提交 db9b00b4 编写于 作者: M martin

6937857: Concurrent calls to new Random() not random enough

Summary: seed uniquifier should use an independent PRNG
Reviewed-by: dl
上级 6c26198a
......@@ -86,8 +86,23 @@ class Random implements java.io.Serializable {
* the seed of the random number generator to a value very likely
* to be distinct from any other invocation of this constructor.
*/
public Random() { this(++seedUniquifier + System.nanoTime()); }
private static volatile long seedUniquifier = 8682522807148012L;
public Random() {
this(seedUniquifier() ^ System.nanoTime());
}
private static long seedUniquifier() {
// L'Ecuyer, "Tables of Linear Congruential Generators of
// Different Sizes and Good Lattice Structure", 1999
for (;;) {
long current = seedUniquifier.get();
long next = current * 181783497276652981L;
if (seedUniquifier.compareAndSet(current, next))
return next;
}
}
private static final AtomicLong seedUniquifier
= new AtomicLong(8682522807148012L);
/**
* Creates a new random number generator using a single {@code long} seed.
......@@ -103,8 +118,11 @@ class Random implements java.io.Serializable {
* @see #setSeed(long)
*/
public Random(long seed) {
this.seed = new AtomicLong(0L);
setSeed(seed);
this.seed = new AtomicLong(initialScramble(seed));
}
private static long initialScramble(long seed) {
return (seed ^ multiplier) & mask;
}
/**
......@@ -127,8 +145,7 @@ class Random implements java.io.Serializable {
* @param seed the initial seed
*/
synchronized public void setSeed(long seed) {
seed = (seed ^ multiplier) & mask;
this.seed.set(seed);
this.seed.set(initialScramble(seed));
haveNextNextGaussian = false;
}
......
......@@ -33,18 +33,54 @@
/*
* @test
* @bug 4949279
* @bug 4949279 6937857
* @summary Independent instantiations of Random() have distinct seeds.
*/
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Random;
public class DistinctSeeds {
public static void main(String[] args) throws Exception {
// Strictly speaking, it is possible for these to randomly fail,
// but the probability should be *extremely* small (< 2**-63).
// but the probability should be small (approximately 2**-48).
if (new Random().nextLong() == new Random().nextLong() ||
new Random().nextLong() == new Random().nextLong())
throw new RuntimeException("Random() seeds not unique.");
// Now try generating seeds concurrently
class RandomCollector implements Runnable {
long[] randoms = new long[1<<17];
public void run() {
for (int i = 0; i < randoms.length; i++)
randoms[i] = new Random().nextLong();
}
}
final int threadCount = 2;
List<RandomCollector> collectors = new ArrayList<RandomCollector>();
List<Thread> threads = new ArrayList<Thread>();
for (int i = 0; i < threadCount; i++) {
RandomCollector r = new RandomCollector();
collectors.add(r);
threads.add(new Thread(r));
}
for (Thread thread : threads)
thread.start();
for (Thread thread : threads)
thread.join();
int collisions = 0;
HashSet<Long> s = new HashSet<Long>();
for (RandomCollector r : collectors) {
for (long x : r.randoms) {
if (s.contains(x))
collisions++;
s.add(x);
}
}
System.out.printf("collisions=%d%n", collisions);
if (collisions > 10)
throw new Error("too many collisions");
}
}
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