spring-controller

Web application logic needs to respond to specific URL patterns, extract parameters from requests, and return data or views to the client. Without a structured approach, each page would manually parse URLs, manage HTTP methods, and construct responses — duplicating effort across handlers.

Spring Controllers are @Controller-annotated classes whose methods handle specific URL patterns and HTTP methods, defined via @RequestMapping and its shortcut annotations (@GetMapping, @PostMapping, etc.). @RestController combines @Controller with @ResponseBody for REST APIs, automatically serializing return values to JSON or XML.

1. @RequestMapping: Maps HTTP method + URL path to a controller method — e.g., @GetMapping("/users/{id}")
2. Method parameters: Automatically bound from request: @RequestParam (query params), @PathVariable (URL segments), @RequestBody (JSON body), @ModelAttribute (form data)
3. Return types: String (view name), ModelAndView, @ResponseBody (JSON/XML), ResponseEntity (full control)
4. @RestController: All methods default to @ResponseBody; no view resolution — returns data directly
5. @Controller vs @RestController: @Controller returns view names; @RestController returns data (JSON/XML)

• @RequestMapping shortcuts: @GetMapping, @PostMapping, @PutMapping, @DeleteMapping, @PatchMapping
• @PathVariable: Binds URL template variables — @GetMapping("/users/{id}") + (@PathVariable Long id)
• @RequestParam: Binds query parameters with optional default values and required flag
• @RequestBody: Deserializes request body (JSON/XML) to Java object using HttpMessageConverter
• @ResponseBody: Serializes return value directly to HTTP response body
• @ResponseStatus: Sets HTTP status code on successful method execution

• Built from: Spring MVC — Controllers are the handler components in Spring MVC
• Built from: DispatcherServlet — DispatcherServlet invokes controller methods
• Related: Spring Form Handling — @ModelAttribute binds form data to model objects
• Related: Spring MVC Exception Handling — @ExceptionHandler in controllers
• Contrasts with: Servlets — Servlets extend HttpServlet; Spring Controllers are POJOs with annotations

• @RequestParam required=true by default: Missing required param returns 400 — use required=false or defaultValue
• @PathVariable vs @RequestParam: Path variables identify resources; request params filter/ paginate
• @ResponseBody + String: Returns the string itself, not a view name — common confusion with @Controller
• Method-level vs class-level @RequestMapping: Class-level is the prefix; method-level defines the specific endpoint