Java / Maven

Creating a Micronaut GraalPy application using a Python package

Learn how to create a simple Micronaut GraalPy application using a third-party Python package.

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:

Some time on your hands
A decent text editor or IDE (e.g. IntelliJ IDEA)
JDK 21 or greater installed with JAVA_HOME configured appropriately

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.

Download and unzip the source

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.

GraalPy Maven Plugin

Insert a <packages> element into the GraalPy Maven Plugin <configuration> element, specifying the Python packages to download and install as shown below:

pom.xml

<plugin>
    <groupId>org.graalvm.python</groupId>
    <artifactId>graalpy-maven-plugin</artifactId>
    <configuration>
      <packages>
        <package>pygal==3.0.4</package>
      </packages>
    </configuration>
    <executions>
      <execution>
        <goals>
          <goal>process-graalpy-resources</goal>
        </goals>
      </execution>
    </executions>
</plugin>

Adding <package>pygal==3.0.4</package> instructs the plugin to download and install the Python Pygal package at build time. The package is included in the Java resources and becomes available to GraalPy.

Python code

We are going to use the StackedBar class from the Pygal package to render a graph in SVG format.

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/PygalModule.java

package example.micronaut;

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

@GraalPyModule("pygal") // 
interface PygalModule {
    StackedBar StackedBar(); // 

    interface StackedBar { // 
        void add(String title, int[] i); // 
        Svg render(); // 
    }

    interface Svg { // 
        String decode(); // 
    }
}

1	The @GraalPyModule annotation indicates that the bean created from the interface is intended to import the `pygal` Python package and expose it to the Java code using the Target type mapping.
2	Java method matching the `StackedBar` Python class constructor.
3	Java interface with methods matching functions from the `StackedBar` Python class we want to use.
4	Java method matching the `StackedBar.add(title, values)` Python function.
5	Java method matching the `StackedBar.render()` Python function.
6	Java interface with methods matching functions from the `Svg` Python class we want to use.
7	Java method matching the `Svg.decode()` Python function.

Controller

To create a microservice that provides the graph rendered by Pygal you also need a controller.

Create a controller:

java/src/main/java/example/micronaut/PygalController.java

package example.micronaut;

import io.micronaut.http.annotation.Controller;
import io.micronaut.http.annotation.Get;
import io.micronaut.http.annotation.Produces;
import io.micronaut.scheduling.TaskExecutors;
import io.micronaut.scheduling.annotation.ExecuteOn;

@Controller("/pygal") // 
class PygalController {

    private final PygalModule pygal;

    PygalController(PygalModule pygal) {  // 
        this.pygal = pygal;
    }

    @ExecuteOn(TaskExecutors.BLOCKING)
    @Get // 
    @Produces("image/svg+xml") // 
    public String index() {
        PygalModule.StackedBar stackedBar = pygal.StackedBar(); // 
        stackedBar.add("Fibonacci", new int[] {0, 1, 1, 2, 3, 5, 8}); // 
        PygalModule.Svg svg = stackedBar.render(); // 
        return svg.decode(); // 
    }

}

1	The class is defined as a controller with the @Controller annotation mapped to the path `/pygal`.
2	Use constructor injection to inject a bean of type `PygalModule`.
3	The @Get annotation maps the `index` method to an HTTP GET request on `/pygal`.
4	By default, a Micronaut response uses `application/json` as Content-Type. We are returning `svg/xml`, so we have to set it.
5	Use the `PygalModule` bean to create a `PygalModule.StackedBar` instance.
6	Add graph title and data.
7	Render the graph.
8	Return the graph’s `svg/xml` representation decoded as a string.

Test

Create a test to verify that when you make a GET request to /pygal you get the svg/xml response:

java/src/test/java/example/micronaut/PygalControllerTest.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.runtime.server.EmbeddedServer;
import io.micronaut.test.extensions.junit5.annotation.MicronautTest;
import jakarta.inject.Inject;
import org.junit.jupiter.api.Test;

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

@MicronautTest // 
class PygalControllerTest {

    @Test
    void testPygalResponse(@Client("/") HttpClient client) { // 
        String response = client.toBlocking().retrieve(HttpRequest.GET("/pygal")); // 
        assertTrue(response.contains("<svg xmlns:xlink"));
        assertTrue(response.contains("<title>Pygal</title>"));
        assertTrue(response.contains("<g class=\"graph stackedbar-graph vertical\">"));
    }
}

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.

Open http://localhost:8080/pygal in a web browser of your choice to execute the endpoint. You should see a stacked bar chart.

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.PygalModule"],
  ["example.micronaut.PygalModule$StackedBar"],
  ["example.micronaut.PygalModule$Svg"]
]

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.

Open http://localhost:8080/pygal in your web browser to execute the endpoint exposed by the native executable. You should see a stacked bar chart.

Next Steps

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).