Creating a unique timestamp in Java

Question

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 1

While searching for a solution I came across ULIB (Universally Unique Lexicographically Sortable Identifier) https://github.com/huxi/sulky/tree/master/sulky-ulid/

It's not a long, but shorter then UUID.

A ULID:

• Is compatible with UUID/GUID's 1.21e+24 unique ULIDs per millisecond (1,208,925,819,614,629,174,706,176 to be exact)
• Lexicographically sortable
• Canonically encoded as a 26 character string, as opposed to the 36 character UUID
• Uses Crockford's base32 for better efficiency and readability (5 bits per character)
• Case insensitive
• No special characters (URL safe)

Answer 2

You could use System.nanoTime(), which is the most precise available system timer, and divide that by million to get milliseconds. While there are no formal guarantees on how often it's updated, I believe it's reasonable to assume that it updates way more (order(s) of magnitude) frequently than once per millisecond. Of course, if you create integer timestamps by less than millisecond interval, then they can't all be unique.

Note that the absolute value nanoTime() is arbitrary. If you want absolute time, calibrate it somehow, i.e. compare it to currentTimeMillis() when starting.

Answer 3

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. ;)

Answer 4

You can use System.nanoTime() for better accuracy

Although I tried below and each time it gives different values, it probably is not guaranteed to be unique all the time.

public static void main(String[] args) {
        long time1 = System.nanoTime();
        long time2 = System.nanoTime();
        long time3 = System.nanoTime();
        System.out.println(time1);
        System.out.println(time2);
        System.out.println(time3);
    }

Another way is to use AtomicInteger/AtomicLong classes for unique numbers if the time is not important for you and you just need unique number, this probably is a btter choice.

Answer 5

Could you perhaps make use of java.util.UUID and it's timestamp() and clockSequence()?

Method Summary
    int clockSequence() 
        The clock sequence value associated with this UUID.
    long timestamp() 
        The timestamp value associated with this UUID.

More details here: http://docs.oracle.com/javase/1.5.0/docs/api/java/util/UUID.html