Building a REST API with Spring Boot

Spring Boot is the dominant framework for building Java backend services — it handles the HTTP server, routing, and dependency injection so a working endpoint is a handful of annotated lines:

@RestController
@RequestMapping("/api/users")
class UserController {
    @GetMapping("/{id}")
    User getUser(@PathVariable int id) {
        return new User(id, "Ada");
    }

    @PostMapping
    User createUser(@RequestBody User user) {
        // save user...
        return user;
    }
}

@RestController marks the class as handling HTTP requests, @GetMapping/@PostMapping map a method to an HTTP verb and path, and @RequestBody automatically deserializes an incoming JSON body into a Java object — Spring wires all of this together via dependency injection and reflection, both covered earlier in this level.