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1.5 Defining Serializable Fields for a Class

The serializable fields of a class can be defined two different ways. Default serializable fields of a class are defined to be the non-transient and non-static fields. This default computation can be overridden by declaring a special field in the Serializable class, serialPersistentFields. This field must be initialized with an array of ObjectStreamField objects that list the names and types of the serializable fields. The modifiers for the field are required to be private, static, and final.

For example, the following declaration duplicates the default behavior.

class List implements Serializable {
    List next;

    private static final ObjectStreamField[] serialPersistentFields
                 = {new ObjectStreamField("next", List.class)};

}

By using serialPersistentFields to define the Serializable fields for a class, there no longer is a limitation that a serializable field must be a field within the current definition of the Serializable class. The writeObject and readObject methods of the Serializable class can map the current implementation of the class to the serializable fields of the class using the interface that is described in Section 1.7, "Accessing Serializable Fields of a Class." Therefore, the fields for a Serializable class can change in a later release, as long as it maintains the mapping back to its Serializable fields that must remain compatible across release boundaries.

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Note - There is, however, a limitation to the use of this mechanism to specify serializable fields for inner classes. Inner classes can only contain final static fields that are initialized to constants or expressions built up from constants. Consequently, it is not possible to set serialPersistentFields for an inner class (though it is possible to set it for static member classes). For other restrictions pertaining to serialization of inner class instances, see section Section 1.10, "The Serializable Interface".

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