Sockets

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Sockets

• How-to

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Ingredients of communication

• For a communication application to work, there
  are two basic ingredients
• Both of them should be present to make a sensible
  communication

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Ingredients of communication

• Ingredients are
• An established communication link
• A protocol

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Communication link

• We will use a client-server socket based
  connection between two machines as a link

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Protocol

• We will write a simple sample protocol to learn
  how protocols work

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What is a Socket

• Socket is a logical entity that can be used for
  following basic operations
• Connect to a remote machine
• Send data
• Receive data
• Close a connection
• Bind to a port
• Listen to incoming connections
• Accept Connections from remote machine on bound
  port

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Socket Architecture
Server
Client
Socket
Socket
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Server Socket

• Server Socket is bound to an port on server
• Then it waits for connections from server
• Whenever a client requests, it accepts a
  connection

Server
Socket
Socket
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Client Socket

• Client socket connects to the server socket
• Then it can start sending and receiving data
  according to the protocol

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Writing a Client Application

• Using java.net.Socket

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Writing a Socket

• Socket is a java class found in java.net package
  and represents the logical socket we just
  discussed
• Socket can be instantiated using one of the
  following constructors
• Socket(String host, int port)
• Socket(InetAddress address, int port)

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Socket(String host, int port)

• First argument to Socket constructor is the
  host-name address of the SERVER that is waiting
  for the clients to connect to it
• Second argument is the port number on which
  server is waiting

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A sample protocol

• Client Request
• Hello
• name
• date
• Anything else

• Server Response
• Hi, how are you?
• Machine name is ____
• Server Date/Time
• Server Error

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Client Program for sample protocol

• Socket s new Socket("basit", 1357)
• System.out.println("Connected to basit on port
  1357")
• InputStream in s.getInputStream()
• OutputStream out s.getOutputStream()
• PrintStream write new PrintStream( out )
• DataInputStream read new DataInputStream(in)
• write.println("name") //REQUEST
• String temp read.readLine() //READING
  RESPONSE
• System.out.println("Response for name is "
  temp)

Server
Protocol Command
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Output of above program

• Connected to basit on port 1357
• Response for name is Machine name is
  BASIT/192.168.12.54

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Exceptions to be caught

• Socket constructor throws UnknownHostException if
  the server is not accessible to the client
  machine
• IOException must be caught that is thrown by
• Socket Constructor
• getInputStream and getOutputStream methods
• Other input and output methods

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Where is the server?

• If protocol is designed by us, we should write
  the server
• But where is the server?
• It is running on the machine 192.168.12.54

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Try out the client first
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Writing the Server Application

• Using java.net.ServerSocket

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Writing the server

• Following steps are typically followed for the
  server to function
• Create a ServerSocket that binds a Socket to a
  port on the server machine
• Wait for the incoming client connections
• When a connection is established, open streams
  for I/O on the socket
• Read the request from incoming stream
• Write back the response to the output stream

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ServerSocket

• ServerSocket is a class found in java.net package
• It is used to bind a socket to a server port and
  wait for the incoming connections from the client

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Server Code Creating Server Socket

• ServerSocket ss new ServerSocket(1357)
• System.out.println("Waiting on port 1357")
• Socket s ss.accept()
• System.out.println("Client connection
  established")

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Server Code Reading Request

• InputStream in s.getInputStream()
• OutputStream out s.getOutputStream()
• DataInputStream read new DataInputStream(in)
• PrintStream write new PrintStream( out )
• // reading request
• String temp read.readLine()

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Server Code Sending response

• String temp read.readLine()// reading
  request
• if(temp.equals("Hello")) //writing back
  response
• write.println("Hi, how are you?")
• else if (temp.equals("name"))
• write.println("Machine name is
  "InetAddress.getLocalHost())
• else if (temp.equals("date"))
• write.println("Machine time is " new
  Date())
• else
• write.println("ERROR from server
  InetAddress.getLocalHost(), bad command")
• s.close()

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Look for the Exceptions

• IOException is thrown by
• Constructor of ServerSocket
• .accept( ) Method
• .getInputStream and getOutputStream methods
• And must be caught

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Protocol

• You can design your own protocols and make them
  work for your purpose
• You can modify a few commands to the existing
  server to check for your protocol design
  capabilities

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END OF TODAYs SESSION