The following code is for TweetController, which caters for retrieving and saving tweets:

@RestController
@RequestMapping("/tweets")
public class TweetController {

    private final TweetService tweetService;
    private final UserService userService;

    public TweetController(TweetService tweetService, UserService 
    userService) {
        this.tweetService = tweetService;
        this.userService = userService;
    }

    @PostMapping
    public Mono<Tweet> save(Principal principal, @RequestBody Tweet 
     tweet) {
        Mono<User> user = 
        userService.getUserByScreenName(principal.getName());
        return user.flatMap(u -> {
                                   tweet.setTweetUser(u);
                                   return tweetService.save(tweet);
                                 });
    }

    @GetMapping
    public Flux<Tweet> getAll(Principal principal) {
        return tweetService.getRelevantTweets(principal.getName());
    }

}

From the preceding code, we can see that the save method uses Principal, which has information about the logged in user, to save a Tweet object sent as the request body of a POST method.

The getAll method uses Principal, which has information about the logged in user, to retrieve all the tweets relevant to that user.

The following code is for UserController, which caters for retrieving the user by screenName and following a user by user ID:

@RestController
@RequestMapping("/users")
public class UserController {

    private final UserService userService;

    public UserController(UserService userService) {
        this.userService = userService;
    }

    @GetMapping("/{screenName}")
    public Mono<User> getUserByScreenName(@PathVariable String 
    screenName) {
        return userService.getUserByScreenName(screenName);
    }

    @PutMapping("/{userId}/follow")
    @ResponseStatus(code = HttpStatus.OK)
    public void followUser(Principal principal, @PathVariable String 
     userId) {
        Mono<User> user = 
        userService.getUserByScreenName(principal.getName());
        user.subscribe(u -> {
            if (!u.getUserId().equalsIgnoreCase(userId)) {
                u.getFollowing().add(userId);
                userService.save(u);
            }
        });
    }
}

The getUserByScreenName method will use screenName sent as part of the URL to retrieve the matching user. 

The followUser method will use the Principal object and the user ID submitted as part of the URL to follow a user.