Table of Contents

Micronaut Gradle plugin

Version 3.0.0

1. What’s new

Note
 Documentation for the 2.x branch can be found here.

The 3.0.0 version of the plugin integrates with the official GraalVM plugin which introduces some breaking changes:

  • the nativeImage task is now replaced with nativeCompile

  • native image configuration happens in the graalvmNative DSL block

  • The GraalVM SDK being used depends on how you configure toolchains

Please refer to the native image section for more help: the toolchain selection mechanism might be surprising to you.

2. The Gradle plugins

A Gradle Plugin which makes development of Micronaut application and libraries a breeze.

This project currently consists of 3 plugins:

3. Quick Start

Template projects are available via Micronaut Launch for each language.

To get started you can use the Micronaut CLI:

$ mn create-app demo --lang java
$ mn create-app demo --lang groovy
$ mn create-app demo --lang kotlin

Or if you don’t have it installed via curl:

# for Java
$ curl https://launch.micronaut.io/demo.zip?lang=java \
  -o demo.zip && unzip demo.zip -d demo && cd demo
# for Groovy
$ curl https://launch.micronaut.io/demo.zip?lang=groovy \
  -o demo.zip && unzip demo.zip -d demo && cd demo
# for Kotlin
$ curl https://launch.micronaut.io/demo.zip?lang=kotlin \
  -o demo.zip && unzip demo.zip -d demo && cd demo

4. Micronaut Library Plugin

Applying the plugin
plugins {
  id 'io.micronaut.library' version '3.0.0'
}
plugins {
  id("io.micronaut.library") version "3.0.0"
}

The Micronaut library plugin applies the following modifications to the build:

The micronaut DSL can be used to configure how this behaves.

The minimum requirement is to set the Micronaut version to use. This can be done by setting micronautVersion in gradle.properties or as follows in build.gradle(.kts):

Selecting the Micronaut version
micronaut {
    version "3.1.1"
}
micronaut {
    version.set("3.1.1")
}

Complete example with the default settings:

Micronaut configuration options
micronaut {
    version "3.1.1"
    processing {
        // Sets whether incremental annotation processing is enabled
        incremental true
        // Sets the module name.
        // This should be the same as the artifactId in the POM
        module project.name
        // Sets the group.
        // This should be th same as the groupId in the POM
        group project.group
        // Sets the Java package names containing any custom Micronaut
        // meta annotations (new annotations annotated with say @Around).
        // Generally used only for advanced cases such as defining new AOP
        // advice. If omitted however, incremental annotation processing
        // will not work correctly
        annotations "com.example.*"
        // additional sourceSets can be configured here to apply the BOM
        // and annotation processors to source sets other than 'main'
        sourceSets(
             sourceSets.main
        )
    }
}
micronaut {
    version.set("3.1.1")
    processing {
        // Sets whether incremental annotation processing is enabled
        incremental.set(true)
        // Sets the module name.
        // This should be the same as the artifactId in the POM
        module.set(project.name)
        // Sets the group.
        // This should be th same as the groupId in the POM
        group.set(project.group)
        // Sets the Java package names containing any custom Micronaut
        // meta annotations (new annotations annotated with say @Around).
        // Generally used only for advanced cases such as defining new AOP
        // advice. If omitted however, incremental annotation processing
        // will not work correctly
        annotations.add("com.example.*")
        // additional sourceSets can be configured here to apply the BOM
        // and annotation processors to source sets other than 'main'
        sourceSets(
             sourceSets.findByName("main")
        )
    }
}
Note
 The Micronaut Library plugin also supports Groovy and Kotlin sources.

4.1. Kotlin Support

For Kotlin, the Kotlin jvm and kapt plugins must be configured:

Configuring Kotlin support
plugins {
    id "org.jetbrains.kotlin.jvm" version "1.5.30"
    id "org.jetbrains.kotlin.kapt" version "1.5.30"
    id "io.micronaut.library" version "3.0.0"
}
plugins {
    id("org.jetbrains.kotlin.jvm") version "1.5.30"
    id("org.jetbrains.kotlin.kapt") version "1.5.30"
    id("io.micronaut.library") version "3.0.0"
}

4.2. Minimal Build

With the io.micronaut.library plugin applied a minimal build to get started writing a library for Micronaut that written in Java and is tested with JUnit 5 looks like:

