Creating a unique timestamp in Java

java concurrency timestamp epoch throttling
Yrlec picture Yrlec · Feb 8, 2012 · Viewed 25.7k times · Source

I need to create a timestamp (in milliseconds) in Java that is guaranteed to be unique in that particular VM-instance. I.e. need some way to throttle the throughput of System.currentTimeMillis() so that it returns at most one results every ms. Any ideas on how to implement that?

Answer

Peter Lawrey picture Peter Lawrey · Feb 8, 2012

This will give a time as close the current time as possible without duplicates.

private static final AtomicLong LAST_TIME_MS = new AtomicLong();
public static long uniqueCurrentTimeMS() {
    long now = System.currentTimeMillis();
    while(true) {
        long lastTime = LAST_TIME_MS.get();
        if (lastTime >= now)
            now = lastTime+1;
        if (LAST_TIME_MS.compareAndSet(lastTime, now))
            return now;
    }
}

One way to avoid the limitation of one id per milli-second is to use a micro-second timestamp. i.e. multiply currentTimeMS by 1000. This will allow 1000 ids per milli-second.

Note: if time goes backwards, eg due to an NTP correction, the time will just progress at 1 milli-second per invocation until time catches up. ;)

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