Virtual threads not supported on JDK <21 about spring-boot

[wilkinsona]

This failure indicates that https://github.com/spring-projects/spring-framework/blob/5012843b1254aefa7d81132619c8fa6e3f79b495/spring-core/src/main/java/org/springframework/core/task/VirtualThreadDelegate.java has been loaded instead of https://github.com/spring-projects/spring-framework/blob/5012843b1254aefa7d81132619c8fa6e3f79b495/spring-core/src/main/java21/org/springframework/core/task/VirtualThreadDelegate.java. The latter should be used automatically on Java 21+ due to spring-core being a multi-release jar file. I cannot tell why the correct version is not being loaded automatically without knowing the version of Spring Boot that you are using and exactly how you have built and run your application.

If you would like us to spend some more time investigating, please spend some time providing a complete yet minimal sample that reproduces the problem. You can share it with us by pushing it to a separate repository on GitHub or by zipping it up and attaching it to this issue.

[smoell]

unicorn-store-spring.tar.gz

Thank you!
I've attached a demo project, you can reproduce the issue with:

1. Unzip the project
2. Build the project using docker buildx build --load -t unicorn-store-spring:latest .
3. cd dockerfiles && docker compose up

[wilkinsona]

The store-spring.jar that you're building is rather unusual as it contains classes from your application's dependencies in BOOT-INF/classes, including VirtualThreadDelegate:

BOOT-INF/classes/org/springframework/core/task/VirtualThreadTaskExecutor.class

I suspect that this is due to your configuration of the maven-shade-plugin. It's breaking the multi-release nature of spring-core when it shades it, causing the wrong version of VirtualThreadTaskExecutor.class to be loaded.

[smoell]

Thank you! We need to Maven Shade plugin to generate an artifact for AWS Lambda as well. We'll stick with standard threads until we find a way to fix this. Thank you!.