ScheduledExecutorService wait for task to complete

Question

I am creating a new thread to check a folder for new files then sleeping for a defined period of time.

My preference would be to use the ScheduledExecutorService, however, I can't find any documentation to clarify if this waits for the currently running task to complete before starting a new one.

For example, in the following code;

Integer samplingInterval = 30;
ScheduledExecutorService executorService = new ScheduledThreadPoolExecutor(10);
executorService.scheduleAtFixedRate(new WatchAgent(agentInfo), 
                                    0, 
                                    samplingInterval, 
                                    TimeUnit.SECONDS);

If the run() of WatchAgent takes longer than 30 seconds, will a new agent be created before it finishes?

Secondly, if I create an instance of WatchAgent, can I keep using the same instance for each periodic run?

Answer 1

Per the javadoc of scheduleAtFixedRate:

If any execution of this task takes longer than its period, then subsequent executions may start late, but will not concurrently execute.

scheduleAtFixedRate by definition takes a single Runnable instance. There is no way for you to provide different instances for each invocation of run. So to answer your question, the same instance will always be used for each periodic run.

Answer 2

Try this test code below.

1) Yes, it seems to wait until run() is finished.

2) Yes, seems it is using the same instance's run method (the one you passed in when you called scheduleAtFixedRate; this makes perfect sense btw).

import java.util.Date;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit;


public class Test006 {

    public static void main(String[] args) {
        Object agentInfo = null;
        Integer samplingInterval = 30;
        ScheduledExecutorService executorService = new ScheduledThreadPoolExecutor(10);
        executorService.scheduleAtFixedRate(new WatchAgent(agentInfo), 0, samplingInterval, TimeUnit.SECONDS);
    }

}


class WatchAgent implements Runnable {

    public WatchAgent(Object info){

    }

    public void run(){
        try{
            System.out.println("Running " + this.hashCode() + " - started on/at " + (new Date()));
            Thread.sleep(60000);
            System.out.println("Running " + this.hashCode() + " - finished on/at " + (new Date()));
        }catch(Exception ex){
            ex.printStackTrace();
        }
    }
}

Output:

Running 1322575120 - started on/at Mon Jul 08 19:58:41 IST 2019
Running 1322575120 - finished on/at Mon Jul 08 19:59:41 IST 2019
Running 1322575120 - started on/at Mon Jul 08 19:59:41 IST 2019
Running 1322575120 - finished on/at Mon Jul 08 20:00:41 IST 2019
Running 1322575120 - started on/at Mon Jul 08 20:00:41 IST 2019
...