[Chapter 9] 9.5 Advanced Serialization
Author: David Flanagan
Date: Thu Jul 31 15:56:49 1997

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| Chapter 9 Object Serialization |

## 9.5 Advanced Serialization

The Object Serialization API also has some advanced features beyond those demonstrated above. One such feature is the `Externalizable` interface. If a class implements this interface, the `ObjectInputStream` and `ObjectOutputStream` classes use an object's `readExternal()` and `writeExternal()` methods to read and write its state from a stream. Most classes that want custom serialization behavior simply implement `readObject()` and `writeObject()` methods to do pre- and post-processing. `Externalizable` objects, on the other hand, take complete control over reading and writing their state. This interface is intended for objects that do not even want to use the basic serialized object data format used for all `Serializable` objects. A word processor object, for example, might "externalize" itself using its own native file format.

Another advanced serialization feature is the ability to register an `ObjectInputValidation` object that can verify that an object graph has been deserialized in a consistent, valid state. Typically, the root of an object graph registers such a validation object by calling the `registerValidation()` method of the `ObjectInputStream` from its own custom `readObject()` method. Then, when the graph has been read completely, the `validateObject()` method of the `ObjectInputValidation` object is called to perform whatever validity checks are necessary.

Finally, you may notice that `ObjectOutputStream` and `ObjectInputStream` have methods like `annotateClass()`, `replaceObject()`, `resolveClass()`, and `resolveObject()`. These are intended for use by subclasses of the object streams that implement special kinds of object serialization behavior.

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