Java Reference
In-Depth Information
Everyserializedobjecthasanidentifier.Thedeserializationmechanismcomparesthe
identifier of the object being deserialized with the serialized identifier of its class (all
serializableclassesareautomaticallygivenuniqueidentifiersunlesstheyexplicitlyspe-
cifytheirownidentifiers)andcauses
InvalidClassException
tobethrownwhen
it detects a mismatch.
Perhaps you've added an instance field to a class, and you want the deserialization
mechanismtosettheinstancefieldtoadefaultvalueratherthanhave
readObject()
throwan
InvalidClassException
instance.(Thenexttimeyouserializetheob-
ject, the new field's value will be written out.)
You can avoid the thrown
InvalidClassException
instance by adding a
static final long serialVersionUID =
long integer value
;
declarationtotheclass.The
long integer value
mustbeuniqueandisknownas
a
stream unique identifier (SUID)
.
During deserialization, the JVM will compare the deserialized object's SUID to its
class'sSUID.Iftheymatch,
readObject()
won'tthrow
InvalidClassExcep-
tion
when it encounters a
compatible class change
(e.g., adding an instance field).
However, it will still throw this exception when it encounters an
incompatible class
change
(e.g., changing an instance field's name or type).
Note
Whenever you change a class in some way, you must calculate a new SUID
and assign it to
serialVersionUID
.
TheJDKprovidesa
serialver
toolforcalculatingtheSUID.Forexample,togen-
Employee.class
and execute
serialver Employee
. In response,
serial-
ver
generates the following output, which you paste (except for
Employee:
) into
Employee.java
:
Employee:
static
final
long
serialVersionUID
=
-6768634186769913248L;
TheWindowsversionof
serialver
alsoprovidesagraphicaluserinterface(GUI)
that youmight find more convenient to use. Toaccess this GUI, specify
serialver
-show
.WhentheGUIappears,enter
Employee
intheFullClassNametextfieldand
click the Show button, as demonstrated in
Figure 8-6
.
