Custom input validation with JAX-RS and JSON-B (Java EE 8)

The state of a POJO (Plain Old JSON Object :-) can be validated after retrieval:

public class Developer {

    public String language;
    public int age;
    //optional field
    public String description;

    public void validate() {
        if (!language.endsWith("va") && age < 200) {
            throw new WebApplicationException(Response.
                    status(400).
                    header("reason", "Learn something cool").
                    build()
            );
        }
    }

}

The DevelopersResource accepts the input, then calls Developer#validate:

@Path("developers")
public class DevelopersResource {

    @POST
    public String newDeveloper(Developer developer) {
        developer.validate();
        return developer.language + " " + developer.age;
    }
}    

Test:

returns

HTTP/1.1 200 OK
X-Powered-By: Servlet/4.0
Content-Type: application/json
Date: Fri, 14 Dec 2018 08:40:24 GMT
Content-Language: en-US
Content-Length: 7

java 42%      

however

yields:

HTTP/1.1 400 Bad Request
X-Powered-By: Servlet/4.0
reason: Learn something cool
Date: Fri, 14 Dec 2018 08:42:43 GMT
Content-Length: 0
Content-Language: en-US
Connection: Close    

ThinWAR size: 4.7kB, deployed with: WAD