 */
	public static UUID getSuffixComb(Random random) {
		return getCombGuid(random, /* prefix = */false);
	}

	/**
	 * Returns prefix or suffix COMB UUID.
	 * 
	 * It uses any instance of {@link Random}.
	 * 
	 * @return a random-based UUID
	 */
	private static UUID getCombGuid(Random random, boolean prefix) {

		long msb = 0;
		long lsb = 0;

		// (3) set bits randomly
		final byte[] bytes = new byte[16];
		random.nextBytes(bytes);
		final long rand0 = (bytes[8] << 8) | (bytes[9] & 0xff);
		final long rand1 = toNumber(bytes, 0, 8);

		// Insert the prefix in the MSB
		final long timestamp = System.currentTimeMillis();
		if (prefix) {
			msb = (rand0 & 0x000000000000ffffL) | ((timestamp & 0x0000ffffffffffffL) << 16);
			lsb = rand1;
		} else {
			msb = rand1;
			lsb = (rand0 << 48) | (timestamp & 0x0000ffffffffffffL);
		}

		// Apply version and variant bits (required for RFC-4122 compliance)
		msb = (msb & 0xffffffffffff0fffL) | ((RANDOM_VERSION & 0x0f) << 12); // apply version bits
		lsb = (lsb & 0x3fffffffffffffffL) | 0x8000000000000000L; // apply variant bits

		// Return the UUID
		return new UUID(msb, lsb);
	}

	private static long toNumber(final byte[] bytes, final int start, final int length) {
		long result = 0;
		for (int i = start; i < length; i++) {
			result = (result << 8) | (bytes[i] & 0xff);
		}
		return result;
	}

	// Holds thread local secure random
	private static class SecureRandomLazyHolder {
		static final ThreadLocal<Random> INSTANCE = ThreadLocal.withInitial(SecureRandom::new);
	}

	/**
	 * For tests!
	 */
	public static void main(String[] args) {

		Random random = new Random();

		System.out.println("// Prefix COMB using thread local `java.security.SecureRandom` (DEFAULT)");
		System.out.println("CombUuidCreator.getPrefixComb()");
		System.out.println();
		for (int i = 0; i < 5; i++) {
			System.out.println(" " + CombUuidCreator.getPrefixComb());
		}
		System.out.println("|----prefix---|----------------------|");

		System.out.println();
		System.out.println("// Prefix COMB using `java.util.Random` (FASTER)");
		System.out.println("CombUuidCreator.getPrefixComb(new Random())");
		System.out.println();
		for (int i = 0; i < 5; i++) {
			System.out.println(" " + CombUuidCreator.getPrefixComb(random));
		}
		System.out.println("|----prefix---|----------------------|");

		System.out.println();
		System.out.println("// Suffix COMB using thread local `java.security.SecureRandom` (DEFAULT)");
		System.out.println("CombUuidCreator.getSuffixComb()");
		System.out.println();
		for (int i = 0; i < 5; i++) {
			System.out.println(" " + CombUuidCreator.getSuffixComb());
		}
		System.out.println("|-----------------------|---suffix---|");

		System.out.println();
		System.out.println("// Suffix COMB using `java.util.Random` (FASTER)");
		System.out.println("CombUuidCreator.getSuffixComb(new Random())");
		System.out.println();
		for (int i = 0; i < 5; i++) {
			System.out.println(" " + CombUuidCreator.getSuffixComb(random));
		}
		System.out.println("|-----------------------|---suffix---|");
	}
}

//	OUTPUT:
//	
//	//Prefix COMB using thread local `java.security.SecureRandom` (DEFAULT)
//	CombUuidCreator.getPrefixComb()
//	
//	01755c83-802d-41c3-9223-6f111718e48f
//	01755c83-802d-42d7-9151-e8f2d8009ec3
//	01755c83-802e-4d9d-8bf8-ff86681e3b33
//	01755c83-802e-46d7-8425-3595e84543aa
//	01755c83-802e-4a93-9316-a344e10a863f
//	|----prefix---|----------------------|
//	
//	//Prefix COMB using `java.util.Random` (FASTER)
//	CombUuidCreator.getPrefixComb(new Random())
//	
//	01755c83-802e-458d-859e-34e4f35925a6
//	01755c83-802e-4848-9052-7f7b811d13e0
//	01755c83-802e-4d77-975e-fb8cfdf73746
//	01755c83-802e-4ee1-9056-70c1c62dc0b9
//	01755c83-802e-4731-8fd2-ce7a89c711a2
//	|----prefix---|----------------------|
//	
//	//Suffix COMB using thread local `java.security.SecureRandom` (DEFAULT)
//	CombUuidCreator.getSuffixComb()
//	
//	6ccdb118-43e0-4e57-837b-01755c83802e
//	99800a56-e6cb-4732-9fde-01755c83802e
//	13fe7ddd-78f0-4a80-8cf9-01755c83802e
//	e0902720-195b-4d70-b6b2-01755c83802e
//	1110dea5-3d2b-4e61-8664-01755c83802e
//	|-----------------------|---suffix---|
//	
//	//Suffix COMB using `java.util.Random` (FASTER)
//	CombUuidCreator.getSuffixComb(new Random())
//	
//	7b9c79ac-e6e0-449d-98c7-01755c83802f
//	04dfe2c4-53bd-4f71-8b76-01755c83802f
//	94767d25-40c7-4493-b981-01755c83802f
//	04e85179-7cdd-4ecd-9963-01755c83802f
//	d1a789ab-d792-4a88-9943-01755c83802f
//	|-----------------------|---suffix---|