Convert String to DTO in the validator of constraint - spring

Question

I have this field in a DTO:
private List<RoleDTO> roles;
It has a validator:
public class InternalUserValidator implements ConstraintValidator<InternalUserConstraint, Object> {
ObjectMapper mapper = new ObjectMapper();
mapper.enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY);
private String[] rolesAsString;
private List<RoleDTO> roles;
.
.
.
#Override
public boolean isValid(final Object value, final ConstraintValidatorContext context) {
//here i get as string, for example: RoleDTO{id=1, name='ADMIN'}
rolesAsString = BeanUtils.getArrayProperty(value, rolesFieldName);//should i use also getproperty again?
//then i try to convert to arraylist:
roles = (List<RoleDTO>) mapper.convertValue(rolesAsString, RoleDTO.class);
but it gives
Cannot construct instance of model.RoleDTO (out of START_ARRAY token
at [Source: UNKNOWN; line: -1, column: -1]
So, while in debug, i also tried this:
(List<RoleDTO>) mapper.convertValue("{\"id\":5,\"name\":\"sad\"}", RoleDTO.class)
This time:
Cannot construct instance of model.RoleDTO (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('{"id":5,"name":"sad"}')
at [Source: UNKNOWN; line: -1, column: -1]
What can i do?
This is RoleDTO:
public class RoleDTO implements Serializable {
private Long id;
private String name;
//getters setters
public RoleDTO() {
}

Answer 1

Cannot construct instance of model.RoleDTO (out of START_ARRAY token at [Source: UNKNOWN; line: -1, column: -1]
This Error Means you are trying to deserialize JSONArray to the RoleDto object.. Which is not possible.
The rolesAsString here has value of something like "[{\"id\":5,\"name\":\"sad\"}]"
Use ObjectMapper with TypeReference like this..
mapper.readValue(rolesAsString, new TypeReference<List<RoleDTO>>() {
});