Name: boT120536 Date: 02/08/2001


java version "1.3.0"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.0-C)
Java HotSpot(TM) Client VM (build 1.3.0-C, mixed mode)

The JDK object streams use the read/writeClassDescriptor methods for two
purposes. This makes overriding these methods very difficult.

In the most common case these methods are used to describe the class of an
object being serialized. In this case the class is always Serializable. This
is important because the only public way to get a ObjectStreamClass is through
ObjectStreamClass.lookup which will only return ObjectStreamClasses for
Serializable classes. Using this one can then implement a version of
writeClassDescriptor which sends the descriptor in a different way and a
version of readClassDescriptor which receives this information and uses
ObjectStreamClass.lookup to find or create the appropriate descriptor.

There is however a less common use of read/writeClassDescriptor which makes
overriding these methods much more difficult (though not quite impossible, see
workaround below). When an instance of a Class object is serialized out it is
handled specially and the stream ends up calling read/writeClassDescriptor for
the class of which the Class object represents. In this case the class may not
be Serializable (the Class object is an instance of java.lang.Class which is
Serializable, but may represent some other class which is not a Serializable).
Because the ObjectOutputStream uses default (package) access methods to
construct the ObjectStreamClass to be written this poses no problem for an
implementation of writeClassDescriptor. However a custom implementation of
readClassDescriptor has no means of creating an ObjectStreamClass for a class
which is not Serializable.

This all makes the ability to override the read/writeClassDescriptor much less
useful than it would otherwise be. It also seems to me to be part of a bigger
question about serializing Class objects. It seems a little strange to allow
this, that is what classloading is for.
(Review ID: 110681)
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