graalpy-spring-boot-pygal-charts

SVG Charts with GraalPy and Spring Boot

This demo illustrates how GraalPy can be used to embed Pygal, a dynamic SVG charting library written in Python, in a Spring Boot application. In particular, this demo shows four different approaches to interact with Pygal from Java.

Preparation

Install GraalVM 25 and set the value of JAVA_HOME accordingly. We recommend using SDKMAN!. (For other download options, see GraalVM Downloads.)

sdk install java 25.0.2-graal

Run the Application

To start the demo, simply run:

./mvnw package spring-boot:run

When the demo runs, open the following URLs in a browser:

URL	Service
http://localhost:8080/java	PyGalServicePureJava
http://localhost:8080/python	PyGalServicePurePython
http://localhost:8080/mixed	PyGalServiceMixed
http://localhost:8080/dynamic	PyGalServiceValueAPIDynamic

Implementation Details

The DemoController uses four services that all render the same XY chart using different implementations:

• PyGalServicePureJava interacts with Pygal and Python using Java interfaces and Value.as(Class<T> targetType). This is the recommended approach.
• PyGalServicePurePython embeds the Python sample code from the Pygal documentation.
• PyGalServiceMixed uses a Python function which is invoked with Java values.
• PyGalServiceValueAPIDynamic uses the Value API from the GraalVM SDK.

The DemoApplicationTests ensures that all four service implementations render the same XY chart. Run it with ./mvnw test.

Note: This demo uses a single GraalPyContext, which can execute Python code in only one thread at a time. Threads running Python code are internally scheduled in round-robin fashion. Pure Python packages including Pygal can be used in multiple GraalPy contexts, for example one context per thread, to improve the throughput of the application. Other demos such as graalwasm-micronaut-photon illustrate how to pool multiple contexts.