private Constructor<?> lookupBuilderForStruct(String structName, Class<?> currentClass) {
        String currentClassName = currentClass.getName();
        Class<?> structClass = getInnerClass(currentClass, currentClassName + "$" + structName);
        if (structClass == null) {
            log.info("Could not find nested class '" + currentClassName + "$" + structName +
                             "'. Ignoring it, assuming it's been added to a newer version of the config.");
            return null;
        }
        return getStructBuilderConstructor(structClass, currentClassName, structName);
    }
Checks if this field is of type 'path', in which
case some special handling might be needed. Caches the result.
config/src/main/java/com/yahoo/vespa/config/ConfigPayloadApplier.java