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

Brian Burkhalter brian.burkhalter at oracle.com
Wed Aug 21 23:37:52 UTC 2013

With respect to this issue


the code of concern from java.lang.Math is

701    private static Random randomNumberGenerator;
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.



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