The Object ClientServer Framework OCSF

1
The Object Client-Server Framework (OCSF)
ConnectionToClient
AbstractServer
AbstractClient
openConnection

sendToClient
listen
sendToServer
stopListening
close
close
closeConnection
setInfo
sendToAllClients
connectionClosed
getClientConnections
getInfo
connectionException
clientConnected
clientDisconnected
connectionEstablished
clientException
handleMessageFromServer
serverStarted
serverStopped
listeningException
serverClosed
Software installed on Client host.
Software installed on Server host.
handleMessageFromClient
2
The Methods Provided By The OCSF

• The OCSF provides three types of methods, which
  are accessible by application developers
• Service methods, which represent the API of the
  OCSF
• Controlling methods.
• Accessing methods.
• Slot methods, also called callback methods
• These must be implemented.
• These methods are declared abstract in the OCSF,
  and, therefore, a concrete implementation is
  enforced.
• Hook methods , also called callback methods
• An application developer may choose optionally to
  implement these methods.
• These methods are declared with empty bodies in
  the OCSF, and, therefore, the application
  developer has the option to override them (or
  not).

3
The API of AbstractClient(Service Methods)

• Controlling methods
• openConnection
• Opens a connection with the server.
• Requires the host name and port number of the
  server.
• closeConnection
• Closes the socket as well as the I/O streams.
• sendToServer
• Sends the specified object to the server using
  the output stream.
• The specified object is typically a string
  message for another client.
• The message could be any type of object that
  supports serialization.

4
The API of AbstractClient(Service Methods)

• Accessing methods
• isConnected
• Returns (boolean) if the client is connected.
• getHost
• Returns the instance variable (String host) host
  name of the server.
• setHost
• Sets the instance variable (String host) the
  host name of the server.
• getPort
• Returns the instance variable (String port) the
  port of the server.
• setPort
• Sets the instance variable (String port) the
  port of the server.
• getInetAddress
• Returns the clients Internet address.

5
The Slot Method of AbstractClient(A Callback
Method)

• handleMessageFromServer
• Must be implemented.
• At any time after a client has been connected to
  a server, the server may send the client a
  message asynchronously.
• The OCSF receives this message from the
  InputStream of the Socket, and then calls the
  method handleMessageFromServer.
• The Client app must provide a concrete
  implementation, and do something useful, like
  displaying the message.

Why is this called a call back?
Why does this method have to be implemented?
6
The Hook Methods of AbstractClient(A Callback
Method)

• connectionEstablished
• Method may be overridden.
• After the openConnection method has been called
  by the client app, the OCSF will call the
  connectionEstablished method when a connection
  has been established.
• The OCSF defines this method with an empty body.
• An application developer can override this method
  and do something interesting, such as let the
  user know a connection has been established.

Why is this called a call back?
7
The Hook Methods of AbstractClient(A Callback
Method)

• connectionClosed
• Method may be overridden.
• After the closeConnection method has been called
  by the client app, the OCSF will call the
  connectionEstablished method when a connection
  has been closed.
• The OCSF defines this method with an empty body.
• An application developer can override this method
  and do something interesting, such as let the
  user know a connection has been closed.

Why is this called a call back?
8
The Hook Methods of AbstractClient(A Callback
Method)

• connectionException
• Method may be overridden.
• If the connection to the server is unexpectedly
  interrupted for some reason, the OCSF catches the
  exception and then calls the connectionException
  method.
• The OCSF defines this method with an empty body.
• An application developer can override this method
  and do something interesting, such as let the
  user know a problem has been encountered.

Why is this called a call back?
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Instance Variables of AbstractClient

• clientSocket
• A Socket which keeps all the information about
  the connection to the server.
• input, output
• Two streams, an ObjectOutputStream and an
  ObjectInputStream.
• clientReader
• A Thread that runs using AbstractClients run
  method to listen and respond to server messages.
• host, port
• The host name and port number of the server.

10
Using AbstractClient

• An application developer, who uses the OCSF to
  develop a client application, would use the OCSF
  as follows
• Create a subclass of AbstractClient.
• Implement handleMessageFromServer slot method.
• Write code that
• Creates an instance of the new subclass.
• Calls openConnection.
• Sends messages to the server using the
  sendToServer service method.
• Optionally Implement the hook methods
• connectionEstablished
• connectionClosed
• connectionException

11
The Server Side AbstractServer

• Provides the basic services to create and open a
  server socket.
• Has a thread which listens for new connections.
• When a connection request from a client is
  accepted, a new ConnectionToClient is created.

ConnectionToClient
AbstractServer

sendToClient
listen
stopListening
close
close
setInfo
sendToAllClients
getClientConnections
getInfo
clientConnected
clientDisconnected
clientException
serverStarted
serverStopped
listeningException
serverClosed
handleMessageFromClient
12
The Server Side ConnectionToClient

• Provides the basic services to exchange data with
  the connected client.
• Manages the connection with the connected client.
• Runs as a thread.

ConnectionToClient
AbstractServer

sendToClient
listen
stopListening
close
close
setInfo
sendToAllClients
getClientConnections
getInfo
clientConnected
clientDisconnected
clientException
serverStarted
serverStopped
listeningException
serverClosed
handleMessageFromClient
13
The API of AbstractServer(Service Methods)

• Controlling methods
• listen
• stopListening
• close
• sendToAllClients
• Accessing methods
• isListening
• getClientConnections
• getPort
• setPort
• setBacklog

14
The API of AbstractServer(listen Method)

• Creates a serverSocket.
• Creates a thread in which the run method listens
  for connection requests through the accept()
  method of the serverSocket object.
• When a connection is accepted, a new client
  Socket is created, and a reference to this object
  is passed to the constructor of
  ConnectionToClient.
• The ConnectionToClient object creates input and
  output streams associated with this Socket
  object, and then starts a thread to exchange data
  and manage the connection with the connected
  client.

15
The Hook Methods of AbstractServer (A Callback
Method)

• Methods that may be overridden
• serverStarted
• clientConnected
• clientDisconnected
• clientException
• serverStopped
• listeningException
• serverClosed

16
The Slot Method of AbstractServer (A Callback
Method)

• Method that must be implemented
• handleMessageFromClient

17
The API of ConnectionToClient (Service Methods)

• Controlling methods
• sendToClient
• close
• Accessing methods
• getInetAddress
• setInfo
• getInfo

18
Using AbstractServer and ConnectionToClient

• An application developer, who uses the OCSF to
  develop a server application, would use the OCSF
  as follows
• Create a subclass of AbstractServer
• Implement the slot method handleMessageFromClient
  
• Write code that
• Creates an instance of the subclass of
  AbstractServer
• Calls the listen method
• Sends messages to clients, using
• the getClientConnections and sendToClient
  service methods
• or sendToAllClients
• Implement one or more of the other callback
  methods

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Internals of AbstractServer and ConnectionToClient

• The setInfo and getInfo methods make use of a
  Java class called HashMap. HashMap is an
  associative array. Imagine you can index an
  array with strings and you are not far off. E.x.
  myHashMaprobots
• Many methods in the server side are synchronized.
• The collection of instances of ConnectionToClient
  is stored using a special class called
  ThreadGroup.
• The server must pause from listening every 500ms
  to see if the stopListening method has been
  called.
• if not, then it resumes listening immediately.

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An Instant Messaging Application GUIChat
AbstractClient
Frame
AbstractServer
listen
SimpleClient
handleMessageFromServer connectionClosed connectio
nException
ClientFrame
close
open
EchoServer
send quit
handleMessageFromClient
ConnectionToClient
serverStarted
1

serverStopped
main