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7071826: Avoid benign race condition in initialization of UUID

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Avoids mostly benign but sometimes expensive race condition on initialization of UUID.numberGenerator which is used by UUID.randomUUID()

Reviewed-by: alanb, chegar
This commit is contained in:
Mike Duigou 2012-05-11 11:31:46 -07:00
parent 6e335b3720
commit 3b77eb7680
2 changed files with 23 additions and 11 deletions
jdk
src/share/classes/java/util
UUID.java
test/java/util/UUID
UUIDTest.java

16
jdk/src/share/classes/java/util/UUID.java

@ -90,9 +90,11 @@ public final class UUID implements java.io.Serializable, Comparable<UUID> {
/*
* The random number generator used by this class to create random
* based UUIDs.
* based UUIDs. In a holder class to defer initialization until needed.
*/
private static volatile SecureRandom numberGenerator = null;
private static class Holder {
static final SecureRandom numberGenerator = new SecureRandom();
}
// Constructors and Factories
@ -137,10 +139,7 @@ public final class UUID implements java.io.Serializable, Comparable<UUID> {
* @return A randomly generated {@code UUID}
*/
public static UUID randomUUID() {
SecureRandom ng = numberGenerator;
if (ng == null) {
numberGenerator = ng = new SecureRandom();
}
SecureRandom ng = Holder.numberGenerator;
byte[] randomBytes = new byte[16];
ng.nextBytes(randomBytes);
@ -255,7 +254,8 @@ public final class UUID implements java.io.Serializable, Comparable<UUID> {
* The variant number has the following meaning:
* <p><ul>
* <li>0 Reserved for NCS backward compatibility
* <li>2 The Leach-Salz variant (used by this class)
* <li>2 <a href="http://www.ietf.org/rfc/rfc4122.txt">IETF&nbsp;RFC&nbsp;4122</a>
* (Leach-Salz), used by this class
* <li>6 Reserved, Microsoft Corporation backward compatibility
* <li>7 Reserved for future definition
* </ul>
@ -265,7 +265,7 @@ public final class UUID implements java.io.Serializable, Comparable<UUID> {
public int variant() {
// This field is composed of a varying number of bits.
// 0 - - Reserved for NCS backward compatibility
// 1 0 - The Leach-Salz variant (used by this class)
// 1 0 - The IETF aka Leach-Salz variant (used by this class)
// 1 1 0 Reserved, Microsoft backward compatibility
// 1 1 1 Reserved for future definition.
return (int) ((leastSigBits >>> (64 - (leastSigBits >>> 62)))

18
jdk/test/java/util/UUID/UUIDTest.java

@ -58,6 +58,12 @@ public class UUIDTest {
List list = new LinkedList();
for (int i=0; i<100; i++) {
UUID u1 = UUID.randomUUID();
if (4 != u1.version()) {
throw new Exception("bad version");
}
if (2 != u1.variant()) {
throw new Exception("bad variant");
}
if (list.contains(u1))
throw new Exception("random UUID collision very unlikely");
list.add(u1);
@ -70,10 +76,16 @@ public class UUIDTest {
List list = new LinkedList();
for (int i=0; i<100; i++) {
byteSource.nextBytes(someBytes);
UUID test = UUID.nameUUIDFromBytes(someBytes);
if (list.contains(test))
UUID u1 = UUID.nameUUIDFromBytes(someBytes);
if (3 != u1.version()) {
throw new Exception("bad version");
}
if (2 != u1.variant()) {
throw new Exception("bad variant");
}
if (list.contains(u1))
throw new Exception("byte UUID collision very unlikely");
list.add(test);
list.add(u1);
}
}