Spring 引导：返回一个空的 JSON 而不是返回空体 object 是 null

Question

I have a RestController and when I call the method:

@RequestMapping(value = "/sigla/{sigla}")
@ResponseBody
public PaisDTO obterPorSigla(@PathVariable String sigla) {
    return service.obterPorSigla(sigla);
}

If a record is found, I get a good JSON response:

{"nome":"Brasil","sigla":"BR","quantidadeEstados":27}

but when nothing is found on database the RestController returns null and I get a empty response, completely blank body.

How can I display a empty JSON instead of a blank response? Like bellow:

{}

Complete Controller:

@RestController
@RequestMapping("/pais")
public class PaisController {

    @Autowired
    private PaisService service;

    @RequestMapping
    public ResponseEntity<List<PaisDTO>> obterTodos() {
        return CreateResponseEntity.getResponseEntity(service.obterTodos());
    }

    @RequestMapping(value = "/sigla/{sigla}", method = RequestMethod.GET, consumes="application/json", produces="application/json")
    public ResponseEntity<PaisDTO> obterPorSigla(@PathVariable String sigla) {
        HttpHeaders headers = new HttpHeaders();
        headers.add("Content-Type", "application/json");
        PaisDTO paisDTO = service.obterPorSigla(sigla);
        if(paisDTO != null) return new ResponseEntity<PaisDTO>(paisDTO, headers, HttpStatus.OK);
        else return new ResponseEntity<PaisDTO>(headers, HttpStatus.OK);
    }

Answer 1

First, if you're using @RestController annotation you don't need the @ResponseBody annotation, get rid of that.

Second if you're trying to have REST Controller, then you're missing a few things, do it like this:

@RequestMapping(value = "/sigla/{sigla}", method = RequestMethod.GET, consumes = "application/json", produces="application/json")
public ResponseEntity<PaisDTO> obterPorSigla(@PathVariable String sigla) {
        HttpHeaders headers = new HttpHeaders();
        headers.add("Content-Type", "application/json");    
        PaisDTO paisDTO = service.obterPorSigla(sigla);
        if(paisDTO != null) return new ResponseEntity<>(paisDTO, headers, HttpStatus.OK);
        else return new ResponseEntity<>(headers, HttpStatus.OK); 
}

In the example above if you'll get null then you'll return an empty response JSON.

Answer 2

The only way that I could find was to create an empty class

@JsonSerialize
public class EmptyJsonBody {
}

Then add this to your response

@PostMapping(value = "/sigla/{sigla}")
public ResponseEntity obterPorSigla(@PathVariable String sigla) {
    HttpHeaders headers = new HttpHeaders();
    headers.add("Content-Type", "application/json");
    PaisDTO paisDTO = service.obterPorSigla(sigla);
    ResponseEntity.BodyBuilder responseBuilder = ResponseEntity.ok().headers(headers);
    if(paisDTO != null) {
        return responseBuilder.body(paisDTO);
    } else {
        return responseBuilder.body(new EmptyJsonBody());
    }
}

Answer 3

Solution 1: You have to implement you entity class with Serializable Solution 2: Your class should have getter and setter In my case the getter and setter were given protected access modifiers. so I changed them to public and vola it worked

Answer 4

I am using this as suggested by marcos almedia..but I am getting java object hashcode response instead json empty {} response

@JsonSerialize public class EmptyJsonBody {}

How can we resolve it?