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 * @key randomness 39 */ 40 41 package test.java.util.Random; 42 43 import java.util.ArrayList; 44 import java.util.HashSet; 45 import java.util.List; 46 import java.util.Random; 47 48 public class DistinctSeeds { main(String[] args)49 public static void main(String[] args) throws Exception { 50 // Strictly speaking, it is possible for these to randomly fail, 51 // but the probability should be small (approximately 2**-48). 52 if (new Random().nextLong() == new Random().nextLong() || 53 new Random().nextLong() == new Random().nextLong()) 54 throw new RuntimeException("Random() seeds not unique."); 55 56 // Now try generating seeds concurrently 57 class RandomCollector implements Runnable { 58 long[] randoms = new long[1<<17]; 59 public void run() { 60 for (int i = 0; i < randoms.length; i++) 61 randoms[i] = new Random().nextLong(); 62 } 63 } 64 final int threadCount = 2; 65 List<RandomCollector> collectors = new ArrayList<>(); 66 List<Thread> threads = new ArrayList<>(); 67 for (int i = 0; i < threadCount; i++) { 68 RandomCollector r = new RandomCollector(); 69 collectors.add(r); 70 threads.add(new Thread(r)); 71 } 72 for (Thread thread : threads) 73 thread.start(); 74 for (Thread thread : threads) 75 thread.join(); 76 int collisions = 0; 77 HashSet<Long> s = new HashSet<>(); 78 for (RandomCollector r : collectors) { 79 for (long x : r.randoms) { 80 if (s.contains(x)) 81 collisions++; 82 s.add(x); 83 } 84 } 85 System.out.printf("collisions=%d%n", collisions); 86 if (collisions > 10) 87 throw new Error("too many collisions"); 88 } 89 } 90