public final boolean isRef() {
        if (sp != 4) {
            return false;
        }

        return charAt(np + 1) == '$' //
                && charAt(np + 2) == 'r' //
                && charAt(np + 3) == 'e' //
                && charAt(np + 4) == 'f';
    }
internal method, don't invoke
@param token
src/main/java/com/alibaba/fastjson/parser/JSONLexerBase.java