Deploy to Google App Engine

Learn how to deploy a Micronaut application to Google App Engine Java Flexible Environment

Authors: Sergio del Amo

Micronaut Version: 3.2.0

1. Getting Started

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

You will deploy a Micronaut application to Google App Engine flexible environment.

2. Costs

This guide uses paid services; you may need to enable Billing in Google Cloud to complete some steps in this guide.

3. 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

4. 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.

5. Writing the Application

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

mn create-app example.micronaut.micronautguide --build=gradle --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.

5.1. Enable annotation Processing

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


5.2. Controller

To respond "Hello World" for the route GET /hello, create a controller:

package example.micronaut

import io.micronaut.http.MediaType
import io.micronaut.http.annotation.Controller
import io.micronaut.http.annotation.Get
import io.micronaut.http.annotation.Produces

@Controller("/hello") (1)
 class HelloController {

     @Produces(MediaType.TEXT_PLAIN) (3)
     fun index(): String {
         return "Hello World" (4)
1 The class is defined as a controller with the @Controller annotation mapped to the path /hello
2 The @Get annotation maps the index method to an HTTP GET request on /hello
3 By default a Micronaut response uses application/json as Content-Type. We are returning a String not a JSON object, so we set it to text/plain.
4 A String "Hello World" is returned as the result

5.3. Test

Create a test to verify that when you make a GET request to /hello you get Hello World as a response:

package example.micronaut

 import io.micronaut.http.HttpRequest
 import io.micronaut.http.client.HttpClient
 import io.micronaut.http.client.annotation.Client
 import io.micronaut.test.extensions.junit5.annotation.MicronautTest
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertNotNull
 import org.junit.jupiter.api.Test
 import jakarta.inject.Inject

 @MicronautTest (1)
 class HelloControllerTest {
     @field:Client("/")  (2)
     lateinit var client : HttpClient

     fun testHello() {
         val request: HttpRequest<Any> = HttpRequest.GET("/hello")  (3)
         val body = client.toBlocking().retrieve(request)
         assertEquals("Hello World", body)
1 Annotate the class with @MicronautTest so the Micronaut framework will initialize the application context and the embedded server. More info.
2 Inject the HttpClient bean and point it to the embedded server.
3 Creating HTTP Requests is easy thanks to the Micronaut framework fluid API.

6. Testing the Application

To run the tests:

$ ./gradlew test
$ open build/reports/tests/test/index.html

7. Google Cloud Platform

7.1. Google Cloud Platform Project

Create a new project:

gcp project

We named the project micronaut-guides

7.2. Cloud SDK

Install Cloud SDK for your operating system.

Cloud SDK includes the gcloud command-line tool. Run the `init command in your terminal:

$ gcloud init

It will prompt you to select the Google account and the project which you want to use.

8. Google App Engine

We are going to deploy the Micronaut application developed in this guide to the Google App Engine flexible environment

App Engine allows developers to focus on doing what they do best: writing code. Based on Google Compute Engine, the App Engine flexible environment automatically scales your app up and down while balancing the load. Microservices, authorization, SQL and NoSQL databases, traffic splitting, logging, versioning, security scanning, and content delivery networks are all supported natively.

Run the command:

gcloud app create

to initialize an App Engine application within the current Google Cloud project.

You will need to choose the region where you want your App Engine Application located.

9. App Engine Build Plugins

To deploy to App Engine, add the Google App Engine Gradle Plugin to your project.

Since the appengine-gradle-plugin is not published to the gradle plugin portal, you must refrence it from the Central Maven repository. Add the maven central resolution to your settings.gradle

pluginManagement {
  repositories {
    // add mavenLocal() if you are using a locally built version of the plugin
  resolutionStrategy {
    eachPlugin {
      if ('')) {

Apply the plugin in your plugins block in your build.gradle

plugins {
  id '' version '2.4.2'

Configure the deployment properties:

appengine {
    stage.artifact = "${buildDir}/libs/${}-${project.version}-all.jar"
    deploy {
        projectId = "changethis" // Project ID from Google Cloud Dashboard
        version = "1"

10. Application Deployment Configuration

To deploy to Google App Engine, add the file src/main/appengine/app.yaml

Then configure the Java 8 runtime in the flexible environment

The Java 8 runtime does not include any web-serving framework. The only requirement is that your app should listen and respond on port 8080. The sample code here shows how you can add your own framework, such as Spring Boot, to the Java 8 runtime.

This describes the application’s deployment configuration:

runtime: java
env: flex

11. App Engine Deploy

You will need to use JDK 8.

java {
    sourceCompatibility = JavaVersion.toVersion("1.8")
    targetCompatibility = JavaVersion.toVersion("1.8")

To deploy the app to Google App Engine run:

$ ./gradlew appengineDeploy

Initial deployment may take a while. When finished, you will be able to access your app:

Deployed service [default] to []

You can stream logs from the command line by running:
  $ gcloud app logs tail -s default

To view your application in the web browser, run:
  $ gcloud app browse

If you go to the Versions section in the App Engine administration panel, you will see the deployed app.

12. Running the App

curl -i
HTTP/2 200
date: Mon, 04 Oct 2021 05:29:04 GMT
content-type: text/plain
content-length: 11
via: 1.1 google
alt-svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000,h3-T051=":443"; ma=2592000,h3-Q050=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,quic=":443"; ma=2592000; v="46,43"

Hello World

13. Logging

For the version you would like to inspect, select Logs in the diagnose dropdown:

google cloud logs

Application log messages written to stdout and stderr are automatically collected and can be viewed in the Logs Viewer.

Check Writing Application Logs documentation to read more about logs in the flexible environment.

14. Cleaning Up

After you’ve finished this guide, you can clean up the resources you created on Google Cloud Platform so you won’t be billed for them in the future. The following sections describe how to delete or turn off these resources.

14.1. Deleting the project

The easiest way to eliminate billing is to delete the project you created for the tutorial.

To delete the project:

Deleting a project has the following consequences:
  • If you used an existing project, you’ll also delete any other work you’ve done in the project.

  • You can’t reuse the project ID of a deleted project. If you created a custom project ID that you plan to use in the future, you should delete the resources inside the project instead. This ensures that URLs that use the project ID, such as an URL, remain available.

  • If you are exploring multiple tutorials and quickstarts, reusing projects instead of deleting them prevents you from exceeding project quota limits.

In the Cloud Platform Console, go to the Projects page.

In the project list, select the project you want to delete and click Delete project. After selecting the checkbox next to the project name, click Delete project

In the dialog, type the project ID, and then click Shut down to delete the project.

Deleting or turning off specific resources

You can individually delete or turn off some of the resources that you created during the tutorial.

14.2. Deleting app versions

To delete an app version:

In the Cloud Platform Console, go to the App Engine Versions page.

Click the checkbox next to the non-default app version you want to delete.

The only way you can delete the default version of your App Engine app is by deleting your project. However, you can stop the default version in the Cloud Platform Console. This action shuts down all instances associated with the version. You can restart these instances later if needed.

In the App Engine standard environment, you can stop the default version only if your app has manual or basic scaling.

Click the Delete button at the top of the page to delete the app version.

15. Next Steps

If you want to learn more about Google Cloud and Micronaut integration, check out the codelab Deploy a Micronaut Application Containerized with Jib to Google Kubernetes Engine.

16. Help with the Micronaut Framework

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