| Interface MultiDoc specifies the interface for an object that supplies more
than one piece of print data for a Print Job. "Doc" is a short,
easy-to-pronounce term that means "a piece of print data," and a "multidoc"
is a group of several docs. The client passes to the Print Job an object
that implements interface MultiDoc, and the Print Job calls methods on
that object to obtain the print data.
Interface MultiDoc provides an abstraction similar to a "linked list" of
docs. A multidoc object is like a node in the linked list, containing the
current doc in the list and a pointer to the next node (multidoc) in the
list. The Print Job can call the multidoc's {@link #getDoc()
getDoc()} method to get the current doc. When it's ready to go
on to the next doc, the Print Job can call the multidoc's {@link #next()
next()} method to get the next multidoc, which contains the
next doc. So Print Job code for accessing a multidoc might look like this:
void processMultiDoc(MultiDoc theMultiDoc) {
MultiDoc current = theMultiDoc;
while (current != null) {
processDoc (current.getDoc());
current = current.next();
}
}
Of course, interface MultiDoc can be implemented in any way that fulfills
the contract; it doesn't have to use a linked list in the implementation.
To get all the print data for a multidoc print job, a Print Service
proxy could use either of two patterns:
-
The interleaved pattern: Get the doc from the current multidoc. Get
the print data representation object from the current doc. Get all the print
data from the print data representation object. Get the next multidoc from
the current multidoc, and repeat until there are no more. (The code example
above uses the interleaved pattern.)
-
The all-at-once pattern: Get the doc from the current multidoc, and
save the doc in a list. Get the next multidoc from the current multidoc, and
repeat until there are no more. Then iterate over the list of saved docs. Get
the print data representation object from the current doc. Get all the print
data from the print data representation object. Go to the next doc in the
list, and repeat until there are no more.
Now, consider a printing client that is generating print data on the fly and
does not have the resources to store more than one piece of print data at a
time. If the print service proxy used the all-at-once pattern to get the
print data, it would pose a problem for such a client; the client would have
to keep all the docs' print data around until the print service proxy comes
back and asks for them, which the client is not able to do. To work with such
a client, the print service proxy must use the interleaved pattern.
To address this problem, and to simplify the design of clients providing
multiple docs to a Print Job, every Print Service proxy that supports
multidoc print jobs is required to access a MultiDoc object using the
interleaved pattern. That is, given a MultiDoc object, the print service
proxy will call {@link #getDoc() getDoc()} one or more times
until it successfully obtains the current Doc object. The print service proxy
will then obtain the current doc's print data, not proceeding until all the
print data is obtained or an unrecoverable error occurs. If it is able to
continue, the print service proxy will then call {@link #next()
next()} one or more times until it successfully obtains either
the next MultiDoc object or an indication that there are no more. An
implementation of interface MultiDoc can assume the print service proxy will
follow this interleaved pattern; for any other pattern of usage, the MultiDoc
implementation's behavior is unspecified.
There is no restriction on the number of client threads that may be
simultaneously accessing the same multidoc. Therefore, all implementations of
interface MultiDoc must be designed to be multiple thread safe. In fact, a
client thread could be adding docs to the end of the (conceptual) list while
a Print Job thread is simultaneously obtaining docs from the beginning of the
list; provided the multidoc object synchronizes the threads properly, the two
threads will not interfere with each other |
