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Extension codestart

This guide explains how to create and configure a Quarkus Codestart for an extension.


"Extension Codestarts" is the name we give to our Quarkus extension "getting started" code generation system. It aims to provide a personalized getting started experience with Quarkus. A Quarkus extension is able to provide one or more well-defined codestarts which will contain the resources and code required/recommended to start using that particular extension.

Extension codestarts are applied by default when using the Quarkus tooling (if the chosen extensions contain any):

  • (find the extensions tagged with [code])

  • The Quarkus Maven plugin:

    mvn io.quarkus.platform:quarkus-maven-plugin:create
  • The Quarkus CLI:

    quarkus create app

How it works

When starting a project, you choose the language, the build tool, the framework, then you add Dockerfiles, CI, dependencies and code.

Codestarts are working the same way when contributing to the generation of a project, they are split in two categories:

The "Base" codestarts (you choose a combination of those):

  • project: The project skeleton (e.g. a Quarkus project)

  • buildtool: The build tool (e.g. Maven, Gradle, Gradle with Kotlin DSL)

  • language: The coding language (e.g. Java, Kotlin, Scala)

  • config: The config type (e.g. yaml, properties)

Extra codestarts (as much as wanted, added on top of the base ones):

  • tooling: Anything that can be added to improve the project (e.g. Dockerfiles, GitHub Actions workflows)

  • code: Any Quarkus extension can provide starter code. The user can decide to activate it or not.

Each codestart consists of:

  1. A codestart unique name, ie my-codestart

  2. A directory for the codestart files, ie my-codestart/

  3. A codestart.yml file

  4. Optionally some templates that are following a common structure and naming conventions

Where are the Quarkus Extension Codestarts located

  • In the Quarkus core repository, the extension codestarts are all in the same module.

  • Quarkus REST (formerly RESTEasy Reactive), RESTEasy and Spring Web extension codestarts are part of the base codestarts.

  • For other extensions, the codestart will typically be located in the runtime module (with special instruction in the pom.xml to generate a separate codestart artifact).

Base codestarts

The base codestarts contains templates to create project, buildtool, languages, config and tooling files.

In addition, Quarkus also provides the following ways to initialize a new extension project with a Codestart:


To create a new extension with a Codestart skeleton provide the --codestart flag to the create extension command:

quarkus create extension --codestart org.acme:greeting-extension
Detected layout type is 'standalone'
Generated runtime artifactId is 'greeting-extension'

applying codestarts...
📚  java
🔨  maven
📦  quarkus-extension
🚀  devmode-test
🚀  extension-base
🚀  extension-codestart
🚀  integration-tests
🚀  unit-test

 👍  extension has been successfully generated in:
--> /Users/.../greeting-extension
Navigate into this directory and get started: quarkus build

For more information about how to install the Quarkus CLI and use it, please refer to the Quarkus CLI guide.


Quarkus provides the create-extension Maven Mojo to initialize an extension project.

To generate a new extension with a Codestart skeleton provide the -DwithCodestart flag to this Mojo:

mvn io.quarkus.platform:quarkus-maven-plugin:3.12.2:create-extension -N \
    -DgroupId=org.acme \
    -DextensionId=greeting-extension \
[INFO] --- quarkus-maven-plugin:3.12.2:create-extension (default-cli) @ standalone-pom ---

Detected layout type is 'standalone'
Generated runtime artifactId is 'greeting-extension'

applying codestarts...
📚  java
🔨  maven
📦  quarkus-extension
🚀  devmode-test
🚀  extension-base
🚀  extension-codestart
🚀  integration-tests
🚀  unit-test

 👍  extension has been successfully generated in:
--> /Users/.../greeting-extension
[INFO] ------------------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  1.638 s
[INFO] Finished at: 2022-10-24T21:27:51+02:00
[INFO] ------------------------------------------------------------------------

Writing an Extension Codestart

Here is a step-by-step guide to write an extension codestart. You may also watch the Quarkus Insight #99 with a live-coding session.

As was mentioned previously, the base project files (pom.xml, Dockerfiles, …​) are already generated by the base codestarts provided by the Quarkus core. Thanks to this, we can only focus on the starter code specific to the extension.

Let’s take io.quarkiverse.aloha:quarkus-aloha as an example extension GAV (don’t look for this extension, it doesn’t exist).

The code

A Codestart is a template for scaffolding new project.

In this tutorial a Codestart project is created from a Quarkus project and adding the needed templates.

Therefore, go to, create a new project with the aloha extension and org.acme as Group (i.e org.acme placeholder for package name). Prepare a nice starter. It should not include any business logic, instead, it should contain some stub data/hello world that compiles and gives an overview of how to use the extension. The idea is to bring code that is the most common starting point for the extension.

Happy with the code? Let’s make a Codestart out of it.

