Jersey's readEntity is serializing my Exceptions into java.lang.Throwable instead of the correct Error exception

[nddipiazza]

In my code I have an Exception that extends IllegalArgumentException:

public class InvalidDataSourceException extends IllegalArgumentException {
  public InvalidDataSourceException(String message, Throwable cause) {
    super(message, cause);
  }
}

And in my Jersey REST web service, all errors are wrapped with a class RESTError so that all rest services can throw this same exception:

public class RESTError extends Throwable {
    public RESTError(Throwable cause) {
        super(cause);
    }
}

So when the InvalidDataSourceException occurs, it is thrown as the cause of a RESTError:

throw new RESTError(invalidDataSourceException);

Then from another Java app, I call create a jersey client and call this service method, triggering the REST Error. Upon reading error status I attempt to readEntity(RESTError.class).

When I do this, the Exception cause is serialized to java.lang.Throwable, instead of my custom InvalidDataSourceException. It down-casted it. Why is that?