Spring MVC — From HTTP Request to Method Call
A member's phone app sends GET /books/42. Somewhere in library-service, a Java method must run and its result must come back as JSON. Spring MVC is the machinery in between, and knowing its path lets you debug a 404 or a 415 in seconds instead of hours.
The rule
One front door, the DispatcherServlet, routes every request to a handler method.
Spring MVC is built on the Java Servlet API, the standard way Java servers handle HTTP. Instead of one servlet per URL, Spring registers a single servlet, the DispatcherServlet, as a front controller. Every request enters through it, and it follows the same path each time:
- Handler mapping — find the controller method whose annotations match the path and HTTP method.
- Argument resolution — fill the method's parameters from the path, query string, headers or body.
- Invoke — call your method.
- Write the response — for a
@RestController, a message converter (Jackson, for JSON) turns the return value into the response body. A classic@Controllerinstead returns a view name, and a view resolver turns it into an HTML page.
@RestController
@RequestMapping("/books")
public class BookController {
private final BookRepository books;
public BookController(BookRepository books) { this.books = books; }
@GetMapping("/{id}")
public ResponseEntity<Book> find(@PathVariable long id) {
return books.findById(id)
.map(ResponseEntity::ok)
.orElse(ResponseEntity.notFound().build());
}
@PostMapping
public ResponseEntity<Book> add(@RequestBody Book book) {
Book saved = books.save(book);
return ResponseEntity.status(HttpStatus.CREATED).body(saved);
}
}(For brevity this controller reads the repository directly; anything with real rules would go through a service.) @RestController is @Controller plus @ResponseBody: every method's return value becomes the body rather than a view name.
The shortcut
Read each request like an address, method + path + parts, and match it against the annotations:
@GetMapping,@PostMapping,@PutMapping,@DeleteMappingmatch the HTTP method.@PathVariabletakes a piece of the path, like42in/books/42.@RequestParamtakes a query parameter, like?author=Le+Guin.@RequestBodytakes the JSON body and converts it to an object.
Front door
The DispatcherServlet receives
POST /books.
When no method matches the path, you get 404. When the path matches but not the HTTP method, 405. When the body's content type cannot be read, 415. Each status tells you which step failed.
Drills
Answer fast — trust the rule.
What is the job of the DispatcherServlet?
For GET /books/42?format=short, which annotation reads the 42?
What does @RestController add compared with @Controller?
A client sends DELETE /books/42, but BookController has only @GetMapping("/{id}") for that path. Which status comes back?
Remember
- One front door, the DispatcherServlet, routes every request to a handler method.
- Read the request as method + path + parts and match the annotations.
- Trap: 404, 405 and 415 fail at different steps — read the status before the code.