Scheduling tasks with Spring

Setup

Scheduling tasks with Spring is a core feature within the Spring framework. This means that you don’t have to add any additional dependencies (except spring-core itself).

You do have to enable this feature though. To do this, add the @EnableScheduling annotation to any configuration class. For example, you could add it to the main application class:

@EnableScheduling // Add this
@SpringBootApplication
public class MySpringBootApplication {

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

}

Using @Scheduled

The most common way to add scheduling to your application is by using the @Scheduled annotation. This annotation has a few properties you can configure to enable scheduling.

Using fixedRate

The fixedRate property allows you to configure an amount of milliseconds in between the start of each run. For example:

@Scheduled(fixedRate = 3600000)
public void method() {
    // TODO: Implement
}

In this case, the method will be invoked every hour, beginning an hour after the application was started.

Using fixedDelay

The fixedDelay property is similar to the fixedRate property as it allows you to provide a time in milliseconds. The difference between fixedRate and fixedDelay is that fixedDelay is the time calculated between the end of the previous run and the start of the next run.

@Scheduled(fixedDelay = 3600000)
public void method() {
    // TODO: Implement
}

For example, let’s say you start your application at 00:00 midnight and your job takes exactly 5 minutes to run. With fixedRate, the job would run at 01:00, 02:00, … while with fixedDelay the job would run at 01:00, 02:05, 03:10, … .

Using cron

The last option within the @Scheduled annotation is the cron property. This allows you to define a schedule as a cron expression. One important difference between crontab and the Spring cron expressions is that the latter also contains a field for seconds.

Some examples:

• To run a job every 30 seconds, you can use */30 * * * * *.
• To run a job every hour, you can use 0 0 * * * *.
• To run a job every night you can use 0 0 0 * * *.

Spring also supports some macros such as @yearly, @monthly, @weekly, @daily and @hourly. More information can be found in the API docs of the CronExpression class.

So if we want to replace our earlier example to run a method every hour, we could write:

@Scheduled(cron = "0 0 * * * *")
public void method() {
    // TODO: Implement
}

Setting an initial delay

Both fixedRate and fixedDelay only start X amount of time after you start the application. If you want to change this behaviour, you can configure an initial delay.

To do this, add the initialDelay property. For example, let’s say we want to run our method on startup and every hour after that, we could write:

@Scheduled(fixedRate = 3600000, initialDelay = 0)
public void method() {
    // TODO: Implement
}

Beware: The cron property does not support the initialDelay as these expressions are time-based and not relative towards when you started the application.

Using Spring’s Expression Language (SpEL)

Another nice feature is that you can use Spring’s Expression Language within the different properties of the @Scheduled annotation. To do this, you need to replace fixedRate, fixedDelay and initialDelay with fixedRateString, fixedDelayString and initialDelayString.

For example, if you want to get the fixedRate property from a bean property, you can use:

@Scheduled(fixedRateString = "#{@myBean.property}")
public void method() {
    // TODO: Implement
}

Spring’s Expression Language is also supported for the cron property. Since this property already expects a string, you don’t need to use a different property:

@Scheduled(cron = "#{@myBean.property}")
public void method() {
    // TODO: Implement
}

Using TaskScheduler

Another alternative to the @Scheduled annotation is to use the TaskScheduler. Autowire this class and call any of its methods such as scheduleWithFixedDelay() or scheduleWithFixedRate().

For example:

@Bean
public ScheduledFuture<?> schedule(TaskScheduler scheduler) {
    return scheduler.scheduleWithFixedRate(() -> {
        // TODO: Implement
    }, Duration.ofHours(1));    
}

The benefits of this approach are:

1. You can write the schedule logic programmatically.
2. You can work with the Duration class from the Java Time API, so you no longer have to calculate the delay in milliseconds.

The TaskScheduler also supports cron expressions by using the CronTrigger class:

@Bean
public ScheduledFuture<?> schedule(TaskScheduler scheduler) {
    Trigger hourlyTrigger = new CronTrigger("0 0 * * * *");
    return scheduler.schedule(() -> {
        // TODO: Implement
    }, hourlyTrigger);
}

Using application properties for scheduling

A question I see often is how you can configure the delay within your application properties. There are a few ways you can implement this.

First of all, we have to create a class using @ConfigurationProperties:

@ConfigurationProperties(prefix = "app")
public record AppProperties(Duration duration) {
    
}

The benefit of using @ConfigurationProperties is that it allows some nice conversions. For example, you can define the duration using:

app.duration=1h

Now we have to register these properties by adding the @EnableConfigurationProperties annotation to any configuration class or the main class:

@EnableConfigurationProperties(AppProperties.class) // Add this
@EnableScheduling // We added this before
@SpringBootApplication
public class MySpringBootApplication {

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

}

Now we have two options to schedule:

1. We can use Spring’s Expression Language (SpEL) in combination with @Scheduled,
2. Or we can use the TaskScheduler we mentioned before.

Option 1: Using @Scheduled

Since the AppProperties will be configured as a bean, you can now access the duration property using Spring’s Expression Language. The name of the bean is the fully qualified name of the properties class, including the package.

This means you can write something like this:

@Scheduled(fixedRateString = "#{@'com.xyz.my.pkg.AppProperties'.duration().toMillis()}")
public void method() {
    // TODO: Implement
}

The reason why we’re using these single quotes is to escape the dots in the package name. If we didn’t do this, Spring would look for a bean called com and call the xyz property on that bean.

Since fixedRateString expects the time in milliseconds, we call the toMillis() method of the Duration class which converts it for us.

Option 2: Using TaskScheduler

With TaskScheduler, we create a new bean and autowire TaskScheduler and AppProperties into our bean definition method:

@Bean
public ScheduledFuture<?> schedule(TaskScheduler scheduler, AppProperties properties) {
    return scheduler.scheduleWithFixedRate(() -> {
        // TODO: Implement
    }, properties.duration());
}

Personally, I like this approach because:

1. We don’t have to write code in a string like with Spring’s Expression Language.
2. We don’t have to include the whole package name in a string either.
3. The TaskScheduler natively supports Duration, and thus we don’t have to call the toMillis() method.

Conclusion

Summarized, you can schedule tasks with both the @Scheduled annotation and the TaskScheduler class. Both allow you to define your trigger as a fixed delay, a fixed rate or a cron expression. They also allow you to customize it programmatically, either by using Spring’s Epxression Language for the @Scheduled annotation or programmatically by using the TaskScheduler class.