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title: Run a Kotlin Spring Boot app on the Google App Engine Flexible Environment description: Learn how to deploy a Kotlin Spring Boot app to the Google App Engine flexible environment. author: hhariri tags: App Engine, Kotlin, Spring Boot date_published: 2018-01-17

The Google App Engine flexible environment is an easy way to deploy your apps to the same infrastructure that powers Google's products. Using Kotlin and Spring Boot, in this tutorial you'll see how to deploy your application to App Engine.

You will create a new Spring Boot application, and then you will learn how to:

  • Deploy your application
  • Update your application

While the tutorial uses Kotlin 1.2 and Spring Boot 2 M7, other releases of Kotlin and Spring Boot should work without any modifications (other than version numbers in Maven files). This tutorial does assume you're familiar with Spring Boot and creating web applications. For simplicity the tutorial responds with JSON to a specific HTTP request, but can be built-on to connect to other Google services and/or databases.

Before you begin

Before running this tutorial, you must set up a Google Cloud Platform project, and you need to have Docker and the Google Cloud SDK installed.

Create a project that will host your Spring Boot application. You can also reuse an existing project.

  1. Use the Google Cloud Platform Console to create a new Cloud Platform project. Remember the project ID; you will need it later. Later commands in this tutorial will use [PROJECT_ID] as a substitution, so you might consider setting the PROJECT_ID environment variable in your shell.

  2. Enable billing for your project.

  3. Install the Google Cloud SDK. Make sure you initialize the SDK and set the default project to the new project you created.

    Version 175.0.0 or later of the SDK is required.

  4. Install JDK 8 or higher if you do not already have it.

  5. Install Maven.

Creating a new app and running it locally

In this section, you will create a new Spring Boot app and make sure it runs. If you already have an app to deploy, you can use it instead.

  1. Use start.spring.io to generate a Spring Boot application using Kotlin as the language, Maven as the build system. Alternatively, you can download the sample application.

  2. Download the generated project and save it to a local folder.

  3. Open the resulting project in your favourite IDE or editor and create a new source file named MessageController.kt with the following contents:

    package com.jetbrains.demo
    import org.springframework.web.bind.annotation.*
    data class Message(val text: String, val priority: String)
    class MessageController {
        fun message(): Message {
            return Message("Hello from Google Cloud", "High")

The package should match that of your group and artifact name.

  1. Make sure you have the right dependencies in your Maven file to import RestController as seen here.

  2. Run the application from the command line using Maven:

    mvn spring-boot:run
  3. Open the browser and make sure you get a valid JSON response when accessing http://localhost:8080/message. The result should be:

      "text": "Hello from Google Cloud",
      "priority": "High"

Deploy your application

To deploy your application, you will use an App Engine plugin for Maven which simplifies some of the process. The plugin is also available for Gradle.

  1. Create a file called app.yaml in a new folder src/main/appengine with the following contents:

    runtime: java
    env: flex
      jdk: openjdk8

    By specifying runtime: java, the runtime image gcr.io/google-appenine/openjdk:8 is automatically selected when you deploy a JAR (*.jar) file. The JDK version is also selected using the jdk field.

  2. Add the following plugin entry to the pom.xml file to configure the Maven plugin.

  3. Run the following command to deploy your app:

    mvn appengine:deploy

    If this is the first time you have deployed to App Engine in this project, gcloud will prompt you for a region. You may choose the default of "us-central", or select a region close to your location.

    Deployment will also take a few minutes to requisition and configure the needed resources, especially the first time you deploy.

    Note: If the command fails with Google Cloud SDK could not be found, make sure the environment variable GOOGLE_CLOUD_SDK_HOME is set to the root directory of where you installed the Google Cloud SDK.

  4. Once the deploy command has completed, you can run

    gcloud app browse

    to see your app running in production on App Engine in the browser. Not however that this application does not respond to the root endpoint. Once the browser is open with the correct URL, you need to append /message to it.

Update your application

Make a simple change and redeploy.

  1. Change the text returned from the message() function to

    "Hello from Google App Engine"

  2. Run the deployment command again:

    mvn appengine:deploy
  3. View your changes live by running:

    gcloud app browse

    Remember to add /message to the URL.

Clean up

After you've finished this tutorial, you can clean up the resources you created on Google Cloud Platform so you won't be billed for them in the future. To clean up the resources, you can delete the project or stop the App Engine service.

Deleting the project

The easiest way to eliminate billing is to delete the project you created for the tutorial. To do so using gcloud, run:

gcloud projects delete [PROJECT_ID]

where [PROJECT_ID] is your Google Cloud Platform project ID.

Warning: 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 appspot.com URL, remain available.

Stopping App Engine services

To disable an App Engine service:

  1. In the Cloud Platform Console, go to the App Engine Versions page.
  2. Make sure your project is selected. If necessary, pull down the project selection dropdown at the top, and choose the project you created for this tutorial.
  3. If you deployed to a service other than "default", make sure it is selected in the Service dropdown.
  4. Select all the versions you wish to disable and click Stop at the top of the page. This will free all of the Google Compute Engine resources used for this App Engine service.

Next steps

See the App Engine documentation for more information on App Engine features including scaling, health checks, and infrastructure customization.

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