Add a unique number to be XORed with the thread-local random sequence

The JVM unique number tries to mitigate the fact that the thread
local random is not seeded with a secure random seed by default.
Their seeds are based on temporal data and predefined constants.
Although the seeds are unique per JVM, they are not across JVMs.
It helps to generate different sequences of numbers even if two
ThreadLocalRandom are by chance instantiated with the same seed.
Of course it doesn't better the output, but doesn't hurt either.

Author: fabiolimace

src/main/java/com/github/f4b6a3/uuid/alt/GUID.java

@@ -29,6 +29,7 @@
 import java.nio.charset.StandardCharsets;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
+import java.security.SecureRandom;
 import java.time.Instant;
 import java.util.Arrays;
 import java.util.Objects;
@@ -208,9 +209,12 @@ public GUID(String string) {
 	 * @return a GUID
 	 */
 	public static GUID v1() {
+
 		final long time = gregorian();
+
 		final long msb = (time << 32) | ((time >>> 16) & (MASK_16 << 16)) | ((time >>> 48) & MASK_12);
-		final long lsb = ThreadLocalRandom.current().nextLong() | MULTICAST;
+		final long lsb = LazyHolder.random() | MULTICAST;
+
 		return version(msb, lsb, 1);
 	}
 
@@ -270,9 +274,8 @@ public static GUID v3(GUID namespace, String name) {
 	 * @return a GUID
 	 */
 	public static GUID v4() {
-		ThreadLocalRandom random = ThreadLocalRandom.current();
-		final long msb = random.nextLong();
-		final long lsb = random.nextLong();
+		final long msb = LazyHolder.random();
+		final long lsb = LazyHolder.random();
 		return version(msb, lsb, 4);
 	}
 
@@ -309,9 +312,12 @@ public static GUID v5(GUID namespace, String name) {
 	 * @return a GUID
 	 */
 	public static GUID v6() {
+
 		final long time = gregorian();
+
 		final long msb = ((time & ~MASK_12) << 4) | (time & MASK_12);
-		final long lsb = ThreadLocalRandom.current().nextLong() | MULTICAST;
+		final long lsb = LazyHolder.random() | MULTICAST;
+
 		return version(msb, lsb, 6);
 	}
 
@@ -327,10 +333,12 @@ public static GUID v6() {
 	 * @return a GUID
 	 */
 	public static GUID v7() {
+
 		final long time = System.currentTimeMillis();
-		ThreadLocalRandom random = ThreadLocalRandom.current();
-		final long msb = (time << 16) | (random.nextLong() & MASK_16);
-		final long lsb = random.nextLong();
+
+		final long msb = (time << 16) | (LazyHolder.random() & MASK_16);
+		final long lsb = LazyHolder.random();
+
 		return version(msb, lsb, 7);
 	}
 
@@ -476,6 +484,22 @@ private static long gregorian() {
 		return time;
 	}
 
+	private static class LazyHolder {
+
+		// The JVM unique number tries to mitigate the fact that the thread
+		// local random is not seeded with a secure random seed by default.
+		// Their seeds are based on temporal data and predefined constants.
+		// Although the seeds are unique per JVM, they are not across JVMs.
+		// It helps to generate different sequences of numbers even if two
+		// ThreadLocalRandom are by chance instantiated with the same seed.
+		// Of course it doesn't better the output, but doesn't hurt either.
+		static final long JVM_UNIQUE_NUMBER = new SecureRandom().nextLong();
+
+		private static long random() {
+			return ThreadLocalRandom.current().nextLong() ^ JVM_UNIQUE_NUMBER;
+		}
+	}
+
 	private static GUID hash(int version, String algorithm, GUID namespace, String name) {
 
 		Objects.requireNonNull(name, "Null name");