Validation of a list of objects in Spring

Question

I have the following controller method:

@RequestMapping(value=\"/map/update\", method=RequestMethod.POST, produces = \"application/json; charset=utf-8\")
@Re         


        

Answer 1

(this answer is in Kotlin, for Java see https://stackoverflow.com/a/64061936)

For those using kotlin and jackson, here is the ValidatedList class that do not require wrapping, that is, it will still be serialized/deserialized as a usual list:

class ValidatedList {
    /**
     * By default, spring-boot cannot validate lists, as they are generic AND do not conform to the Java Bean definition.
     * This is one work-around: create a wrapper that fits the Java Bean definition, and use Jackson annotations to
     * make the wrapper disappear upon (de)serialization.
     * Do not change anything (such as making the _value field private) or it won't work anymore !
     * 
     * Usage:
     * ```
     * @PostMapping("/something")
     * fun someRestControllerMethod(@Valid @RequestBody pojoList: ValidatedList){
     *     // access list with:
     *     pojoList.values
     *}
     * ```
     */

    @JsonValue
    @Valid
    @NotNull
    @Size(min = 1, message = "array body must contain at least one item.")
    var _values: List? = null

    val values: List
        get() = _values!!

    @JsonCreator
    constructor(vararg list: E) {
        this._values = list.asList()
    }
}

Advantages:

• no need for the @Validated annotation
• will throw an error if the body is an empty array (see @Size)
• the exception will be mapped correctly to 400 Bad Request (which is not the case when using javax and @Validated annotation)

---

Example:

data class N(
    @field:Min(value = 0, message = "id must be positive.")
    val id: Long? = null,

    @field:NotNull
    @field:Size(min = 2, max = 32, message = "wrong size: should be 32 chars long.")
    val token: String? = null
)

@RestController
class XController {
    @PostMapping("/ns")
    fun getNs(@Valid @NotNull @RequestBody wrap: ListWrapper) = wrap
}

Submit ok:

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

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

Submit empty body:

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

{
   "timestamp" : "2020-09-25T08:49:30.324+00:00",
   "message" : "Validation failed for object='listWrapper'. Error count: 1",
   "error" : "Bad Request",
   "path" : "/ns",
   "status" : 400,
   "exception" : "org.springframework.web.bind.MethodArgumentNotValidException",
   "trace":"org.springframework.web.bind.MethodArgumentNotValidException: Validation failed for argument [0] in public com.example.demo.test.XController$ListWrapper com.example.demo.test.XController.getNs(com.example.demo.test.XController$ListWrapper): [Field error in object 'listWrapper' on field '_values': rejected value [[]]; codes [Size.listWrapper._values,Size._values,Size.java.util.List,Size]; [...]"
}

Submit invalid items:

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

{
   "message" : "Validation failed for object='listWrapper'. Error count: 2",
   "path" : "/ns",
   "exception" : "org.springframework.web.bind.MethodArgumentNotValidException",
   "timestamp" : "2020-09-25T08:49:54.505+00:00",
   "error" : "Bad Request",
   "status" : 400,
   "trace":"org.springframework.web.bind.MethodArgumentNotValidException: Validation failed for argument [0] in public com.example.demo.test.XController$ListWrapper com.example.demo.test.XController.getNs(com.example.demo.test.XController$ListWrapper) with 2 errors: [...]"
}