public Set<String> getFieldNames(Object object) throws Exception {
        Set<String> fieldNames = new HashSet<String>();
        for (FieldSerializer getter : sortedGetters) {
            Object value = getter.getPropertyValueDirect(object);
            if (value != null) {
                fieldNames.add(getter.fieldInfo.name);
            }
        }
        return fieldNames;
    }
Get field names of not null fields. Keep the same logic as getSize.

@param object the object to be checked
@return field name set
@throws Exception
@see #getSize(Object)
src/main/java/com/alibaba/fastjson/serializer/JavaBeanSerializer.java