Java IO.pdf - Nguyen Dang Binh

Java I/O
• All changes to transient fields—changing their type, their names, adding or removing
them, etc. Serialization ignores all transient fields.
• Adding an instance field. When an instance of the older version of the class is
deserialized, the new field will merely be set to its default value (0 for numeric types,
false for booleans, null for object types) when an object in that class is deserialized.
• Adding or removing an interface (except the Serializable interface) from a class.
Interfaces say nothing about the instance fields of a class.
• Adding or removing inner classes, provided no nontransient instance field has the type
of the inner class.
• Changing the access specifiers of a field. Serialization does not respect access
protection.
• Changing a field from static to nonstatic or transient to nontransient. This is the same
as adding a field.
The following incompatible changes prevent deserialization of serialized objects:
• Changing the name of a class.
• Changing the type of an instance field.
• Changing the name of an instance field. This is the same as removing the field with
the old name.
• Changing a field from nonstatic to static or nontransient to transient. This is the same
as removing the field.
• Changing the superclass of a class. This may affect the inherited state of an object.
• Changing the writeObject() or readObject() method (discussed later) in an
incompatible fashion.
• Changing a class from Serializable to Externalizable (discussed later) or
Externalizable to Serializable.
11.7.2 Version IDs
To help identify compatible or incompatible classes, each class may have a stream unique
identifier, SUID for short. This is a long calculated by a special hash function and stored in
the class in a static field called serialVersionUID like this:
public class UnicodeApplet extends Applet {
static final long serialVersionUID = 5913267123532863320L;
// ...
Every time you release a new version of a class that makes an incompatible change, you
should change the serialVersionUID field. The serialver tool, included with the JDK,
calculates the appropriate hash of the class's name and fields. For example:
% serialver UnicodeApplet
UnicodeApplet: static final long serialVersionUID =
5913267123532863320L;
There's also a GUI interface available with the -show flag, as shown in Figure 11.2.
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