Using Executors in Java to run code repeatedly at given intervals
problem
You wish to run a piece of code in your Java application repeatedly on a given interval, for example every 10 minutes. In this post we will see how to achieve this by using ExecutorService provided by Java.
SOLUTION
Java offers Executor and ExecutorService classes for achieving this from the java.util.concurrent package. Moreover, we want a Task class to be executed repeatedly by the executor service. Below we will make a Timer app that will print the current time every second.
ScheduledExecutorService scheduledExecutorService = Executors.newSingleThreadScheduledExecutor();
long delay = 0; //delay to start execution
long period = 1; //periodic interval, i.e. 1 second
TimeUnit timeUnit = TimeUnit.SECONDS;
PrintTimeTask printTimeTask = new PrintTimeTask();
ScheduledFuture<?> scheduledFuture = scheduledExecutorService.scheduleAtFixedRate(printTimeTask, delay, period, timeUnit);
import java.time.LocalTime;
public class PrintTimeTask implements Runnable {
@Override
public void run() {
System.out.println(LocalTime.now());
}
}
run() method: add code that prints the current time.
Let’s put the code in a Runner class with a main method so it can be executed. The complete code:
import java.util.concurrent.*;
public class Runner {
static void main() {
ScheduledExecutorService scheduledExecutorService = Executors.newSingleThreadScheduledExecutor();
long delay = 5; //initial delay to start execution. i.e. wait for 5 seconds
long period = 1; //periodic interval, i.e. repeat every 1 second
TimeUnit timeUnit = TimeUnit.SECONDS;
System.out.println("Starting in 5 seconds");
PrintTimeTask printTimeTask = new PrintTimeTask();
ScheduledFuture<?> scheduledFuture = scheduledExecutorService.scheduleAtFixedRate(printTimeTask, delay, period, timeUnit);
}
}
In the code, I added an initial delay of 5 seconds. The program executes and 5 seconds later, the executor service will invoke the run() method. Then, repeat every second.
running
I executed it via IntelliJ
output
It periodically executes the run() method in the PrintTimeTask object. It does not stop executing in this case.
Create a new Task to stop it
In order to stop the executor service from our program, we need to shut it down. Also, the scheduledFuture needs to be cancelled.
StopTimerTask< class with the same approach to PrintTimeTask, implementing Runnable. This task will accept 2 objects as constructor parameters: the executorService and scheduledFuture.
import java.util.concurrent.ExecutorService;
import java.util.concurrent.ScheduledFuture;
public class StopTimerTask implements Runnable {
private final ExecutorService executorService;
private final ScheduledFuture<?> scheduledFuture;
public StopTimerTask(ExecutorService executorService, ScheduledFuture<?> scheduledFuture) {
this.executorService = executorService;
this.scheduledFuture = scheduledFuture;
}
@Override
public void run() {
System.out.println("Stopping timer task and service!");
scheduledFuture.cancel(true);
executorService.shutdown();
}
}
In this case, passing the 2 parameters in the constructor, it's called dependency injection (via constructor).
Then, we initialize them to class fields and have them available in the run() method in which we will invoke the cancel and shutdown methods on the objects respectively.
Then expect it will stop gracefully executing the PrintTimeTask and exit the program.
Schedule a new Task with delay
Now, let’s schedule this new task using the predefined scheduler service. This time, we will use the schedule() method and not the scheduleAtFixedRate() method. This will start the StopTimerTask after an initial delay and it will not repeat. I set the delay to 7 seconds, meaning, the PrintTimerTask will be stopped after 7 seconds by the StopTimerTask.
long delayToStop = 7;
System.out.println("Stopping after 7 seconds");
scheduledExecutorService.schedule(new StopTimerTask(scheduledExecutorService, scheduledFuture), delayToStop, timeUnit);
running
Recompile and Re – run it in IntelliJ
The complete Runner class code
import java.util.concurrent.*;
public class Runner {
static void main() {
ScheduledExecutorService scheduledExecutorService = Executors.newSingleThreadScheduledExecutor();
long delay = 0; //no initial delay
long period = 1; //periodic interval, i.e. repeat every 1 second
TimeUnit timeUnit = TimeUnit.SECONDS;
System.out.println("Starting..");
PrintTimeTask printTimeTask = new PrintTimeTask();
ScheduledFuture<?> scheduledFuture = scheduledExecutorService.scheduleAtFixedRate(printTimeTask, delay, period, timeUnit);
long delayToStop = 7;
System.out.println("Stopping after 7 seconds");
scheduledExecutorService.schedule(new StopTimerTask(scheduledExecutorService, scheduledFuture), delayToStop, timeUnit);
}
}
delay for PrintTimeTask for this example so it starts immediately printing the local time and stops after 7 seconds. If the initial delay was left at 5 seconds, then it would only print the time for 2 seconds. output
Correct . We can see the process finishes with exit code 0 as expected!
conclusion
In this post we saw how to use the Executors and in particular the ScheduledExecutorService. Methods used:
- To run periodically:
scheduleAtFixedRate() - To run once:
schedule()
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Ellion
Professional IT consultant, writer, programmer enthusiast interested in all sorts of coding.
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