与完全不可序列化祖先的可序列化子级相比，在完全可序列化的层次结构中序列化类的好处/缺点是什么？

Question

That is

(1)

class A 
B extends A
C extends B
D extends C 
E extends D implements Serializable

vs.

(2)

A implements Serializable
B extends A
C extends B
D extends C 
E extends D

(1)In the first case, serializing E will result in the state of E being saved, and A through D being reconstructed upon deserialization through a call to D's default constructor.

(2)In the second case, serializing E will result in the state of A through E all being saved, and no reconstruction required upon deserialization (since everything would be seriailizable due to inheriting from a serializable base class.)

Are there any dangers to doing it the first way? Would there be circumstances in which the savings of data transferred during serialization/deserialization by doing it the first way would be sufficient to make it a preferred approach, or is it always best to fully serialize the class hierarchy?

Thanks in advance.

Answer 1

It is not usually considered a best practice to implement Serializable in a superclass, particularly if it is intended as a base class in a hierarchy of an exported API (however, your base class should be designed to anticipate subclasses that implement Serializable; see below). The reason is that it forces the clients of your API to make their subclasses serializable whether or not it useful for them to do so (Essential Java, 2nd Ed).

The exception to this rule is when the base class itself participates in a framework for which Serializable is necessary.

Regardless, there are problems that can arise if a subclass is serializable and its parent isn't. In such a case the subclass is responsible for serializing and deserializing the state of its parent. The drawbacks are:

• the parent fields can not be final
• if the parent fields are not accessible to the subclass, then a correct serialized representation may not be possible
• parent classes that don't implement Serializable themselves require no-arg constructors

There is an excellent discussion of the pitfalls and recommended practices of serializing Java instances in Essential Java, 2nd Ed.