Effective Java

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CHAPTER 12
SERIALIZATION
366
The serialization proxy pattern has two limitations. It is not compatible with
classes that are extendable by their users (Item 19). Also, it is not compatible with
some classes whose object graphs contain circularities: if you attempt to invoke a
method on such an object from within its serialization proxy’s 
readResolve
method, you’ll get a 
ClassCastException
because you don’t have the object yet,
only its serialization proxy.
Finally, the added power and safety of the serialization proxy pattern are not
free. On my machine, it is 14 percent more expensive to serialize and deserialize
Period
instances with serialization proxies than it is with defensive copying.
In summary, consider the serialization proxy pattern whenever you find
yourself having to write a 
readObject
or 
writeObject
method on a class that is
not extendable by its clients. This pattern is perhaps the easiest way to robustly
serialize objects with nontrivial invariants.

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367
A P P E N D I X
Items Corresponding to Second Edition