CS 596 Client-Server Programming
Java Networking

Internet Protocol addresses are always numeric.

We generally don't use IP addresses:

Did you know that rohan's address is 130.191.143.100?

DNS is a service that translates between host names and IP addresses.

Example lookup programs (clients):

nslookup
host

Example:

rohan%  host ender
ender.sdsu.edu          A       130.191.13.2
rohan%  host 130.191.13.2
Name: ender.sdsu.edu
Address: 130.191.13.2
rohan%

Java interface to DNS

The InetAddress class performs the lookups.

Methods:

String getHostName()
byte[] getAddress()
int hashCode()
boolean equals(Object)
String toString()
static InetAddress getByName(String)
static InetAddress[] getAllByName(String)
static InetAddress getLocalHost()

Example use of InetAddress class

import java.net.InetAddress;

public class SimpleDNS
{
  public static void main(String args[])      
                     throws Exception
  {
      InetAddress address = 
         InetAddress.getByName(args[0]);
      byte IP[] = address.getAddress();
	
      for (int index = 0; index < IP.length;
                                     index++)
      {
         if (index > 0)
           System.out.print(".");
         System.out.print(((int)IP[index])& 0xff);
      }
    System.out.println();
  }
}

Use:

rohan% java SimpleDNS www.sdsu.edu
130.191.13.5
rohan%

Java interface to known services

Sorry, there is none...

Could easily write a class that reads /etc/services but that introduces platform dependencies.

Why?

Trivial TCP client example

import java.net.Socket;
import java.io.InputStream;

public class SimpleTelnet
{
  public static void main(String args[])
                 throws Exception
  {
    String	serverName = args[0];
    int	portNumber = 
              Integer.parseInt(args[1]);

    Socket	server = null;
    server = new Socket(serverName,
                         portNumber);

    InputStream	input = 
                  server.getInputStream();
    int		byteCount;
    byte		inputBuffer[] = new byte[1024];
	
    while ((byteCount = 
      input.read(inputBuffer, 0, 1024)) != -1)
    {
      String	stuff = new String(
                inputBuffer, 0, 0, byteCount);
          System.out.println("Received: " + 
                            stuff);
    }
  }
}

Trivial TCP client example (continued)

This client only reads... We'll test it with a service that only writes.

The daytime service is assigned to port 13.

moria% java SimpleTelnet moria.sdsu.edu 13
Received: Thu Feb 22 14:55:18 1996
moria%

UDP Networking

UDP == User Datagram Protocol

Features:

no reliable delivery
no reliable ordering
little overhead
no connection

Steps in using datagrams:

Create a packet
Send or receive the packet

The java.net.DatagramPacket class

Two constructors:

DatagramPacket(byte[], int) for receiving
DatagramPacket(byte[], int, InetAddress, int) for sending

Methods:

InetAddress getAddress()
byte[] getData()
int getLength()
int getPort()

The java.net.DatagramSocket class

Constructors:

DatagramSocket()
DatagramSocket(int)

If no port is specified in the constructor, a random port is picked.

Methods:

void close()
void finalize()
int getLocalPort()
void receive(DatagramPacket)
void send(DatagramPacket)

receive() will block until a packet is actually received.

Example UDP client

import java.net.*;
class DatagramTest
{
  public static void main(String[] args)
                             throws  Exception
  {
    DatagramSocket  socket;
    DatagramPacket  packet;
    InetAddress     address;
    byte[]          message = new byte[256];
    int             port = 13;

    //
    // Send empty request
    //
    socket = new DatagramSocket();
    address = InetAddress.getByName("moria.sdsu.edu");
    packet = new DatagramPacket(message, message.length,
                                address, port);
    socket.send(packet);

    //
    // Receive reply and print
    //
    packet = new DatagramPacket(message,
                                message.length);
    socket.receive(packet);
    String received = new String(packet.getData(), 0);
    System.out.println("Received: " + received);
    socket.close();
  }
}

Example UDP client sample run

rohan% java DatagramTest
Received: Mon Feb 26 23:07:06 1996

rohan%

If either the request packet or reply packet were to get dropped/lost, the program would hang. Why?

Possible solutions?

Status so far

With the information from the Network Services lecture and this one, we can now:

Write a program that provides a TCP service through STDIO
Write a TCP client that can connect and communicate with a service.
Write a UDP client that can communicate with a service.

We're done... End of the semester!

Sorry... No such luck

We have only seen the tip of the iceberg.

Major topics still to cover

In no particular order:

Different server types
Protocols
Concurrency and threading

More Java networking:

TCP and UDP servers
URL and URLConnection
Security Manager

CS 596 Client-Server Programming Java Networking

Contents of TITLE Lecture

CS 596 Client-Server Programming
Java Networking