Spring MVC - @Valid on list of beans in REST service

Question

In a Spring MVC REST service (json), I have a controller method like this one :

@RequestMapping(method = RequestMethod.POST, value = { \"/doesntmatter\" })
         


        

Answer 1

Given Spring-Boot + Jackson for JSON serialization + org.springframework.boot:spring-boot-starter-validation (must be included manually for spring boot >= 2.3.0)

Using built-ins

• add @Validated to your controller
• use @Valid @NotNull @RequestBody List<@Valid Pojo> pojoList in your controller method signature

This will throw a javax.validation.ConstraintViolationException error on invalid beans though, which is mapped to 500 Internal Error by default. Hence, ensure you have a ControllerAdvice for this as well !

Using a wrapper

A list wrapper is nice (that is, a class with a single field of type List), but from the responses above you will have to change the JSON as well ({"list": []} vs []), which is not nice...

Solution:

• in the wrapper, use @JsonValue annotation on the wrapped list field
• add a constructor taking a list as argument, and annotate it with @JsonCreator
• in your controller method, use @Valid @RequestBody ListWrapper tokenBodies

This works, is elegant, and doesn't require anything more. Moreover, it will throw the usual org.springframework.web.bind.MethodArgumentNotValidException on invalid beans.

---

Wrapper Example (java):

(For a full example in Kotlin, see https://stackoverflow.com/a/64060909)

public class ValidList {
    @JsonValue
    @Valid
    @NotNull
    @Size(min = 1, message = "array body must contain at least one item.")
    private List values;

    @JsonCreator
    public ValidList(E... items) {
        this.values = Arrays.asList(items);
    }

    public List getValues() {
        return values;
    }

    public void setValues(List values) {
        this.values = values;
    }
}

public class SomePojo {
    @Min(value = 1)
    int id;

    @Size(min = 2, max = 32)
    String token;

    // getters and setters
}

@RestController
public class SomeController {

    @PostMapping("/pojos")
    public ValidList test(@Valid @RequestBody ValidList pojos) {
        return pojos;
    }
}

Submit ok:

curl -H "Content-Type: application/json" -X POST http://localhost:8080/pojos -d '[{"id": 11, "token": "something"}]'

[{"token" : "something", "id" : 11}]

Submit empty body:

curl -H "Content-Type: application/json" -X POST http://localhost:8080/ns -d '[]'

{
   "timestamp" : "2020-09-25T09:55:05.462+00:00",
   "error" : "Bad Request",
   "message" : "Validation failed for object='validList'. Error count: 1",
   "exception" : "org.springframework.web.bind.MethodArgumentNotValidException",
   "path" : "/pojos",
   "status" : 400,
   "trace": "org.springframework.web.bind.MethodArgumentNotValidException: Validation failed for argument [0] in public com.example.demo.ValidList com.example.demo.SomeController.test(com.example.demo.ValidList): [Field error in object 'validList' on field 'values': rejected value [[]]; codes [Size.validList.values,Size.values,Size. [...]"
}

Submit invalid items:

curl -H "Content-Type: application/json" -X POST http://localhost:8080/ns -d '[{"id": -11, "token": ""}]'

{
   "timestamp" : "2020-09-25T09:53:56.226+00:00",
   "error" : "Bad Request",
   "message" : "Validation failed for object='validList'. Error count: 2",
   "exception" : "org.springframework.web.bind.MethodArgumentNotValidException",
   "path" : "/pojos",
   "status" : 400,
   "trace": "org.springframework.web.bind.MethodArgumentNotValidException: Validation failed for argument [0] in public com.example.demo.ValidList com.example.demo.SomeController.test(com.example.demo.ValidList) with 2 errors: [Field error in object 'validList' on field 'values[0].id': rejected value [-11]; co [...]"
}