Schedule periodic tasks inside your Micronaut applications

Learn how to schedule periodic tasks inside your Micronaut microservices.

Authors: Sergio del Amo

Micronaut Version: 3.1.0

1. Getting Started

In this guide, we will create a Micronaut application written in Kotlin.

Nowadays it is pretty usual to have some kind of cron or scheduled task that needs to run every midnight, every hour, a few times a week,…​

In this guide you will learn how to use Micronaut capabilities to schedule periodic tasks inside a Micronaut microservice.

2. What you will need

To complete this guide, you will need the following:

  • Some time on your hands

  • A decent text editor or IDE

  • JDK 1.8 or greater installed with JAVA_HOME configured appropriately

3. Solution

We recommend that you follow the instructions in the next sections and create the application step by step. However, you can go right to the completed example.

4. Writing the Application

Create an application using the Micronaut Command Line Interface or with Micronaut Launch.

mn create-app example.micronaut.micronautguide --build=maven --lang=kotlin
If you don’t specify the --build argument, Gradle is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.

The previous command creates a Micronaut application with the default package example.micronaut in a directory named micronautguide.

4.1. Enable annotation Processing

If you use Java or Kotlin and IntelliJ IDEA, make sure to enable annotation processing.

annotationprocessorsintellij

4.2. Set log level INFO for package demo

In this guide, we will use several log statements to show Job execution.

Add this statement to the end of logback.xml:

src/main/resources/logback.xml
    <logger name="example.micronaut" level="INFO"/>

The above line configures a logger for package example.micronaut with log level INFO.

4.3. Creating a Job

Create a file and add the following:

src/main/kotlin/example/micronaut/HelloWorldJob.kt
package example.micronaut

import io.micronaut.scheduling.annotation.Scheduled
import org.slf4j.LoggerFactory
import java.text.SimpleDateFormat
import java.util.Date
import jakarta.inject.Singleton

@Singleton (1)
class HelloWorldJob {

    @Scheduled(fixedDelay = "10s") (3)
    fun executeEveryTen() {
        LOG.info("Simple Job every 10 seconds: {}", SimpleDateFormat("dd/M/yyyy hh:mm:ss").format(Date()))
    }

    @Scheduled(fixedDelay = "45s", initialDelay = "5s") (4)
    fun executeEveryFourtyFive() {
        LOG.info("Simple Job every 45 seconds: {}", SimpleDateFormat("dd/M/yyyy hh:mm:ss").format(Date()))
    }

    companion object {
        private val LOG = LoggerFactory.getLogger(HelloWorldJob::class.java) (2)
    }
}
1 Use jakarta.inject.Singleton to designate a class as a singleton.
2 Inject a Logger.
3 Create trigger every 10 seconds
4 Create another trigger every 45 seconds with an initial delay of 5 seconds (5000 millis)

Now start the application. Execute the ./mvnw mn:run command, which will start the application on port 8080.

After a few seconds, you will see the following output:

... Simple Job every 10 seconds :15/5/2018 12:48:02 (1)
... Simple Job every 45 seconds :15/5/2018 12:48:07 (2)
... Simple Job every 10 seconds :15/5/2018 12:48:12 (3)
... Simple Job every 10 seconds :15/5/2018 12:48:22
... Simple Job every 10 seconds :15/5/2018 12:48:32
... Simple Job every 10 seconds :15/5/2018 12:48:42
... Simple Job every 45 seconds :15/5/2018 12:48:52 (4)
... Simple Job every 10 seconds :15/5/2018 12:48:52
1 First execution of 10 seconds Job after the application starts
2 The 45 seconds Job starts 5 seconds after the application starts
3 Second execution of 10 seconds Job 10 seconds after the first execution
4 Second execution of 45 seconds Job 45 seconds after the first execution

4.4. Business logic in dedicated Use Cases

Although the previous example is valid, usually you don’t want to put your business logic in a Job. A better approach is to create an additional bean that the Job invokes. This approach decouples your business logic from the scheduling logic. Moreover, it facilitates testing and maintenance. Let’s see an example:

Create the following use case:

src/main/kotlin/example/micronaut/EmailUseCase.kt
package example.micronaut

import org.slf4j.LoggerFactory
import java.text.SimpleDateFormat
import java.util.Date
import jakarta.inject.Singleton

@Singleton
class EmailUseCase {

    fun send(user: String, message: String) {
        LOG.info("Sending email to {} : {} at {}", user, message, SimpleDateFormat("dd/M/yyyy hh:mm:ss").format(Date()))
    }

    companion object {
        private val LOG = LoggerFactory.getLogger(EmailUseCase::class.java)
    }
}

And then the Job:

src/main/kotlin/example/micronaut/DailyEmailJob.kt
package example.micronaut

import io.micronaut.scheduling.annotation.Scheduled
import jakarta.inject.Singleton

@Singleton (1)
class DailyEmailJob(protected val emailUseCase: EmailUseCase) { (2)

    @Scheduled(cron = "0 30 4 1/1 * ?") (3)
    fun execute() {
        emailUseCase.send("john.doe@micronaut.example", "Test Message") (4)
    }
}
1 Use jakarta.inject.Singleton to designate a class as a singleton.
2 Constructor injection.
3 Trigger the Job once a day at 04:30 AM
4 Call the injected use case

4.5. Scheduling a Job Manually

Consider the following scenario. You want to send every user an email two hours after they register on your application and ask them about their experiences during this first interaction.

For this guide, we will schedule a Job to trigger after one minute.

