Table of Contents

Creating a Micronaut GraalPy application using a Python package

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

Authors: Tomas Stupka

Micronaut Version: 4.10.2

1. Getting Started

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

2. What you will need

To complete this guide, you will need the following:

3. 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.

4. 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
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.

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.

4.1. 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:

src/main/../../pom.xml
      <plugin>
          <groupId>org.graalvm.python</groupId>
          <artifactId>graalpy-maven-plugin</artifactId>
          <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.

4.2. Python code

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

4.3. 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:

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

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

@GraalPyModule("pygal") (1)
interface PygalModule {
    StackedBar StackedBar(); (2)

    interface StackedBar { (3)
        void add(String title, int[] i); (4)
        Svg render(); (5)
    }

    interface Svg { (6)
        String decode(); (7)
    }
}
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.

4.4. Controller

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

Create a controller:

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") (1)
class PygalController {

    private final PygalModule pygal;

    PygalController(PygalModule pygal) {  (2)
        this.pygal = pygal;
    }

    @ExecuteOn(TaskExecutors.BLOCKING)
    @Get (3)
    @Produces("image/svg+xml") (4)
    public String index() {
        PygalModule.StackedBar stackedBar = pygal.StackedBar(); (5)
        stackedBar.add("Fibonacci", new int[] {0, 1, 1, 2, 3, 5, 8}); (6)
        PygalModule.Svg svg = stackedBar.render(); (7)
        return svg.decode(); (8)
    }

}
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 graphs svg/xml representation decoded as a string.

4.5. Test

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

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 (1)
class PygalControllerTest {

    @Test
    void testPygalResponse(@Client("/") HttpClient client) { (2)
        String response = client.toBlocking().retrieve(HttpRequest.GET("/pygal")); (3)
        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.

5. Testing the Application

To run the tests:

./mvnw test

6. 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 execute the endpoint. You should see a stacked bar chart.

graalpy pygal

7. 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 23
sdk install java 23-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 23
sdk install java 23-graalce

7.1. 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

8. 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.

8.1. Native Image Configuration

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

Create a proxy configuration file:

src/main/resources/META-INF/native-image/proxy-config.json
[
  ["example.micronaut.PygalModule"],
  ["example.micronaut.PygalModule$StackedBar"],
  ["example.micronaut.PygalModule$Svg"]
]

8.2. 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. endpoint exposed by the native executable. You should see a stacked bar chart.

9. Next Steps

Read more about Micronaut testing.

10. Help with the Micronaut Framework

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

11. License

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