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

[Mike Duigou]

On Aug 21 2013, at 17:01 , David M. Lloyd wrote:

> 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();
>    }
> 
> -- 
> - DML


David's suggestion seems very reasonable.


Mike