Java / Maven

Creating your first Micronaut GraalPy application

Learn how to create a Hello World Micronaut GraalPy application with a controller and a functional test.

Tomas Stupka
On this guide
In this section

Getting Started

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

What you will need

To complete this guide, you will need the following:

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.

Writing the Application

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

mn create-app example.micronaut.micronautguide \
    --features=graalpy \
    --build=maven \
    --lang=java \
    --test=junit
Note
 If you don’t specify the --build argument, Gradle with the Kotlin DSL is used as the build tool.
If you don’t specify the --lang argument, Java is used as the language.
If you don’t specify the --test argument, JUnit is used for Java and Kotlin, and Spock is used for Groovy.

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

If you use Micronaut Launch, select Micronaut Application as application type and add graalpy features.

Note
 If you have an existing Micronaut application and want to add the functionality described here, you can view the dependency and configuration changes from the specified features, and apply those changes to your application.

Diff

Setup IntelliJ IDEA to develop Micronaut Applications.

Python code

Part of the "Hello World" functionality will be provided by Python.

Create a Python file with the following code:

java/src/main/resources/org.graalvm.python.vfs/src/hello.py
def hello(txt): # 
    return f"Hello {txt}"
1 The Python function we are going to call from Java

All files in the src/main/resources/org.graalvm.python.vfs/ directory will be accessible to the GraalPy runtime environment. The src subdirectory is automatically added to the Python search path, making it the ideal location for custom Python files in your application.

Java binding

Python code can be accessed programmatically using the GraalVM SDK Polyglot API, which enables you to embed Python into your applications.

In order to make it work, we first need a Java interface providing the intended binding to Python.

Create a Java interface with the following code:

java/src/main/java/example/micronaut/HelloModule.java
package example.micronaut;

import io.micronaut.graal.graalpy.annotations.GraalPyModule;

@GraalPyModule("hello") // 
public interface HelloModule {
    String hello(String txt); // 
}
1 The @GraalPyModule annotation indicates that the bean created from the interface

is intended to import the hello.py Python module into GraalPy and expose it to the Java code using the Target type mapping. <2> Java method matching the Python function hello(txt) in hello.py.

Controller

To create a microservice that responds with "Hello World" you also need a controller.

Create a controller with the following code:

java/src/main/java/example/micronaut/HelloController.java
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") // 
class HelloController {

    private final HelloModule hello;

    HelloController(HelloModule hello) {  // 
        this.hello = hello;
    }

    @Get // 
    @Produces(MediaType.TEXT_PLAIN) // 
    String index() {
        return hello.hello("World"); // 
    }
}
1 The class is defined as a controller with the @Controller annotation mapped to the path /hello.
2 Use constructor injection to inject a bean of type HelloModule.
3 The @Get annotation maps the index method to an HTTP GET request on /hello.
4 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 with the @Produces annotation.
5 Use the HelloModule bean to call the Python function hello(txt) in hello.py.

Test

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

java/src/test/java/example/micronaut/HelloControllerTest.java
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.Test;

import static org.junit.jupiter.api.Assertions.assertEquals;

@MicronautTest // 
class HelloControllerTest {
    @Test
    void testHelloResponse(@Client("/") HttpClient client) { // 
        String response = client.toBlocking().retrieve(HttpRequest.GET("/hello")); // 
        assertEquals("Hello World", response);
    }

}
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.

Testing the Application

To run the tests:

./mvnw test

Running the Application

To run the application, use the ./mvnw mn:run command, which starts the application on port 8080.

Python Runtime Compilation

When using the GraalVM JDK, Python code executed with the GraalPy engine is just-in-time compiled. With other JDKs, the Python code is interpreted, resulting in reduced performance.

The easiest way to install GraalVM on Linux or Mac is to use SDKMan.io.

Java 25
sdk install java 25.0.2-graal

For installation on Windows, or for a manual installation on Linux or Mac, see the GraalVM Getting Started documentation.

The previous command installs Oracle GraalVM, which is free to use in production and free to redistribute, at no cost, under the GraalVM Free Terms and Conditions.

Alternatively, you can use the GraalVM Community Edition:

Java 25
sdk install java 25.0.2-graalce

GraalVM JDK Compatibility

To enable runtime compilation, you must use versions of GraalPy and the Polyglot API dependencies that are compatible with the specified GraalVM JDK version. If you see errors like the following:

Your Java runtime '23.0.1+11-jvmci-b01' with compiler version '24.1.1' is incompatible with polyglot version '24.1.0'.

You need to override the versions of the following dependencies according to the error message:

org.graalvm.polyglot:polyglot
org.graalvm.python:python
org.graalvm.python:python-embedding
org.graalvm.python:graalpy-maven-plugin

Generate a Micronaut Application Native Executable with GraalVM

Starting with GraalVM for JDK 23, Micronaut applications that use GraalPy or any Truffle language can be compiled into native executables using GraalVM Native Image.

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

Native Image Configuration

GraalVM Native Image compilation requires metadata to correctly run code that uses dynamic proxies.

Create a proxy configuration file:

java/src/main/resources/META-INF/native-image/proxy-config.json
[
  ["example.micronaut.HelloModule"]
]

Native Executable Generation

To generate a native executable using Maven, run:

./mvnw package -Dpackaging=native-image

The native executable is created in the target directory and can be run with ./target/micronautguide.

You can execute the endpoint exposed by the native executable:

curl localhost:8080/hello
Hello World

Next Steps

Read more about Micronaut Graalpy integration.

Help with the Micronaut Framework

The Micronaut Foundation sponsored the creation of this Guide. A variety of consulting and support services are available.

License

Note
 All guides are released with an Apache License 2.0 for the code and a Creative Commons Attribution 4.0 license for the writing and media (images).