Java serialization: readObject() vs. readResolve()

Question

The book Effective Java and other sources provide a pretty good explanation on how and when to use the readObject() method when working with serializable Java classes. The readResolve() method, on the other hand, remains a bit of a mystery. Basically all documents I found either mention only one of the two or mention both only individually.

Questions that remain unanswered are:

• What is the difference between the two methods?
• When should which method be implemented?
• How should readResolve() be used, especially in terms of returning what?

I hope you can shed some light on this matter.

Answer 1

readResolve is used for replacing the object read from the stream. The only use I've ever seen for this is enforcing singletons; when an object is read, replace it with the singleton instance. This ensures that nobody can create another instance by serializing and deserializing the singleton.