What Spring controller method return type to use If I want to return type X or Y depending on condition?

Question

I want to create method in Spring controller that returns String or SseEmitter depending on condition.

What return type I should specify in method signature?

@GetMapping("/test")
public WHAT_TO_WRITE_HERE test() {
    if (MyCondition) {
        SseEmitter emitter = new SseEmitter();
        //Fill SseEmitter with data, not related to question
        return emitter;
    } else {
        return "String";
    }
}

Answer 1

You shouldn't do it because it will confuse the API clients. But say you want to do it for testing purposes. Then, you can simply set an Object as a return type.

Here is an example

@RestController
@RequestMapping("/api")
public class DemoController {


    @GetMapping("/demo")
    public Object demo() {
        if (new Random().nextBoolean()) {
            return "Hello, World!";
        } else {
            return new User("John Doe", "[email protected]");
        }
    }
}

@Data
@AllArgsConstructor
class User {
    private String name;
    private String email;
}

Depending on condition, the endpoint will return

GET http://localhost:8080/api/demo

HTTP/1.1 200 
Content-Type: application/json
Transfer-Encoding: chunked
Date: Fri, 21 Jun 2024 05:20:41 GMT

{
  "name": "John Doe",
  "email": "[email protected]"
}

or

GET http://localhost:8080/api/demo

HTTP/1.1 200 
Content-Type: text/plain;charset=UTF-8
Content-Length: 13
Date: Fri, 21 Jun 2024 05:24:56 GMT

Hello, World!