Java multithreading. Same System.currentTimeMillis() values on different threads

Question

I want to use the result of System.currentTimeMillis() in the multithreaded appplication to generate unique IDs. This is my code:

    class MyRunnableClass implements Runnable{
        @Override
        public void run() {
            long thisValueShouldBeUnique = System.currentTimeMillis ();
        }
    }

I start 10 threads which run MyRunnableClass object run() method. Is it true, that sometimes the result of System.currentTimeMillis () in different threads may be the same?

highly possible, you may concate some thread identifier to your unique value — Anas EL KORCHI, Jul 13 '16 at 10:05
If you're the one starting the threads, why not give them a unique ID upon initialization? You can also use Thread.currentThread().getId() — D Levant, Jul 13 '16 at 10:13

score 6 · Accepted Answer · answered Jul 13 '16 at 10:02

6

The function System.currentTimeMillis() returns the current time in milliseconds. So if your threads run during the same millisecond, yes this function will return the same result. So you can generate UUIDs like that: UUID.randomUUID()

answered Jul 13 '16 at 10:02

Creart

146
1
10

1

or more simply an `atomicLong.getAndIncrement();` – assylias Jul 13 '16 at 10:16
Yes you can, if you want just a simple incrementable ID – Creart Jul 13 '16 at 14:37

Yoav Gur · Answer 2 · 2023-07-24T11:22:40.020

3

Today's CPUs can process around 100,000,000,000 instructions per second, so the answer is definitely "Yes, It is true that sometimes the result of System.currentTimeMillis() in different threads may be the same".

edited Jul 24 '23 at 11:22

answered Jul 13 '16 at 10:01

Yoav Gur

1,366
9
15

Java multithreading. Same System.currentTimeMillis() values on different threads

2 Answers2