The error below when configure context attribute replication in tomcat cluster:

NotSerializableException: org.zkoss.web.util.resource.ClassWebResource$CWC

After further investigation with the test code attached, found the result below:
Can not be serialized:
class org.zkoss.web.util.resource.ResourceCache
class org.zkoss.zk.au.http.DHtmlUpdateServlet
class org.zkoss.zk.ui.http.WebManager
class java.util.Collections$SynchronizedMap
class java.util.Collections$SynchronizedMap

Can be serialized:
class java.util.concurrent.ConcurrentHashMap
class java.lang.String
class java.lang.String
class java.io.File
class java.util.concurrent.ConcurrentHashMap