This project demonstrates a Spring Feign client lazy initialization error, when using CompletableFuture.supplyAsync() to trigger Feign client initialization.

The project contains a feign client that fetches http://www.google.com. This client is used directly by a rest controller. By design, Feign clients in Spring are lazily initialized, thus leaving initialization to the first caller. In this case, the calling rest controller uses CompletableFuture.supplyAsync() to handle the outbound Feign call asynchronously.

The project can be built using:

./gradlew clean bootBuildImage

The Spring Boot application contained in this project can be tested successfully, as follows:

./gradlew clean bootRun
curl http://localhost:8080/api/sample

This should yield the output from http://www.google.com, fetched by the Feign client.

However, when running the very same application from within Docker, as follows

./gradlew clean bootBuildImage
docker-compose up
curl http://localhost:8080/api/sample

the Feign client initialization will fail with

java.lang.ClassNotFoundException: org.springframework.boot.autoconfigure.condition.OnPropertyCondition

When looking at the stack-trace, it is obvious that the ForkJoinWorkerThread in a Docker image built using the Spring Boot Build Image plugin seems to cause this error when trying to lazily initialize the Feign client. The error seems to be strongly related to the Spring Boot Build Image, since using jib instead, does not cause this error.

The same is true for running the Docker image in certain Kubernetes environments, although not all environments seem to be affected.