private static String mapExpression(String attribute) {
        final int indexOf = attribute.indexOf("[");
        return toGetMethod(attribute.substring(0, indexOf))
            + "(" + attribute.substring(indexOf + 1, attribute.length() - 1) + ")";
    }
@return empty if this attribute is not type of map.
oap-server/oal-rt/src/main/java/org/apache/skywalking/oal/rt/util/ClassMethodUtil.java