r2dbc-spi | Service Provider Interface for R2DBC Implementations | Reactive Programming library

This might have to do with you not using Generics with your java Collections

Alright mate, firstly, why exactly does this "Due" entity need to be its own entity? I mean, a task may have a deadline and it may be recurring. Keep it simple.

Regarding the code, irrespective of domain modelling aspects:

Use Long or UUID as ID field types. Personally, I've never seen int in production.

Create a CrudRepo for those "Due" entities to check out whether you can query them on their own. Write a test to verify that.

My guess is that the "JoinTable" annotation is somehow redundant and/or misconfigured. Try "Due.ID" instead of "Due_ID" to tell it which field of the Due object it should take.

Looks like you are missing the reactor-netty-0.9.8.RELEASE.jar which has the constant DEFAULT_SHUTDOWN_QUIET_PERIOD defined in it's LoopResources interface.

The most important thing that I notice in your POM is that you have got both spring-webmvc and spring-boot-starter-webflux as dependency.

As explained in the Spring Boot reference documentation, Spring Boot will auto-configure a Spring MVC application if both MVC and WebFlux are available since Spring MVC cannot run on Netty server.

That is why it is advisable not to mix the web dependencies with webflux. If you are trying to build a reactive microservice, just use the spring-boot-starter-webflux and remove web, webmvc and all.

Also as @thomas mentioned, there is no point to separately include spring-core as it comes with all spring boot starter dependencies. To see that happening, use mvn dependency:tree or ./gradlew :dependencies and check the dependency tree.