Spring Boot Controllers

When learning Spring Boot, one of the most confusing things is seeing controllers written in many different ways. Sometimes a method returns a DTO. Sometimes ResponseEntity . Sometimes void , String , ModelAndView , CompletableFuture , Mono , and more. At first it feels random. But each return type has a specific purpose. This article is a practical cheat sheet to quickly understand the most common controller patterns in Spring Boot REST APIs. What Is a Controller? A controller handles HTTP requ