The Codestart (Quarkiverse or Standalone extensions)

In your extension:

  • Create the runtime/src/main/codestarts/quarkus/aloha-codestart directory

  • Move the src/main/java from your generated project to runtime/src/main/codestarts/quarkus/aloha-codestart/java/src/main/java

  • (Optional) Move the config using this convention: application config application.yml.

  • Create a codestart.yml file in runtime/src/main/codestarts/quarkus/aloha-codestart:

    name: aloha-codestart
    ref: aloha
    type: code
    tags: extension-codestart
      title: Aloha
      description: Start to code with the Aloha extension.
      path: /aloha # (optional) for web extensions providing HTTP resources
  • Add the Maven build plugin configuration in runtime/pom.xml (to generate the codestart artifact: /target/quarkus-aloha-VERSION-codestarts.jar):

  • Add the codestart binding in the extension metadata runtime/src/main/resources/META-INF/quarkus-extension.yaml. Without this, your codestart won’t be added when your extension is picked:

    name: ...
    description: ...
        name: "aloha"
        - "java"
        artifact: "io.quarkiverse.aloha:quarkus-aloha:codestarts:jar:${project.version}"
  • Add the readme section template in base/

    {#include readme-header /}
  • Run mvn clean install in the extension root (or just runtime).

  • Now we can check that the codestart actually works by creating a project consuming our extension (make sure the snapshot version is correct):

    quarkus create app aloha-app -x=io.quarkiverse.aloha:quarkus-aloha:999-SNAPSHOT
    applying codestarts...
    📚  java
    🔨  maven
    📦  quarkus
    📝  config-properties
    🔧  dockerfiles
    🔧  maven-wrapper
    🚀  aloha-codestart <<<<<<<<<<<<<<<<


  • Add this dependency to the integration-tests:

  • Create a AlohaCodestartTest in the integration-tests:

    public class AlohaCodestartTest {
        public static QuarkusCodestartTest codestartTest = QuarkusCodestartTest.builder()
        void testContent() throws Throwable {
        void buildAllProjects() throws Throwable {

Ir más allá

  • If the extension provides some web resources, add the base/src/main/resources/META-INF/resources/index.entry.qute.html template (index.html and web extension codestarts).

  • Add another language (it is recommended to provide Java and Kotlin).

  • You may add some other resources (in the ./base directory if they are not language-specific).

Extensions codestarts in Quarkus Core

  • The codestarts are all grouped in a specific module.

  • No extra Maven configuration is needed.

  • The extension metadata references the artifact containing all the core codestarts.

  • The tests are also grouped. You don’t need to test the build as there is a specific grouped test for it. e.g.:

    public class ConfigYamlCodestartTest {
        public static QuarkusCodestartTest codestartTest = QuarkusCodestartTest.builder()
                .languages(JAVA, KOTLIN)
        void testContent() throws Throwable {
            codestartTest.assertThatGeneratedFileMatchSnapshot(JAVA, "src/main/resources/application.yml");
        @EnabledIfSystemProperty(named = "build-projects", matches = "true")
        void buildAllProjectsForLocalUse() throws Throwable {

Specific topics

org.acme placeholder for package name

You have to use org.acme as the package name in your extension codestart sources. In the generated project, the user specified package (or Group) will be used (and automatically replace org.acme).

The package will be automatically replaced in all the source files (.java, .kt, .scala). The package directory will also be automatically adjusted. If for some reason, another type of file needs the user package name then you should use a Templates (Qute) for it and the {project.package-name} data placeholder (find an example in the grpc proto file).


# codestart unique name
name: resteasy-example
# codestart reference, use the extension id
ref: resteasy
# use 'code' (other types are for base codestarts)
type: code
# use 'extension-codestart'
tags: extension-codestart
# public metadata for this example (accessible as data in the templates e.g. {title})
  title: RESTEasy Jakarta REST example
  description: Rest is easy peasy with this Hello World RESTEasy resource.
  # (optional) use this in web extensions with a specific path (and also add the index page)
  path: /some-path

Directory Structure

codestart.yml is the only required file.
  • codestart.yml must be at the root of the codestart

  • ./base contains all the files that will be processed independently of the specified language

  • ./[java/kotlin/scala] contains all the files that will be processed only if the specified language has been selected (overriding base)

Dynamic Config Keys in Codestart

gen-info.time = generation time (in milliseconds)
input.selected-extensions[].name|description|guide = list of selected extensions with info
input.selected-extensions-ga = Set of Strings containing the list of extensions groupId:artifactId, useful for dynamic codestarts depending on selected extensions
input.provided-code[].name|tags|title|description|related-guide: list of selected codestarts with info

