Java Reference
In-Depth Information
communication between Client and MyServer . In particular, DSocketActivity
receives a message with the name of a requested service, creates the
corresponding DParser object, which decodes the message and starts
MyService .
Phase 2: Client writes a message on the socket including a command to
MyService . DSocketActivity receives the message, decodes the command
and invokes the corresponding method of MyService .
Phase 3: DSocketActivity is listening to events raised by MyService . When it
notifies an event, DSocketActivity encodes the event in a socket message
and sends it to Client .
Decision point
How does the control module support HTTP-based communication?
Since the HTTP protocol does not support “push” operations, the remote
client, i.e. a web browser running on a desktop computer or on a mobile
phone, cannot be notified of events raised by the control module. Thus, we
need first to extend the MMI framework in order to add support for the pull
model of event notification. When Client requests the activation of a service
(e.g. MyService ) it specifies whether it supports the push model or not. If it
does not support this model, class DSocketActivity catches all the events
raised by MyService and registers them in an event list. Client is in charge of
querying DSocketActivity periodically about registered events.
The bridge between the HTTP communication protocol and the TCP
Socket mechanism is implemented using a servlet component that needs to
be implemented for each specific client application. The servlet runs inside
a web server, implements the basic doPOST() and doGET() methods to inter-
act with the remote web browser, and communicates with DSocketActivity
through sockets. Whenever the web browser refreshes the HTTP connection
with the web server, the servlet writes a query message on the socket.
DSocketActivity parses the message, extracts registered events from the event
list, and composes them into a response message that it writes back to the
servlet. Finally, the servlet composes a new web page according to the
received events and forwards it to the web browser.
The implementation of class DSocketListener , class DSocketActivity and
DParser is quite simple. It should be noted that method step() of class
DSocketActivity uses reflection to create an instance of the parser associated
with the requested service and that the DSocketActivity thread dies when the
remote client closes the socket. When this occurs, it catches a SocketException
and invokes method stopActivity() of class DProcess . Finally, method quit() of
class SocketActivity , closes the socket's input
output buffers and stops the
Search WWH ::

Custom Search