serialization - Why should Java's value-based classes not be serialized?

Question

Since Version 8 Java has the concept of value-based classes. This is in preparation of a future version which will most likely allow the definition of value types. Both definitions/descriptions mention serialization (bold face added by me):

About the existing value-based classes:

A program may produce unpredictable results if it attempts to distinguish two references to equal values of a value-based class, whether directly via reference equality or indirectly via an appeal to synchronization, identity hashing, serialization, or any other identity-sensitive mechanism.

About future value types:

The default identity-based hash code for object, available via System.identityHashCode, also does not apply to value types. Internal operations like serialization which make identity-based distinctions of objects would either not apply to values (as they do not apply to primitives) or else they would use the value-based distinction supplied by the value type’s hashCode method.

Because future JVM implementations might not use object headers and reference pointers for value-based classes, some of the limitations are clear. (E.g. not locking on an identity which the JVM must not uphold. A reference on which is locked could be removed and replaced by another later, which makes releasing the lock pointless and will cause deadlocks).

But I don't get how serialization plays into this. Why is it considered an "identity-sensitive mechanism"? Why does it "make identity-based distinctions of objects"?

Answer 1

Serialization uses System.identityHashCode (via IdentityHashMap) to ensure that the topology of the object graph resulting from deserialization is topologically equivalent to that of the input graph.