Grails session-scoped Service throwing NotSerializable Exception - session

Question

I have a Grails 3 application (3.1.1) that implements a Breadcrumb system. I based it on this plugin for an earlier version of Grails.
It runs fine locally (using Grails command line command run-app), however, when I build and run on Tomcat 7, I get the following exception:
Jul 08, 2016 5:10:34 PM org.apache.catalina.ha.session.DeltaManager requestCompleted
SEVERE: Unable to serialize delta request for sessionid [6605EAC9C04DF7E2512493CABDFB1AD2]
java.io.NotSerializableException: au.packagename.BreadCrumbService
The above named service has a scope set to session, and is specified as a proxy. I have even added the Serializable implementation, however that does not seem to help.
class BreadCrumbService implements Serializable {
static transactional = false
static scope = 'session'
static proxy = true
}
I have an Interceptor that uses the Service to write the current controller and action as a Crumb object to the session. I have added the Serializable implementation to the Crumb and BreadCrumbType Enum (used in the Controllers annotation) that gets written to the session.
Is anyone able to indicate to me why this might be happening (I understand the concept of Serializable) and how I might go about resolving it?

Answer 1

#Hoàng Long's comment put me the right track.
When writing the Crumb object (all of which were made Serializable) to the session, I was using a static variable in the BreadCrumbService for the session map's key.
session[BreadCrumbService.NAMESPACE].crumb = new Crumb()
When I moved this variable to the Crumb object, the issue was resolved.
Even though the Service itself hasn't being written to the session. The mere reference to a variable inside the Service in the writing to the session required the service and every member to be Serializable.