How do you create a RESTful web service in Spring Boot?

You create a REST service by adding the web starter and writing a class annotated with @RestController. Each handler method maps an HTTP method and a path to Java code, and Spring converts the return value to JSON.

1. Add the web starter, spring-boot-starter-webmvc, named spring-boot-starter-web before Spring Boot 4. It brings Spring MVC, the Jackson JSON library and an embedded Tomcat web server.
2. Annotate a class with @RestController and give it a base path with @RequestMapping.
3. Map each operation with a shortcut annotation that has the HTTP method built in, such as @GetMapping or @PostMapping.
4. Read input with @PathVariable for a path segment and @RequestBody for a JSON body.

@RestController
@RequestMapping("/api/users")
public class UserController {
    @Autowired
    private UserService userService;
    @GetMapping
    public List<User> getAllUsers() {
        return userService.findAll();
    }
    @GetMapping("/{id}")
    public User getUser(@PathVariable Long id) {
        return userService.findById(id);
    }
    @PostMapping
    public User createUser(@RequestBody User user) {
        return userService.save(user);
    }
    // ...
}

Start the @SpringBootApplication main class, and a GET request to /api/users/1 returns that user as JSON. To choose the status code yourself, return a ResponseEntity, for example 201 Created from the POST method.