To test it, we will call a new use case named RegisterUseCase twice when the application starts.

Modify Application.kt:

src/main/kotlin/example/micronaut/Application.kt
package example.micronaut

import io.micronaut.context.event.ApplicationEventListener
import io.micronaut.runtime.Micronaut.build
import io.micronaut.runtime.server.event.ServerStartupEvent
import jakarta.inject.Singleton

fun main(args: Array<String>) {
    build()
        .args(*args)
        .packages("example.micronaut")
        .start()
}

@Singleton (1)
class Application(val registerUseCase: RegisterUseCase) (2)
    : ApplicationEventListener<ServerStartupEvent> { (3)

    override fun onApplicationEvent(event: ServerStartupEvent) {  (4)
        try {
            registerUseCase.register("harry@micronaut.example")
            Thread.sleep(20000)
            registerUseCase.register("ron@micronaut.example")
        } catch (e: InterruptedException) {
            e.printStackTrace()
        }
    }
}
1 Use jakarta.inject.Singleton to designate a class as a singleton.
2 Listen to the event ServerStartupEvent
3 Constructor injection of RegisterUseCase
4 onApplicationEvent is invoked when the application starts

As you see in the previous example, subscribing to an event is as easy as implementing ApplicationEventListener.

Create a runnable task EmailTask.kt

src/main/kotlin/example/micronaut/EmailTask.kt
package example.micronaut

class EmailTask(private val emailUseCase: EmailUseCase, private val email: String, private val message: String) : Runnable {

    override fun run() {
        emailUseCase.send(email, message)
    }

}

Create RegisterUseCase.kt, which schedules the previous task.

src/main/kotlin/example/micronaut/RegisterUseCase.kt
package example.micronaut

import io.micronaut.scheduling.TaskExecutors
import io.micronaut.scheduling.TaskScheduler
import org.slf4j.LoggerFactory
import java.text.SimpleDateFormat
import java.time.Duration
import java.util.Date
import jakarta.inject.Named
import jakarta.inject.Singleton

@Singleton
class RegisterUseCase(private val emailUseCase: EmailUseCase, (1)
                      @param:Named(TaskExecutors.SCHEDULED) private val taskScheduler: TaskScheduler) { (2)

    fun register(email: String) {
        LOG.info("saving {} at {}", email, SimpleDateFormat("dd/M/yyyy hh:mm:ss").format(Date()))
        scheduleFollowupEmail(email, "Welcome to the Micronaut framework")
    }

    private fun scheduleFollowupEmail(email: String, message: String) {
        val task = EmailTask(emailUseCase, email, message) (3)
        taskScheduler.schedule(Duration.ofMinutes(1), task)  (4)
    }

    companion object {
        private val LOG = LoggerFactory.getLogger(RegisterUseCase::class.java)
    }
}
1 Constructor injection of EmailUseCase
2 Inject the TaskScheduler bean
3 Create a Runnable task
4 Schedule the task to run a minute from now

If you execute the above code, you will see the Job being executed one minute after we schedule it and with the supplied email address.

INFO  example.micronaut.RegisterUseCase - saving harry@micronaut.example at 15/5/2018 06:25:14
INFO  example.micronaut.RegisterUseCase - saving ron@micronaut.example at 15/5/2018 06:25:34
INFO  example.micronaut.EmailUseCase - Sending email to harry@micronaut.example : Welcome to Micronaut at 15/5/2018 06:26:14
INFO  example.micronaut.EmailUseCase - Sending email to ron@micronaut.example : Welcome to the Micronaut framework at 15/5/2018 06:26:34

5. Summary

During this guide, we learned how to configure Jobs using the @Scheduled annotation using fixedDelay, initialDelay, and cron, as well as manually configuring Jobs with a TaskScheduler and tasks.

6. Generate a Micronaut Application Native Image with GraalVM

We will use GraalVM, the polyglot embeddable virtual machine, to generate a native image of our Micronaut application.

Compiling native images ahead of time with GraalVM improves startup time and reduces the memory footprint of JVM-based applications.

Only Java and Kotlin projects support using GraalVM’s native-image tool. Groovy relies heavily on reflection, which is only partially supported by GraalVM.

6.1. Native image generation

The easiest way to install GraalVM is to use SDKMan.io.

Java 11
$ sdk install java 21.3.0.r11-grl
Java 17
$ sdk install java 21.3.0.r17-grl
If you still use Java 8 use the GraalVM JDK11 version.

You need to install the native-image component, which is not installed by default.

$ gu install native-image

To generate a native image using Maven run:

$ ./mvnw package -Dpackaging=native-image

The native image is created in target/application and can be run with ./target/application.

Execute the native image, and you will see the log message of the Jobs appear:

$ 10:16:20.803 [pool-2-thread-1] INFO  example.micronaut.HelloWorldJob - Simple Job every 10 seconds: 09/12/2019 10:16:20
$ 10:16:20.806 [main] INFO  example.micronaut.RegisterUseCase - saving harry@micronaut.example at 09/12/2019 10:16:20
$ 10:16:25.804 [pool-2-thread-2] INFO  example.micronaut.HelloWorldJob - Simple Job every 45 seconds: 09/12/2019 10:16:25
$ 10:16:30.807 [pool-2-thread-4] INFO  example.micronaut.HelloWorldJob - Simple Job every 10 seconds: 09/12/2019 10:16:30

7. Next steps

Explore more features with Micronaut Guides.

8. Help with the Micronaut Framework

Object Computing, Inc. (OCI) sponsored the creation of this Guide. A variety of consulting and support services are available.