1 /*
   2  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   3  *
   4  * This code is free software; you can redistribute it and/or modify it
   5  * under the terms of the GNU General Public License version 2 only, as
   6  * published by the Free Software Foundation.
   7  *
   8  * This code is distributed in the hope that it will be useful, but WITHOUT
   9  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  10  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  11  * version 2 for more details (a copy is included in the LICENSE file that
  12  * accompanied this code).
  13  *
  14  * You should have received a copy of the GNU General Public License version
  15  * 2 along with this work; if not, write to the Free Software Foundation,
  16  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  17  *
  18  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  19  * or visit www.oracle.com if you need additional information or have any
  20  * questions.
  21  */
  22 
  23 /*
  24  * This file is available under and governed by the GNU General Public
  25  * License version 2 only, as published by the Free Software Foundation.
  26  * However, the following notice accompanied the original version of this
  27  * file:
  28  *
  29  * Written by Doug Lea with assistance from members of JCP JSR-166
  30  * Expert Group and released to the public domain, as explained at
  31  * http://creativecommons.org/publicdomain/zero/1.0/
  32  */
  33 
  34 /*
  35  * @test
  36  * @bug 4949279 6937857
  37  * @summary Independent instantiations of Random() have distinct seeds.
  38  */
  39 
  40 import java.util.ArrayList;
  41 import java.util.HashSet;
  42 import java.util.List;
  43 import java.util.Random;
  44 
  45 public class DistinctSeeds {
  46     public static void main(String[] args) throws Exception {
  47         // Strictly speaking, it is possible for these to randomly fail,
  48         // but the probability should be small (approximately 2**-48).
  49         if (new Random().nextLong() == new Random().nextLong() ||
  50             new Random().nextLong() == new Random().nextLong())
  51             throw new RuntimeException("Random() seeds not unique.");
  52 
  53         // Now try generating seeds concurrently
  54         class RandomCollector implements Runnable {
  55             long[] randoms = new long[1<<17];
  56             public void run() {
  57                 for (int i = 0; i < randoms.length; i++)
  58                     randoms[i] = new Random().nextLong();
  59             }
  60         }
  61         final int threadCount = 2;
  62         List<RandomCollector> collectors = new ArrayList<>();
  63         List<Thread> threads = new ArrayList<Thread>();
  64         for (int i = 0; i < threadCount; i++) {
  65             RandomCollector r = new RandomCollector();
  66             collectors.add(r);
  67             threads.add(new Thread(r));
  68         }
  69         for (Thread thread : threads)
  70             thread.start();
  71         for (Thread thread : threads)
  72             thread.join();
  73         int collisions = 0;
  74         HashSet<Long> s = new HashSet<Long>();
  75         for (RandomCollector r : collectors) {
  76             for (long x : r.randoms) {
  77                 if (s.contains(x))
  78                     collisions++;
  79                 s.add(x);
  80             }
  81         }
  82         System.out.printf("collisions=%d%n", collisions);
  83         if (collisions > 10)
  84             throw new Error("too many collisions");
  85     }
  86 }