A minimal build file
plugins {
    id 'io.micronaut.library' version '3.0.0'
}

version "0.1"
group "com.example"

repositories {
    mavenCentral()
}

micronaut {
    version = "3.1.1"
}

dependencies {
    testImplementation("io.micronaut.test:micronaut-test-junit5")
    testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine")
}
plugins {
    id("io.micronaut.library") version "3.0.0"
}

version = "0.1"
group = "com.example"

repositories {
    mavenCentral()
}

micronaut {
    version.set("3.1.1")
}

dependencies {
    testImplementation("io.micronaut.test:micronaut-test-junit5")
    testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine")
}

5. Micronaut Application Plugin

Applying the Micronaut Application plugin
plugins {
  id "io.micronaut.application" version "3.0.0"
}
plugins {
  id("io.micronaut.application") version "3.0.0"
}

The Micronaut application plugin extends the Micronaut Library plugin and adds the following customizations:

  • Instead of the java-library plugin the plugin applies the Gradle application plugin

  • Applies the io.micronaut.graalvm plugin

  • Correctly configures Gradle for continuous build

The following additional tasks are provided by this plugin:

  • buildLayers - Builds application layers for use in a Docker container

  • dockerfile - Builds a Docker File for a Micronaut application

  • dockerBuild - Builds a Docker Image using the Docker Gradle plugin

  • dockerfileNative - Builds a Docker File for for GraalVM Native Image

  • dockerBuildNative - Builds a Native Docker Image using GraalVM Native Image

  • nativeCompile - Builds a GraalVM Native Image

  • testNativeImage (since 1.1.0) - Builds a GraalVM Native Image, starts the native server and runs tests against the server

  • dockerPush - Pushes a Docker Image to configured container registry

  • dockerPushNative - Pushes a Docker Image built with GraalVM Native Image to configured container registry

To run an application with continuous build use the run task with the -t parameter:

$ ./gradlew run -t

5.1. Minimal Build

With the io.micronaut.application plugin applied a minimal build to get started with a Micronaut server application that is written in Java and tested with JUnit 5 looks like:

plugins {
    id 'io.micronaut.application' version '3.0.0'
}

version "0.1"
group "com.example"

repositories {
    mavenCentral()
}

micronaut {
    version = "3.1.1"
}

dependencies {
    implementation("io.micronaut:micronaut-http-server-netty")
    runtimeOnly("ch.qos.logback:logback-classic")
    testImplementation("io.micronaut.test:micronaut-test-junit5")
    testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine")
}

application {
    mainClass = "example.Application"
}
plugins {
    id("io.micronaut.application") version "3.0.0"
}

version = "0.1"
group = "com.example"

repositories {
    mavenCentral()
}

micronaut {
    version.set("3.1.1")
}

dependencies {
    implementation("io.micronaut:micronaut-http-server-netty")
    runtimeOnly("ch.qos.logback:logback-classic")
    testImplementation("io.micronaut.test:micronaut-test-junit5")
    testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine")
}

application {
    mainClass.set("example.Application")
}

5.2. Kotlin Support

The most simple Kotlin build using a build.gradle(.kts) file looks like:

plugins {
    id "org.jetbrains.kotlin.jvm" version "1.5.30"
    id "org.jetbrains.kotlin.kapt" version "1.5.30"
    id "org.jetbrains.kotlin.plugin.allopen" version "1.5.30"
    id "io.micronaut.application" version "3.0.0"
}

version "0.1"
group "com.example"

repositories {
    mavenCentral()
}

micronaut {
    version = "3.1.1"
}

dependencies {
    implementation "io.micronaut:micronaut-http-server-netty"
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:1.5.30"
    implementation "org.jetbrains.kotlin:kotlin-reflect:1.5.30")
    runtimeOnly "ch.qos.logback:logback-classic")
    testImplementation("io.micronaut.test:micronaut-test-junit5")
    testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine")
}

application {
    mainClass = "example.ApplicationKt"
}
plugins {
    id("org.jetbrains.kotlin.jvm") version "1.5.30"
    id("org.jetbrains.kotlin.kapt") version "1.5.30"
    id("org.jetbrains.kotlin.plugin.allopen") version "1.5.30"
    id("io.micronaut.application") version "3.0.0"
}

version = "0.1"
group = "com.example"

repositories {
    mavenCentral()
}

micronaut {
    version.set("3.1.1")
}

dependencies {
    implementation("io.micronaut:micronaut-http-server-netty")
    implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8:1.5.30")
    implementation("org.jetbrains.kotlin:kotlin-reflect:1.5.30")
    runtimeOnly("ch.qos.logback:logback-classic")
    testImplementation("io.micronaut.test:micronaut-test-junit5")
    testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine")
}

application {
    mainClass.set("example.ApplicationKt")
}

5.3. GraalVM Native Image

Since version 3.0.0, the Micronaut plugins rely on the official GraalVM plugin to build native images.

