writeExternal() , readExternal()

I'm not understanding.

I'm trying to restore the contents of a topComponent at startup to its 
closing state.  Its persistence type is TopComponent.PERSISTENCE_ALWAYS 
as per the @TopComponent.Description annotation but writeExternal and 
readExternal aren't being called which I thought would happen at 
application startup and shutdown respectively.

Looking at TopComponent it looks to me as if the only place these are 
called is from an inner class Replacer from writeObject and readObject 
but these methods don't seem to be called by anybody.

I suppose I can call my overridden methods myself from ModuleInstall 
restored() and close() but that hardly seems the right way to go about it.

Can anyone point me in the right direction please?

NetBeans IDE 8.1
Java 1.8.0_66 64bit
Ubuntu 14.04