How to accept GET argument in Spring boot Controller and return appropriate object - spring-mvc

Question

I am very new to spring boot and I can't figure out the #Controller class. What should I pass if I can't find a particular object in my DB in spring boot? Is it better if I declare my return type as Response Entity and send a null User object?
//Get single user
#GetMapping("/users/{id}")
public User getUser(#PathVariable String id){
try {
Long i = Long.parseLong(id);
} catch (NumberFormatException ex) {
return ???? //Can't figure out what to return here.
}
return userService.getUser(id);
}
I want the consumer to know that they've sent an invalid string.
2) Also, User's variable id is of Long type. So should I take the argument as Long in the getUser function or take a String and parse it? Taking a Long would crash my server if a string was sent in the link.

Answer 1

This is my typical code of a REST controller for 'get user by id' request:
#RestController
#RequestMapping("/users") // 1
public class UserController {
private final UserRepo userRepo;
public UserController(UserRepo userRepo) {
this.userRepo = userRepo;
}
#GetMapping("/{id}") // 2
public ResponseEntity getById(#PathVariable("id") Long id) { // 3
return userRepo.findById(id) // 4
.map(UserResource::new) // 5
.map(ResponseEntity::ok) // 6
.orElse(ResponseEntity.notFound().build()); // 7
}
}
Where:
1 - is a common starting path for all requests handled by this controller
2 - a path variable pattern of GET request (/users/{id}).
3 - provide the name of path variable which name is correspond to the parameter in GetMapping. The type of the parameter in getById method is corresponds to the type of User ID.
4 - I use findById method of my UserRepo that return Optional
5 - Here I transform User to some type of DTO - UserResource (it's optional step)
6 - return OK response if User was found
7 - or return Not Found response otherwise.

Answer 2

I'm also using the controller-service-repository pattern in a couple of projects, and this is how I lay it out:
Controller.java
#RestController // 1
#RequestMapping(value = "/users") // 2
public class UserController {
private final UserService userService;
#Autowired // 3
public UserController(UserService userService) {
this.userService = userService;
}
#RequestMapping(value = "/{user_id}", method = RequestMethod.GET) //4
#ResponseStatus(HttpStatus.OK) //5
public UserModel getUser(#PathVariable(value="user_id") long user_id) { //6
return userService.getUserById(user_id);
}
#RequestMapping(method = RequestMethod.POST) // 7
#ResponseStatus(HttpStatus.CREATED) // 8
public UserModel getUser(#ResponseBody UserModel userModel) { // 9
return userService.createUser(usermodel);
}
}
1) #RestController is a combination of #Controller and #ResponseBody, which essentially means every method in your class will have a response body.
2) Prefix #RequestMapping values in this class with /users
3) Autowiring in the Constructor is the safest approach to injecting beans.
4) This method will be accessible via a GET request to /users/{user_id}
5) This method will return HttpStatus.OK status code on success (200)
6) Extracts the path variable "user_id" from the Request. Use the same numeric type as your user id's here (i.e. int or long).
7) This method will be accessible via a POST request to /users
8) This method will return HttpStatus.CREATED status code on success (201)
9) Extracts a UserModel from the request body (should have the same structure as the json given later).
There are no real differences to Cepr0 and my approach, it's purely a style preference.
UserModel can be a class like this:
UserModel.java
public class UserModel {
private String username;
// Constructor, Getter, Setter...
}
And this will return a JSON object in the body of the response like this:
{
"username":"username"
}
If you would like to handle Exceptions within your controller (and even control the data returned by an exception, you can use #ExceptionHandler like so:
#ExceptionHandler(Exception.class)
public ResponseEntity<ErrorResponseWrapper> handleGenericException(Exception ex){
return ResponseEntity
.status(HttpStatus.I_AM_A_TEAPOT)
.body(new CustomExceptionWrapper(ex)));
}
Where CustomExceptionHandler converts exceptions thrown by your application into a format you decide (This can also be a POJO and Spring Boot will transform it into JSON for you!)
To answer your questions more specifically:
1) You should throw an exception if the user isn't found that will include the response status 404 (NOT FOUND). Returning null is typically a bad idea as it could mean a lot of things.
1.1?) If your user sends an invalid string, you can look up which exception it causes in your server, and use the exception handler to deal with it and return an appropriate response (BAD_REQUEST maybe?)
2) Yes use long if your use id's are longs.
Check out the baeldung site, would really recommend them for learning Spring Boot.