How to validate a form in play framework

I want do validate a form after filling with a json object. By stepping throw the code I see the correct values in the Post Object but I will get no errors if the title is under 5 chars. Why, had some one an idea?

@Constraints.Required
@Constraints.MinLength(5)
private String title;

Post post = Json.fromJson(json, Post.class);                            
if(postForm.hasErrors()){

/*
example
*/
private FormFactory formFactory;

    @Inject
    YourContructor(FormFactory formFactory){
        this.formFactory
    }

    @BodyParser.Of(value = BodyParser.Json.class)

 public static Result create() {

        JsonNode json = request().body().asJson();

       Form<Post> post= formFactory.form(Post.class).bind(json);
        if(post.hasErrors()){
            return badRequest(post.errorsAsJson());
    }

        return ok(json);
    }