How to schedule a cron job in spring boot without using @Scheduled() annotation

Question

In spring boot, can I schedule a spring job by not using @Scheduled annotation to a method?

I am working with spring job in the spring boot. I want to schedule a job by using cron expression, but without using @Scheduled(cron = " ") annotation to the method.

I know that I can schedule a job inside this method as below.

@Scheduled (cron = "0 10 10 10 * ?")

public void execute() { 

   / * some job code * /

}

But I want it to be dynamic so that I can take a cron expression as input from the user and schedule it.

Answer 1

I came up with a working example since I found your question interesting and have been interested in this problem before. It's based entirely on the source code so I have no idea if it comes close to following best practice. Nonetheless, you may be able to tune it to your needs. FYI, you don't necessarily need to create a new ScheduledTaskRegistrar object - I figured that since your objective is a dynamic scheduler, you wouldn't be interested in defining your tasks purely in the overwritten method.

@SpringBootApplication
public class TaskScheduler implements SchedulingConfigurer, CommandLineRunner {

    public static void main(String[] args){SpringApplication.run(TaskScheduler.class, args);}

    List<CronTask> cronTasks;

    @Override
    public void run(String... args) throws Exception {
        CronTask task = this.createCronTask(new Runnable() {
            @Override
            public void run() {
                System.out.println(LocalDateTime.now());
            }
        }, "1/10 * * * * *");

        ScheduledTaskRegistrar taskRegistrar = new ScheduledTaskRegistrar();
        taskRegistrar.addCronTask(task);
        configureTasks(taskRegistrar);
        Thread.sleep(51);

        taskRegistrar.destroy();
        taskRegistrar = null;

        ScheduledTaskRegistrar taskRegistrar2 = new ScheduledTaskRegistrar();
        taskRegistrar2.addCronTask(task);
        configureTasks(taskRegistrar2);

    }

    @Override
    public void configureTasks(ScheduledTaskRegistrar taskRegistrar) {
        // "Calls scheduleTasks() at bean construction time" - docs
        taskRegistrar.afterPropertiesSet();
    }

    public CronTask createCronTask(Runnable action, String expression) {
        return new CronTask(action, new CronTrigger(expression));
    }

}

I have experience using cron jobs in Azure and other places. Programming in Java, I have typically used @Scheduled with fixed times just for the sake of simplicity. Hope this is useful to you though.