Concurrent calls to new Random() not random enough

[Doug Lea]

On 03/24/10 13:42, Xueming Shen wrote:
> 6937857: Concurrent calls to new Random() not random enough
>
> Martin Buchholz wrote:
>> [+fy, jeremymanson]
>>
>> Here's a much better test case,
>> and a proposed fix:
>>
>> http://cr.openjdk.java.net/~martin/webrevs/openjdk7/RandomSeedCollisions
>>
>> This adds some initialization overhead, but also removes some
>> since
>> new Random()
>> no longer invokes a synchronized method.

Sorry for the long delay in looking at this, but if
this is still in need of a reviewer OK, then I think
it is fine.

-Doug


>>
>> ----
>> import java.util.*;
>>
>> public class RandomSeed {
>> public static void main(String[] args) throws Throwable {
>> class RandomCollector implements Runnable {
>> long[] randoms = new long[1<<22];
>> 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);
>> }
>> }
>>
>>
>> On Tue, Mar 23, 2010 at 15:50, Martin Buchholz <martinrb at google.com>
>> wrote:
>>> Hi Sherman,
>>>
>>> This is a bug report (sorry, no fix this time)
>>>
>>> Synopsis: Concurrent calls to new Random() not random enough
>>> Description:
>>> new Random() promises this:
>>> /**
>>> * Creates a new random number generator. This constructor sets
>>> * the seed of the random number generator to a value very likely
>>> * to be distinct from any other invocation of this constructor.
>>> */
>>>
>>> but if there are concurrent calls to new Random(), it does not
>>> do very well at fulfilling its contract.
>>>
>>> The following program should print out a number much closer to 0.
>>>
>>> import java.util.*;
>>>
>>> public class RandomSeed {
>>> public static void main(String[] args) throws Throwable {
>>> class RandomCollector implements Runnable {
>>> int[] randoms = new int[1<<21];
>>> public void run() {
>>> for (int i = 0; i < randoms.length; i++)
>>> randoms[i] = new Random().nextInt();
>>> }};
>>> 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<Integer> s = new HashSet<Integer>();
>>> for (RandomCollector r : collectors) {
>>> for (int x : r.randoms) {
>>> if (s.contains(x))
>>> collisions++;
>>> s.add(x);
>>> }
>>> }
>>> System.out.println(collisions);
>>> }
>>> }
>>> ---
>>> ==> javac -source 1.6 -Xlint:all RandomSeed.java
>>> ==> java -esa -ea RandomSeed
>>> 876
>>>
>