JDK 8 RFC 6470700: Math.random() / Math.initRNG() uses "double checked locking"

[David M. Lloyd]

On 8/21/13 5:37 PM, Brian Burkhalter wrote:
> With respect to this issue
>
> http://bugs.sun.com/view_bug.do?bug_id=6470700
>
> the code of concern from java.lang.Math is
>
> 701    private static Random randomNumberGenerator;
> 702
> 703    private static synchronized Random initRNG() {
> 704        Random rnd = randomNumberGenerator;
> 705        return (rnd == null) ? (randomNumberGenerator = new Random()) : rnd;
> 706     }
>
> 731    public static double random() {
> 732        Random rnd = randomNumberGenerator;
> 733        if (rnd == null) rnd = initRNG();
> 734        return rnd.nextDouble();
> 735    }
>
> where the class variable "randomNumberGenerator" is not used anywhere else in the class. The complaint is that this is an instance of the broken double-checked locking pattern. While at first glance this might appear to be the case, it does not seem so to me. It looks more like an attempt to avoid calling a synchronized method if "randomNumberGenerator" has already been initialized.
>
> A more typical pattern would be
>
>      private static Random randomNumberGenerator;
>
>      private static synchronized Random getRNG() {
>          if (randomNumberGenerator == null) {
>              randomNumberGenerator = new Random();
>          }
>          return randomNumberGenerator;
>      }
>
>      public static double random() {
>          return getRNG().nextDouble();
>      }
>
> Comments, please.

I don't think you'd want to introduce the overhead of synchronization 
here.  It may be better in this case to use this kind of lazy init pattern:

     static final class Holder {
         static final Random RNG = new Random();
     }

     public static double random() {
         return Holder.RNG.nextDouble();
     }

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