Static Config Keys in Codestart = BOM groupId
quarkus.platform.artifact-id = BOM artifactId
quarkus.platform.version = BOM version = Project groupId
project.artifact-id = Project artifactId
project.version = Project version = Project name (if specified)
project.description = Project description (if specified)
project.package-name = Project package name = Quarkus Maven plugin groupId
quarkus.maven-plugin.artifact-id = Quarkus Maven plugin artifactId
quarkus.maven-plugin.version = Quarkus Maven plugin version = Quarkus Gradle pluginId
quarkus.gradle-plugin.version = Quarkus Gradle plugin version
quarkus.version = Quarkus version
java.version = Java version
kotlin.version = Kotlin version
scala.version = Scala version
scala-maven-plugin.version = Scala Maven plugin version
maven-compiler-plugin.version = Maven compiler plugin version
maven-surefire-plugin.version = Maven Surefire plugin version

Naming Convention for files

  • .tpl.qute will be processed with Qute and can use data (.tpl.qute will be removed from the output file name).

  • certain common files, such as, src/main/resources/application.yml, src/main/resources/META-INF/resources/index.html are generated from the collected fragments found in the selected codestarts for the project

  • other files are copied.

Templates (Qute)

Codestarts may use Qute templates for dynamic rendering.

Those templates are able to use data which contains:

  • The data (and public metadata) of the codestart to generate (specified in the codestart.yml)

  • A merge of the shared-data from the all the codestarts used to generate the project

  • The user input

  • Some dynamically generated data (e.g. dependencies and test-dependencies)

You may add a or in the base directory, it will be appended to the others. So just add the info relative to your extension codestart.


{#include readme-header /}

[Optionally, Here you may add information about how to use the example, settings, ...]
The {#include readme-header /} will use a template located in the Quarkus project codestart which displays standard info from the codestart.yml metadata.

application config application.yml

As a convention, you should always provide the Quarkus configuration as a yaml file (base/src/main/resources/application.yml).

It is going to be:

  • merged with the other extension codestarts configs

  • automatically converted to the selected config type (yaml or properties) at generation time depending on the selected extensions

index.html and web extension codestarts

Extension codestarts may provide a snippet for the generated index.html by adding this file:


{#include index-entry /}
The {#include index-entry /} will use a template located in the Quarkus project codestart which displays standard info from the codestart.yml metadata.

Integration test

An extension is available to help test extension codestarts QuarkusCodestartTest:


It provides a way to test:

  • the generated project content (with immutable mocked data) using snapshot testing

  • the generated project build/run (with real data) with helpers to run the build

Before all the tests, the extension will generate Quarkus projects in the specified languages with the given codestart using mocked data and real data. You can find those generated projects in the target/quarkus-codestart-test directory. You can open the real-data ones in your IDE or play with them using the terminal. The real data is the easiest way to iterate on your extension codestart development.

The extension provides helpers to test that the projects build buildAllProjects or just a specific language project buildProject(Language language). It also provides helpers to test the content with Snapshot testing.

The ConfigYamlCodestartTest is a good example in Quarkus core.

Snapshot testing

Snapshot testing is a way to make sure the content generated by a test doesn’t change from one revision to another, i.e. between commits. That means, the generated content for each commit needs to be immutable and deterministic (this is the reason for using mocked data). To be able to perform such checks, we auto-generate snapshots of the generated content and commit them as the references of the expected output for subsequent test runs. When the templates change, we also commit the induced snapshots changes. This way, during the review, we can make sure the applied code changes have the expected effects on the generated output.

The extension provides helpers to check the content:

  • checkGeneratedSource() validate a class against the snapshots for all languages (or a specific one).

  • checkGeneratedTestSource() validate a test class against the snapshots for all languages (or a specific one).

  • assertThatGeneratedFileMatchSnapshot() check a project file against the snapshot.

  • You can use AbstractPathAssert.satisfies(checkContains("some content")) or any Path assert on the return of the methods above to also check the file contains a specific content.

  • assertThatGeneratedTreeMatchSnapshots() lets you compare the project file structure (tree) for a specific language against its snapshot.

In order to first generate or update existing snapshots files on your local filesystem, you need to add -Dsnap when running the tests locally while developing the codestart. They need to be added as part of the commit, else the tests will not pass on the CI.

Writing tips

  • Your extension codestart must/should be independent of buildtool and dockerfiles.

  • Extension codestarts should be able to work alongside each other without interference (in combination).

  • Make sure your class names are unique across all extension codestarts.

  • Only use org.acme as package name.

  • Use a unique path /[unique] for your REST paths

  • Write the config in yml src/main/resources/application.yml.

    It is going to be merged with the other codestarts config and automatically converted to the selected config type (yaml or properties).

  • You can start with java and add kotlin later in another PR (create an issue so you don’t forget).

  • If you have a question, ping me @ia3andy on

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