protected char writeBefore(JSONSerializer jsonBeanDeser, //
                            Object object, char seperator) {
        
        if (jsonBeanDeser.beforeFilters != null) {
            for (BeforeFilter beforeFilter : jsonBeanDeser.beforeFilters) {
                seperator = beforeFilter.writeBefore(jsonBeanDeser, object, seperator);
            }
        }
        
        if (this.beforeFilters != null) {
            for (BeforeFilter beforeFilter : this.beforeFilters) {
                seperator = beforeFilter.writeBefore(jsonBeanDeser, object, seperator);
            }
        }
        
        return seperator;
    }
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