scheduleAtFixedRate vs scheduleWithFixedDelay

Question

What\'s the main difference between scheduleAtFixedRate and scheduleWithFixedDelay methods of ScheduledExecutorService?

scheduler.s         


        

Answer 1

Let's write a simple program:

import java.util.concurrent.Executors
import java.util.concurrent.TimeUnit

var time = 0L
var start = System.currentTimeMillis()
val executor = Executors.newScheduledThreadPool(1)
executor.scheduleWithFixedDelay({
    if (time >= 12_000L) {
        executor.shutdown()
    } else {
        Thread.sleep(2000L)
        val now = System.currentTimeMillis()
        time += now - start
        System.out.println("Total $time delay ${now - start}\n")
        start = now
    }
}, 0L, 1000L, TimeUnit.MILLISECONDS)

And see the results:

| scheduleWithFixedDelay |   scheduleAtFixedRate  |
|:----------------------:|:----------------------:|
| Total 2001 delay 2001  | Total 2003 delay 2003  |
| Total 5002 delay 3001  | Total 4004 delay 2001  |
| Total 8003 delay 3001  | Total 6004 delay 2000  |
| Total 11003 delay 3000 | Total 8004 delay 2000  |
| Total 14003 delay 3000 | Total 10005 delay 2001 |
|          ---           | Total 12005 delay 2000 |

NOTICE the execution time is bigger than waiting

scheduleWithFixedDelay keeps delay
scheduleAtFixedRate removes delay