Those plugins make use of the Gradle toolchains support, which means that the SDK which is used to build the native is decorrelated from the JVM which is used to launch Gradle itself. Said differently, you can run Gradle with OpenJDK, while still building native images using the GraalVM SDK.

The Micronaut Gradle plugin will automatically configure the toolchains support for you, but there are a few things that you should be aware of:

  • running Gradle with a GraalVM SDK doesn’t necessarily imply that Gradle will use the same SDK to build native images

  • Gradle will try to locate a compatible GraalVM toolchain to build images. You can tweak what GraalVM version to use by following the official documentation.

Important
 While the toolchain selection will properly select a GraalVM SDK which matches your language version requirements, it will not let you pick a particular GraalVM version (say, prefer 21.3 over 21.1). If your application depends on a specific GraalVM version, you will have to disable automatic detection like explained below.

If you have several GraalVM installations available, or that you want to disable the automatic toolchain recognition, we recommend that you do the following:

  • setup an environment variable named GRAALVM_HOME pointing to your GraalVM installation

  • edit your gradle.properties file to add the following options:

# Disable Gradle automatic download of Java SDKs
org.gradle.java.installations.auto-download=false
# Disable auto-detection of Java installations
org.gradle.java.installations.auto-detect=false
# Setup explicitly that the Java version to use
# should be the one from the JAVA_HOME environment variable
org.gradle.java.installations.fromEnv=JAVA_HOME

Alternatively you can pass those options from the command line:

./gradlew -Porg.gradle.java.installations.auto-download=false \
  -Porg.gradle.java.installations.auto-detect=false \
  -Porg.gradle.java.installations.fromEnv=JAVA_HOME \
  build

You can build a native image by running the following task:

$ ./gradlew nativeCompile

And you can run it by calling the following task:

$ ./gradlew nativeRun

You can tweak the native image options by configuring the graalvmNative extension as explained in the plugin documentation.

For example you can add options to the main image by doing:

graalvmNative {
    binaries {
        main {
            buildArgs << "-H:-DeleteLocalSymbols"
            buildArgs << "-H:+PreserveFramePointer"
        }
    }
}
graalvmNative {
    binaries {
        named("main") {
            buildArgs.add("-H:-DeleteLocalSymbols")
            buildArgs.add("-H:+PreserveFramePointer")
        }
    }
}
Important
 If you update an existing Micronaut application that contains the file src/main/resources/META-INF/native-image/xxxxx/native-image.properties, please make sure to delete the properties -H:Name and -H:Class from the file because they are managed automatically by the plugin.

5.3.1. Build "mostly static" native images

Since GraalVM 21.0 it is possible to create "mostly static" native images that can run in a distroless docker image. You only need to configure the appropriate baseImage and the plugin will automatically configure GraalVM:

tasks.named('dockerfileNative') {
    baseImage('gcr.io/distroless/cc-debian10')
}
tasks.named<io.micronaut.gradle.docker.NativeImageDockerfile>("dockerfileNative") {
    baseImage("gcr.io/distroless/cc-debian10")
}

In case you want to use another base image you need to set the appropriate GraalVM flag:

tasks.named('dockerfileNative') {
    baseImage(...)
    args('-H:+StaticExecutableWithDynamicLibC')
}
tasks.named<io.micronaut.gradle.docker.NativeImageDockerfile>("dockerfileNative") {
    baseImage(...)
    args("-H:+StaticExecutableWithDynamicLibC")
}

5.4. Testing Native Images

Note
 This feature is independent from the official GraalVM testing support, which actually runs a test suite within a native image. Micronaut native test support launches a JVM test suite against a native image server.

Since 1.1.x of the plugin, you can also use the testNativeImage task to start the Micronaut native server and run tests against it.

Important
 This feature only works in combination with micronaut-test-core versions 2.2.1 or above. Make sure your test classpath includes at least this version of Micronaut Test.

Using this task will replace the regular embedded server used for tests with the natively built executable:

./gradlew testNativeImage

It is important to note that there are some limitations to this approach in that the native server is no longer "embedded" in the test. This has the following implications:

  • It is not possible to mock components using @MockBean or replace beans using @Replaces since the native server starts in a separate process and beans injected into or defined by the test are no longer shared with the application under test since it is running in a separate process.

