Which one is recommended: Instant.now().toEpochMilli() or System.currentTimeMillis()

Question

In Java, we can have many different ways to get the current timestamp, but which one is recommended: Instant.now().toEpochMilli() or System.currentTimeMillis()

Answer 1

Both are fine. And neither is recommended except for a minority of purposes.

What do you need milliseconds since the epoch for?

In Java, we can have many different ways to get the current timestamp,

For current timestamp just use Instant.now(). No need to convert to milliseconds.

Many methods from the first years of Java, also many in the standard library, took a long number of milliseconds since the epoch as argument. However, today I would consider that old-fashioned. See if you can find — or create — or more modern method that takes for instance an Instant as argument instead. Go object-oriented and don’t use a primitive long. It will make your code clearer and more self-explanatory.

As Eliott Frisch said in a comment, if this is for measuring elapsed time, you may prefer the higher resolution of System.nanoTime().

If you do need milliseconds since the epoch

Assuming that you have good reasons for wanting a count of milliseconds since the epoch, …

which one is recommended: Instant.now().toEpochMilli() or System.currentTimeMillis()[?]

Opinions differ. Some will say that you should use java.time, the modern date and time API, for all of your date and time work. This would imply Instant here. Unsg java.time is generally a good habit since the date and time classes from Java 1.0 and 1.1 (Date, Calendar, TimeZone, DateFormat, SimpleDateFormat and others) are poorly designed and now long outdated, certainly not any that we should use anymore. On the other hand I am not aware of any design problem with System.curremtTimeMillis() in particular (except what I mentioned above about using a long count of milliseconds at all, which obviously is intrinsic to both Instant.now().toEpochMilli() and System.currentTimeMillis()).

If there is a slight performance difference between the two, I have a hard time imagining the situation where this will matter.

Take the option that you find more readable and less surprising in your context.

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Answer 2

As per my understanding Instant.now().toEpochMilli() is better as Java-8 onward usage of Instant has been recommended.

Also, it works based on timeline and instant represents a specific moment on that timeline.

In case of java.lang.System.currentTimeMillis() method it returns the current time in milliseconds. The granularity of the value depends on the underlying operating system and may be larger.

Hence, to be consistent altogether use Instant.

Answer 3

For what it's worth, I've done a quick non-ideal performance test comparing the two methods.

On my system (Ubuntu 20.04, OpenJDK 17.0.4), running System.currentTimeMillis ten million times takes cca 230ms while running Instant.now().toEpochMilli() ten million times takes cca 370ms.

import java.time.Instant;

public class A { 
   public static void main(String[] args) {
       long a = 0;
       long start = System.currentTimeMillis();
       for (int i = 0; i < 10_000_000; i++) {
           //a += Instant.now().toEpochMilli();
           a += System.currentTimeMillis();
       }
       System.out.println(a);
       System.out.println(System.currentTimeMillis() - start);
   }   
}

Answer 4

I want to add that System.nanoTime() is less about precision but more about accuracy.

System.currentTimeMillis() is based on the system clock, which is, most of the time, based on a quartz clock inside a computer. It is not accurate and it drifts. (VM is even worse since you don't have a physical clock and have to sync with the host) When your computer syncs this quartz clock with a global clock, you might even observe your clock jumps backward/forward because your local clock is too fast or slow.

On the other hand, System.nanoTime() is based on a monotonic clock. This clock has nothing to do with the actual time we humans speak. It only moves forward at a constant pace. It does not drift like the quartz clock and there is no sync required. This is why it is perfect for measuring elapses.