Web Layer: Controllers & Request Mapping

Spring MVC uses one thread per request from Tomcat's pool (default 200 threads); singleton beans are shared by ALL threads, so mutable instance fields cause race conditions; always set timeouts on downstream calls to prevent pool exhaustion.

@RestController returns JSON directly; @PathVariable extracts from URL path, @RequestParam from query string, @RequestBody deserializes JSON body via Jackson; @Valid triggers bean validation automatically.

Controller handles HTTP, Service handles business logic, Repository handles data access, and auto-configuration creates beans automatically based on classpath and properties via @Conditional annotations.