Rest application in Spring 2.5 (java 6)

[must Janik]

Hi guys. I have a question regarding spring 2.5 (with java 6). I would like to rewrite web application based on REST which is written in newest version of Spring Boot.
The question is... is it even possible? I mean, I do not want to create any front (returning jsp files), just want to create backend.

Some sample from controller I want to rewrite:

@RestController
public class UserCrudActivitiesController {

    private final UserCrudActivitiesService userCrudActivitiesService;

    public UserCrudActivitiesController(UserCrudActivitiesService userCrudActivitiesService) {
        this.userCrudActivitiesService = userCrudActivitiesService;
    }

    @PreAuthorize("hasRole('ADMIN')")
    @GetMapping("/users")
    public List<User> getAllUsers() {
        return userCrudActivitiesService.getAllUsers();
    }

    @PostMapping("/createUser")
    public List<String> createUser(@RequestBody User user) {
        return userCrudActivitiesService.createUser(user);
    }

    @PutMapping("/user")
    public List<String> updateUser(@RequestBody User user) {
        return userCrudActivitiesService.updateUser(user);
    }
}

As you can see I just return response in JSON. That's all I want. I read that I need some RestTemplate or something, but it is available since Spring 3.0, so I would need some custom mapper to return json, but it is not that hard I think.
Could you tell me whether is it possible to do it? All tutorials are about returning jsp files which I do not want.
Some sample of service/controller would be extremely useful for me if possible.
Thanks.

[must Janik]

To be clear.I want to rewrite application which is written in Spring Boot . I will have 2 application which are the same, but written in two different technologies. It is for my engineering thesis