  • The native server starts with the test environment active, however the classpath of the application is the runtime classpath not the test classpath. This has the implication that certain testing features (like for example Testcontainers' usage of JDBC URLs to start containers) won’t work and you have to explicitly start any test containers in the test itself.

If you wish to split your native image tests from your regular tests you can create an additional source set for integration tests and the plugin will add an additional task suffixed with *NativeImage to run the native image tests, for example: gradle integrationTestNativeImage.

5.5. Docker Support

The Micronaut plugin includes integration with the Gradle Docker plugin allowing you to easily build applications and native images using Docker containers.

Applications are built as layered JARs using the buildLayers task ensuring optimized Docker images for Java applications.

To build a regular Java application into a Docker container that is ready to be deployed and exposes ports 8080 you can simply do:

$ ./gradlew dockerBuild

The default uses an openjdk:17-alpine base image, however you can easily switch the base image to use by using the baseImage property of the dockerfile task:

tasks.named("dockerfile") {
  baseImage = "oracle/graalvm-ce:20.3.0-java11"
}
tasks.named<MicronautDockerfile>("dockerfile") {
  baseImage.set("oracle/graalvm-ce:20.3.0-java11")
}

The above examples switches to use GraalVM CE 20.3.0 as a base image.

To build the application into a Native Image you can run:

$ ./gradlew dockerBuildNative

Note that for this to work you must build the application with the same GraalVM SDK as used to build the image.

To push the container to the currently configured container registry you can use either dockerPush or dockerPushNative for the native image:

$ ./gradlew dockerPush

To configure the image names to push you can use the images setting of the dockerBuild task.

For example the following configures dockerPush to use Oracle Container Registry:

tasks.named("dockerBuild") {
    images = ["eu-frankfurt-1.ocir.io/xyzzyz/repo/my-image:$project.version"]
}

tasks.named("dockerBuildNative") {
    images = ["eu-frankfurt-1.ocir.io/xyzzyz/repo/my-image-native:$project.version"]
}
tasks.named<DockerBuildImage>("dockerBuild") {
    images.add("eu-frankfurt-1.ocir.io/xyzzyz/repo/my-image:$project.version")
}

tasks.named<DockerBuildImage>("dockerBuildNative") {
    images.add("eu-frankfurt-1.ocir.io/xyzzyz/repo/my-image-native:$project.version")
}

Notice that you can supply two different image names to push to for the JVM version and the native version of the application.

If you wish to customize the docker builds that are used, the easiest way is to run ./gradlew dockerfile (or dockerfileNative for the native version) and copy the generated Dockerfile from build/docker to your root directory and modify as required.

If you wish to customize the JVM arguments or native image arguments then it is possible to do so with the args method of the dockerfile and dockerfileNative tasks:

tasks.named("dockerfile") {
   args("-Xmx128m")
}
tasks.named("dockerfileNative") {
   args("-Xmx64m")
}
tasks.named<MicronautDockerfile>("dockerfile") {
   args("-Xmx128m")
}
tasks.named<io.micronaut.gradle.docker.NativeImageDockerfile>("dockerfileNative") {
   args("-Xmx64m")
}

The above configuration uses a max heap setting of 128m for Java and 64m for native image for the application.

To add additional docker instructions to the generated Dockerfile, such as adding a HEALTHCHECK, you can do the following. The additional instructions will be added at the end of the Dockerfile just before the ENTRYPOINT.

tasks.named("dockerfile") {
 args("-Xmx128m")
 instruction """HEALTHCHECK CMD curl -s localhost:8090/health | grep '"status":"UP"' """
}
tasks.named("dockerfileNative") {
 args("-Xmx64m")
 instruction """HEALTHCHECK CMD curl -s localhost:8090/health | grep '"status":"UP"'"""
}
tasks.named<Dockerfile>("dockerfile") {
 args("-Xmx128m")
 instruction("""HEALTHCHECK CMD curl -s localhost:8090/health | grep '"status":"UP"' """)
}
tasks.named<io.micronaut.gradle.docker.NativeImageDockerfile>("dockerfileNative") {
 args("-Xmx64m")
 instruction("""HEALTHCHECK CMD curl -s localhost:8090/health | grep '"status":"UP"'""")
}

You can also add any of the other instructions/commands that the docker plugin supports, see the Dockerfile task documentation.

