Assignment

Task

Try to introduce every single Design Pattern you can think of. The goal is to experiment and play.

Disclaimer: I would discourage such an attitude in production systems, but for learning, try to add in here all the patterns you can imagine.

If lacking ideas, consider:

Adapter + a new Domain class to return from it
Implement Dependency Inversion between different packages (move the Adapter in an infra package and use ArchUnit to check that domain does NOT depend on infra)
Repository encapsulating the (very ugly) raw JDBC access
Exposing Dtos from your REST API (@GetMapping method), instead of central Entities
Isolate the filtering rules in a class that only works on structures that I control (that is, in a Domain Service)
Introduce a Facade to orchestrate the main flow (3-5 lines in its method)
Factory method
Decorator around the extracted Adapter that logs every method call
Proxy if you are brave enough to try implement a Spring @Aspect (with the same goal: logging method calls + parameters) [hard]
"Manual" singleton around the configuration (despite the wonderful Spring @Value annotation) to avoid re-reading it every time
Fluent Builder for the Lemon to ease its construction when read from DB
Observer Pattern to insert the audit "out of band", without direct coupling of our code to it
Command Pattern/DTO to encapsulate the two input parameters as an object
Filters that remove (or allow) a Lemon to pass the search criteria

God smiled upon this project so there is ONE end-to-end smoke test going through all the code. Try to keep it green.

If you want start the app and test it manually: