1) The Serializable interface is just a marker. It means that objects can be serialized, i.e. they can be represented as a bit string and restored from that bit string.
For example, both of your classes are serializable because they can be represented as a bit string (or regular string). On the other hand, a class that represents a file handle given out by the operating system can not be serialized: As soon as the program is finished, that handle is gone, and there is no way to get it back. Reopening the file by file name is not guaranteed to work, since it may have been deleted/moved/changed permissions in the meantime.
2) Serializing objects that don't implement the Serializable interface will result in a NotSerializableException being thrown.
3) According to the documentation:
mappedBy
This element is only specified on the inverse (non-owning) side of the association.