5.6. Micronaut Runtimes

A higher level concept of "runtimes" is included in the Micronaut Gradle plugin which essentially allows the plugin to decide which server runtime to include in the dependencies of the application when building the application. For example consider this minimal build:

plugins {
     id 'io.micronaut.application' version '3.0.0'
}
version "0.1"
group "com.example"

repositories {
    mavenCentral()
}

micronaut {
    version = "3.1.1"
    runtime "netty"
}

dependencies {
    runtimeOnly("ch.qos.logback:logback-classic")
}

application {
    mainClass = "example.Application"
}
plugins {
     id("io.micronaut.application") version "3.0.0"
}
version = "0.1"
group = "com.example"

repositories {
    mavenCentral()
}

micronaut {
    version.set("3.1.1")
    runtime.set("netty")
}

dependencies {
    runtimeOnly("ch.qos.logback:logback-classic")
}

application {
    mainClass = "example.Application"
}

Here the only dependency declared is on the logging framework to use however runtime is to netty resulting in an application that can be built and run.

If you wish to take the same and build or run it with a different runtime you can pass the micronaut.runtime property for the build. For example:

./gradlew run -Pmicronaut.runtime=google_function

The above example run the application as a Google Cloud Function.

The available runtimes are:

  • netty - A Netty server runtime

  • jetty - A Jetty server runtime

  • tomcat - A Tomcat server runtime

  • undertow - An Undertow server runtime

  • lambda - Allows building the application into an AWS Lambda

  • oracle_function - A Project.fn runtime for deploying Oracle Functions

  • google_function - A runtime for deploying Google Functions.

  • azure_function - A runtime for deploying Azure Functions

The advantage of allowing your dependencies to be dictated by the runtime is that you can potentially take the same application and deploy it to any of the above runtimes without changes.

5.6.1. Deploying to AWS Lambda as GraalVM native image

If you are interested in deploying your Micronaut application to AWS Lambda using GraalVM you only need to set the runtime to lambda and execute ./gradlew buildNativeLambda. This task will generate a GraalVM native image inside a Docker container and then it will create the file build/libs/your-app.zip file ready to be deployed to AWS Lambda using a custom runtime. See more information in Micronaut AWS documentation.

5.7. Packaging the application

By default the plugin doesn’t create a runnable fatjar when running ./gradlew assemble. There are a couple of options:

5.7.1. Layered application

The plugin creates a "layered" application in build/layers and from that directory you can run java -jar myapp.jar. It works because that directory contains a lib directory with all the libraries and a resources directory with the configuration. Keep in mind that copying the only .jar file to another directory won’t work.

5.7.2. Add Shadow plugin

You can add Gradle Shadow plugin so when running ./gradlew assemble a runnable fatjar is created in build/libs directory.

plugins {
    ...
    id "com.github.johnrengelman.shadow" version "7.0.0"
    ...
}
plugins {
    ...
    id("com.github.johnrengelman.shadow") version "7.0.0"
    ...
}

6. Micronaut GraalVM Plugin

The Micronaut GraalVM plugin is applied automatically by the Micronaut application plugin (see below) and it provides tasks to generate a GraalVM native image and also creates the GraalVM resource-config.json automatically with all the resources from the application.

This plugin can be applied separately if you use the application plugin without the io.micronaut.application plugin (but we strongly recommend to switch to the io.micronaut.application plugin in this case).

7. Upgrade notes

7.1. Upgrading from 2.x

When upgrading from the 2.x version of the plugins, you will need to change the configuration of the GraalVM native image builds if you use them.

Typically, instead of configuring image compilation using the task:

nativeImage {
    imageName.set("custom")
}

You now need to use the graalvmNative extension. This extension supports building multiple native images, and the main one is named main (there is another one for tests, called test, which runs unit tests natively):

graalvmNative {
    binaries {
        named("main") {
            imageName.set("custom")
        }
    }
}

Similarly, to compile the native image, you now need to run nativeCompile instead of nativeImage.

In addition, the official GraalVM plugin makes use of Gradle toolchains support, which can lead to surprising behavior if you are used to switching between local JDKs. If you are facing errors like this one:

> No compatible toolchains found for request filter: {languageVersion=11, vendor=matching('GraalVM'), implementation=vendor-specific} (auto-detect true, auto-download true)

then we recommend tweaking toolchain detection as described in this section of the documentation.

In any case, make sure to follow the configuration instructions.