public static Object getField(ConfigBuilder builder, String fieldName) {
        try {
            Field f = builder.getClass().getDeclaredField(fieldName);
            f.setAccessible(true);
            return f.get(builder);
        } catch (NoSuchFieldException | SecurityException | IllegalArgumentException | IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }
Gets the value of a private field on a Builder.
@param builder a {@link com.yahoo.config.ConfigBuilder}
@param fieldName a config field name
@return the value of the private field
config/src/main/java/com/yahoo/config/subscription/ConfigInstanceUtil.java