Uploaded image for project: 'JDK'
  1. JDK
  2. JDK-7051516

ThreadLocalRandom seed is never initialized so all instances generate the same sequence

    XMLWordPrintable

Details

    • b94
    • 7
    • b01
    • generic
    • generic
    • Verified

    Backports

      Description

        From: "Aleksey Shipilev" <###@###.###>
        Date: Fri, 3 Jun 2011 00:29:12 +1000
        To: "concurrency-interest" <###@###.###>

        Hi,

        I've been stumbled upon ThreadLocalRandom seed behavior. JavaDoc
        reads: "ThreadLocalRandom is initialized with an internally generated
        seed that may not otherwise be modified." I would expect TLR called in
        several threads concurrently to have different global values. But
        apparently the internal seed in TLR always has the same seed (which is
        default value for long).

        Is this intentional? Or just oversight that should be fixed?

        I.e. I would expect my test [1] print all-different values per thread,
        like regular Random does.

        That's what happens now:

        Regular thread-local Random
        780
        4307
        9112
        4368
        6673
        ========
        4143
        4905
        2331
        154
        2887
        ========
        9586
        6161
        9948
        4179
        3608
        ========

        ThreadLocalRandom
        0
        6118
        1895
        7186
        7366
        ========
        0
        6118
        1895
        7186
        7366
        ========
        0
        6118
        1895
        7186
        7366
        ========

        Thanks,
        Aleksey.

        [1]
        import java.util.Random;
        import java.util.concurrent.ThreadLocalRandom;

        public class Main {

            public static ThreadLocal<Random> random = new ThreadLocal<Random>() {
                @Override
                protected Random initialValue() {
                    return new Random();
                }
            };

            public static void main(String[] args) throws InterruptedException {

                System.out.println("Regular thread-local Random");
                for (int i = 0; i < 3; i++) {
                    Thread t = new Thread(new Runnable() {
                        @Override
                        public void run() {
                            for (int c = 0; c < 5; c++) {
                                System.out.println(random.get().nextInt(10000));
                            }
                            System.out.println("========");
                        }
                    });
                    t.start();
                    t.join();
                }

                System.out.println("ThreadLocalRandom");
                for (int i = 0; i < 5; i++) {
                    Thread t = new Thread(new Runnable() {
                        @Override
                        public void run() {
                            for (int c = 0; c < 5; c++) {

        System.out.println(ThreadLocalRandom.current().nextLong(10000));
                            }
                            System.out.println("========");
                        }
                    });
                    t.start();
                    t.join();
                }

            }

        }

        Attachments

          Issue Links

            Activity

              People

                chegar Chris Hegarty
                dholmes David Holmes
                Votes:
                0 Vote for this issue
                Watchers:
                1 Start watching this issue

                Dates

                  Created:
                  Updated:
                  Resolved:
                  Imported:
                  Indexed: