I have developed a web application usin JSP technology.
It works fine in local, but when I deploy it to the server I find in Tomcat's logs:

java.io.WriteAbortedException: writing aborted; java.io.NotSerializableException: it.gtagic.prs.bip.daos.PianoImplementation
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1278)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:324)
at java.util.ArrayList.readObject(ArrayList.java:547)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at java.io.ObjectStreamClass.invokeReadObject(ObjectStreamClass.java:838)
at java.io.ObjectInputStream.readSerialData(ObjectInputStream.java:1746)
at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1646)
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1274)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:324)
at org.apache.catalina.session.StandardSession.readObject(StandardSession.java:1369)
at org.apache.catalina.session.StandardSession.readObjectData(StandardSession.java:864)
at org.apache.catalina.session.StandardManager.load(StandardManager.java:440)
at org.apache.catalina.session.StandardManager.start(StandardManager.java:655)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:3570)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1188)
etc
Caused by: java.io.NotSerializableException: it.gtagic.prs.bip.daos.PianoImplementation
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1054)
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:278)
at java.util.ArrayList.writeObject(ArrayList.java:529)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:324)
at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:809)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1296)
at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1247)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1052)
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:278)
at org.apache.catalina.session.StandardSession.writeObject(StandardSession.java:1439)
at org.apache.catalina.session.StandardSession.writeObjectData(StandardSession.java:881)
at org.apache.catalina.session.StandardManager.unload(StandardManager.java:536)
at org.apache.catalina.session.StandardManager.stop(StandardManager.java:691)
at org.apache.catalina.core.StandardContext.stop(StandardContext.java:3665)
at org.apache.catalina.core.ContainerBase.removeChild(ContainerBase.java:1036)
at org.apache.catalina.core.StandardHostDeployer.remove(StandardHostDeployer.java:470)
at org.apache.catalina.core.StandardHost.remove(StandardHost.java:852)
at org.apache.catalina.startup.HostConfig.undeployApps(HostConfig.java:758)
at org.apache.catalina.startup.HostConfig.stop(HostConfig.java:738)
etc

I'm getting crazy!!!
Any help?
Thanks in advance BYE