@Override
        public void ensureValid(String name, Object value) {
            if (!(value instanceof List)) {
                throw new ConfigException("Invalid value type for listeners (expected list of URLs , ex: http://localhost:8080,https://localhost:8443).");
            }

            List<?> items = (List<?>) value;
            if (items.isEmpty()) {
                throw new ConfigException("Invalid value for listeners, at least one URL is expected, ex: http://localhost:8080,https://localhost:8443.");
            }

            for (Object item : items) {
                if (!(item instanceof String)) {
                    throw new ConfigException("Invalid type for listeners (expected String).");
                }
                if (Utils.isBlank((String) item)) {
                    throw new ConfigException("Empty URL found when parsing listeners list.");
                }
            }
        }
Add the properties related to an internal-only server to the given {@link ConfigDef}.
@param configDef the {@link ConfigDef} to add the properties to; may not be null
connect/runtime/src/main/java/org/apache/kafka/connect/runtime/rest/RestServerConfig.java