Java IO.pdf - Nguyen Dang Binh

Java I/O
For example, valid XML documents are essentially trees of elements combined with a
document type definition (DTD). The DTD defines a grammar the document must follow. [3]
The Document Object Model (DOM) defines a means of representing XML (and HTML)
documents as instances of Java classes and interfaces, including XMLNode, EntityReference,
EntityDeclaration , DocumentType, ElementDefinition , AttributeDefinition , and
others.
An XML document could be saved as a set of these serialized objects. In that case, when you
deserialized the document, you would want to check that the deserialized document is still
valid; that is, that the document adheres to the grammar given in the DTD. You can't do this
until the entire document—all its elements, and its entire DTD—has been read. There are also
a number of smaller checks you might want to perform. For instance, well-formedness (wellformedness
is a slightly less stringent requirement than validity) requires that all entity
references like &date; be defined in the DTD. To check this, it's not enough to have
deserialized the EntityReference object. You must also have deserialized the corresponding
DocumentType object that contains the necessary EntityDeclaration objects.
You can use the ObjectInputStream class's registerValidation() method to specify an
ObjectInputValidation object that will be notified of the object after its entire graph has
been reconstructed but before readObject() has returned it. This gives the validator an
opportunity to make sure that the object doesn't violate any implicit assertions about the state
of the system.
public synchronized void registerValidation(ObjectInputValidation oiv,
int priority) throws NotActiveException, InvalidObjectException
This method is invoked inside the readObject() method of the object that needs to be
validated. Every time the readObject() method is called to read an object, that object is
registered with the stream as needing to be validated when the rest of the graph is available.
Invoking the registerValidation() method from anywhere except the readObject()
method throws a NotActiveException. The oiv argument is the object that implements the
ObjectInputValidation interface and that will validate deserialized objects. Most of the
time, this is the object that has the readObject() method; that is, objects tend to validate
themselves. The priority argument determines the order in which objects will be validated
if there's more than one registered ObjectInputValidation object for the class. Validators
with higher priorities are invoked first.
The ObjectInputValidation interface declares a single method, validateObject():
public abstract void validateObject() throws InvalidObjectException
If the object is invalid, validateObject() throws an InvalidObjectException.
For example, let's suppose an application maintains a hashtable of Person objects, each of
which is identified primarily by its Social Security Number. Let's further suppose that the
application doesn't allow two Person objects with the same Social Security Number to exist
at the same time. You can use an ObjectInputValidation to ensure that this doesn't happen.
Example 11.6 demonstrates.
3 For more details, see XML: Extensible Markup Language, by Elliotte Rusty Harold, IDG Books, 1998.
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