Schedulers in Java

Scheduler is a mechanism that supports executing of tasks periodically or at certain periods of time, without need to run tasks manually. For example, sometimes in an application we need to run some analysis every one hour, or we need to send an email notification every Monday at 08:00 AM, but we don’t want to be responsible for running these tasks each time. Using a scheduler would help implement these kinds of requirements.

In this Tech Bite will be illustrated Java Spring Boot support for scheduling tasks that is simple to implement even in the smallest applications.

1. Enable support for scheduling

Scheduling support will be simply enabled by adding annotation @EnableScheduling to the main class:

@SpringBootApplication
@EnableScheduling
public class SchedulerApplication {

    public static void main(String[] args) {

        SpringApplication.run(SchedulerApplication.class, args);

    }
}

2. Add scheduler service

Scheduler service class will hold scheduled methods. These methods will perform tasks that need to be executed in a certain period of time or periodically and have to be annotated with @Scheduled

In this case, method printCurrentTime() will print current time in the console each time that task is executed.

@Component
public class SchedulerService {

    @Scheduled
    public void printCurrentTime() {
        LocalTime currentTime = LocalDateTime.now().toLocalTime();

        System.out.println("Current time is: " + currentTime);
    }
}

3. Schedule task

Final step is to properly set attributes of @Scheduledannotation, depending on when we want a task to run. If we want task to run every second we set:

@Scheduled(fixedRate = 1000)

This annotation also supports cron expressions, so if we want to schedule task every Monday at 08:00 AM we set:

@Scheduled(cron = “0 0 8 ? * MON *”)

More about each attribute can be found in official documentation.

4. Read attributes from configuration file (optional)

When we want to change configuration for a scheduler, usually we don’t want to redeploy the entire application. So, good practice for values that are configuring schedulers would be to put them in the application properties file. In that case we can change configuration easily.

@Scheduled(fixedRateString = "${project.scheduler.fixedRate}")

or

@Scheduled(cron = "${project.scheduler.cronExpression}")