Object Serialization Enhancements
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Documentation Contents
Strings longer than 64K can now be serialized (since 1.3)
Prior to 1.3, an attempt to serialize a string longer than 64K would result in a being thrown. In 1.3, the serialization protocol has been enhanced to allow strings longer than 64K to be serialized. Note that if a 1.2 (or earlier) JVM attempts to read a long string written from a 1.3-compatible JVM, the 1.2 (or earlier) JVM will receive a

Serialization performance enhancements (since 1.3)
Several changes have been made to serialization to improve overall performance:
  • UTF string reads/writes have been optimized to reduce unnecessary memory allocation and synchronization/method call overhead.
  • Code for reading and writing primitive data arrays has been streamlined. Float and double array reads/writes have been reimplemented to minimize the number of calls to native methods.
  • Internal buffering has been improved.
  • Reflective operations for getting/setting primitive field values have been batched to minimize the number of separate native method calls.

Improved exception reporting (since 1.3)
If a class cannot be found during the class resolution process of deserialization, the original java.lang.ClassNotFoundException is thrown instead of a generic one so that more information about the failure is available. Also, deserialization exception reporting now includes maintaining the name of the original class that could not be found instead of reporting a higher-level class that was being deserialized. For example, if (in an RMI call) the stub class can be found but the remote interface class cannot, the serialization mechanism will now report correctly that the interface class was the class that could not be found instead of erroneously reporting that the stub class could not be found., (since 1.3)
The writeClassDescriptor and readClassDescriptor methods have been added to provide a means of customizing the serialized representation of class descriptors. writeClassDescriptor is called when an instance of needs to be serialized, and is responsible for writing the ObjectStreamClass to the serialization stream. Conversely, readClassDescriptor is called when the ObjectInputStream expects an ObjectStreamClass instance as the next item in the serialization stream. By overriding these methods, subclasses of ObjectOutputStream and ObjectInputStream can transmit class descriptors in an application-specific format. For more information, refer to sections 2.1 and 3.1 of the Java Object Serialization Specification., (since 1.3)
These methods are similar in purpose to ObjectOutputStream.annotateClass and ObjectInputStream.resolveClass, except that they apply to dynamic proxy classes (see java.lang.reflect.Proxy), as opposed to non-proxy classes. Subclasses of ObjectOutputStream may override annotateProxyClass to store custom data in the stream along with descriptors for dynamic proxy classes. ObjectInputStream subclasses may then override resolveProxyClass to make use of the custom data in selecting a local class to associate with the given proxy class descriptor. For details, see section 4 of the Java Object Serialization Specification.

The javadoc tool tags @serial, @serialField, and @serialData (since 1.2)
The javadoc tags @serial, @serialField, and @serialData have been added to provide a way to document the serialized form of a class. Javadoc generates a serialization specification based on the contents of these tags. For details, refer to section 1.6 of the Java Object Serialization Specification.

Protocol versioning (since 1.2)
Prior to 1.2, object serialization used a protocol that did not support skipping over objects implementing the interface if the classes for those objects were not available. In 1.2, a new protocol version was added which addressed this deficiency. For backwards compatibility, ObjectOutputStream and ObjectInputStream can read and write serialization streams written in either protocol; the protocol version used can be selected by calling the ObjectOutputStream.useProtocolVersion method. For details and a discussion of compatibility issues, see section 6.3 of the Java Object Serialization Specification.

Class-defined writeReplace and readResolve methods (since 1.2)
Since 1.2, classes can define writeReplace and readResolve methods which allow instances of the given classes to nominate replacements for themselves during serialization and deserialization. The required signatures of these methods, along with further details, are described in sections 2.5 and 3.6 of the Java Object Serialization Specification., (since 1.2)
Since 1.2, subclasses of ObjectOutputStream and ObjectInputStream can implement a custom serialization protocol by overriding the writeObjectOverride and readObjectOverride methods. Note that these methods will only be called if the ObjectOutputStream/ObjectInputStream subclasses possess the permission"enableSubclassImplementation"), and call the no-argument constructors of ObjectOutputStream/ObjectInputStream. See sections 2.1 and 3.1 of the Java Object Serialization Specification for more information.

Security permission checks (since 1.2)
Subclasses of ObjectOutputStream and ObjectInputStream may override inherited methods to obtain "hooks" into certain aspects of the serialization process. Since 1.2, object serialization uses the 1.2 security model to verify that subclasses possess adequate permissions to override certain hooks. The permissions"enableSubclassImplementation") and"enableSubstitution") govern whether or not the methods ObjectOutputStream.writeObjectOverride, ObjectOutputStream.replaceObject, ObjectInputStream.readObjectOverride, and ObjectInputStream.resolveObject will be called during the course of serialization. See sections 2.1 and 3.1 of the Java Object Serialization Specifications for more information.

Defining serializable fields for a class (since 1.2)
By default, the values of all non-static and non-transient fields of a serializable class are written when an instance of that class is serialized. In 1.2, a new mechanism was introduced to allow classes finer control of this process. By declaring a special field serialPersistentFields, serializable classes can dictate which fields will be written when instances of the class (or subclasses) are serialized. This feature also enables classes to "define" serializable fields which do not correspond directly to actual fields in the class. Used in conjunction with the serializable fields API (described below), this capability allows fields to be added or removed from a class without altering the serialized representation of the class. See sections 1.5 and 1.7 of the Java Object Serialization Specification for details.

Serializable fields API (since 1.2)
Introduced in 1.2, the serializable fields API allows class-defined writeObject/readObject methods to explicitly set and retrieve serializable field values by name and type. This API is particularly useful for classes that need to maintain backwards compatibility with older class versions; in some cases, the older version of the class may have defined a set of serializable fields that cannot be mapped directly to the fields of the current class. In this case, newer versions of the class can define custom writeObject and readObject methods that convert the internal state of a given instance of the (new) class into the "old" serialized form, and vice versa. For more information, see section 1.7 of the Java Object Serialization Specification.
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