Networking

Intro

• Client/server relationship
  • client requests some information or action
  • server performs the action and responds to the client
• Connection-based service: a client and a server form a connection and communicate back and forth until the connection is given up
  • The telephone is a good physical analogy. Connection is made, both parties talk
  • TCP (Transmission Control Protocol): Most common protocol for connection-based transmission
  • Example: SSH is connection-oriented
• Connectionless service: A client and server communicate by sending individual packets back and forth.
  • A good analogy is the US Postal Service. Send a letter to another person. No connection required
  • Note that the packets do not have to arrive in the same order they are sent!
  • UDP (User Datagram Protocol): Most common protocol for connectionless communication
  • Example: E-mail is typically connectionless
• socket -- a software construct that represents one endpoint of a connection.
  • Writing to (reading from) a socket is similar to writing to (reading from) a file
• Package java.net -- contains Java's fundamental networking capabilities
  • Java uses stream sockets for TCP communication
  • Java uses datagram sockets for UDP communication

Working With URLs

• URL - Uniform Resource Locator
• java.net.URL - a Java class for wrapping up a URL and working with it
  • Can use a URL object to help create connections, retrieve the web page or file it points to, and query other info about it (many get methods)

Opening a web page with a URL object, from Java

• From inside an applet
  • Retrieve a reference to the applet's context (usually the browser window) with the getAppletContext() method (class Applet)
  • Call the showDocument method (which takes a URL as a parameter), from the AppletContext inferface
• From inside an application using javax.swing.JEditorPane
  • JEditorPane is a swing component that supports plain text, HTML text, and RTF (rich text) already
  • It also supports the processing of hyperlink events, using interface HyperLinkListener
  • Call method SetPage (from JEditorPane) and pass in the URL object as the parameter

Using Stream Sockets

Server:

1. Create a ServerSocket object:
   • The port number identifies the application on the host computer. (web = port 80, for example)
   • queue length = max number of clients that can connect to server

    ServerSocket serv = new ServerSocket(portNumber, queueLength);
   

2. Listen for a client connection. Call method accept():

     Socket connection = serv.accept();
   

3. Retrieve the OutputStream and InputStream objects associated with the socket:

    OutputStream os = connection.getOutputStream();
    InputStream is = connection.getInputStream();
   

   If we desire, we can wrap these up in other types of streams:

     ObjectInputStream objIn = new ObjectInputStream(is);	// etc
   

4. Process data with the stream objects (like writing to and reading from files)
5. close the connections by invoking the close() method on the streams and the Socket

     connection.close();
   

Client:

1. Create a socket to connect to the server:

     Socket connection = new Socket(serverAddress, port);
   

2. Obtain the socket's InputStream and OutputStream references (like step 3 of server)
3. Process data
4. Close the socket

Using Datagram Sockets

• Datagram sockets are used for connectionless communication between programs
• class DatagramSocket -- used for creation of datagram sockets. Examples:

    DatagramSocket s1, s2;
  
    s1 = new DatagramSocket(12345);	// bind to port 12345
    s2 = new DatagramSocket();		// bind to any available port
  					//   on local host machine
  

• class DatagramPacket -- used to build packets of information to send over datagram sockets. Examples:

    DatagramPacket p1, p2;
  
    byte[] data1 = new byte[500];
    p1 = new DatagramPacket(data1, data1.length);
    // creates a packet for storing up to 500 bytes of data
    //   will be used to receive a datagram packet
  
    String message = "Hello, World";
    byte[] data2 = message.getBytes();	// convert string to byte array
    p2 = new DatagramPacket(data2, data2.length, 
  			  InetAddress.getLocalHost(), 1234);
    // creates a packet of outgoing data (the "hello world" message),
    //  setting the port number to 1234
  

• Send a packet by calling the send method:

    s2.send(p2);
  

• Receive a datagram packet with the receive method:

    s1.receive(p1);
  

Deitel Examples