CS 424524: Introduction to Java Programming

1
CS 424/524 Introduction to Java Programming

• Lecture 25
• Spring 2002
• Department of Computer Science
• University of Alabama
• Joel Jones

2
Overview

• Questions
• Brief Introduction to Networking
• Network Programming in Java
• Reading for next time

3
Brief Introduction to Networking

• Network Layers

Application
Presentation
Process layer
http, ftp, telnet
Session
Transport
UDP/TCP
Network
IP
Data Link
Ethernet
Physical
802.3 Ethernet/CAT5, modems
4
Brief Introduction to Networking (cont.)

• Packets
• Address Information (header)
• Users data (payload), 1500 bytes
• Datagrams (User Datagram Protocol, UDP)
• Packets sent one by one
• No verification by Transport layer of receipt
• stateless

5
Brief Introduction to Networking (cont.)

• Connection based (Transmission Control Protocol,
  TCP)
• Connection established, data sent, connection
  closed
• Packets streamed and acknowledged
• Transport layer ensures transmission through
  retransmission
• Has state

6
Brief Demonstration of Differences

• Datagrams
• Connection-based

7
Java Networking

• Worlds shortest web browser!
• See handout, Browser.java
• So, there is a java.net package
• What are the components of the model
• What classes implement which components
• Details

8
Java Networking (cont.)

• Java supports both datagram and connection-based
• Datagram DatagramSocket, DatagramPacket
• Connection URL

9
Datagrams DatagramSocket

• Constructors
• DatagramSocket() - binds to any available port
• DatagramSocket(int port) - binds to specified
  port
• DatagramSocket(int port, InetAddress laddr) -
  bind to specified local address
• Transmission
• receive(DatagramPacket p) - receive a packet
• send(DatagramPacket p) - send a packet

10
Datagrams DatagramPacket

• Constructors
• DatagramPacket(byte buf, int length) -
  receiving
• DatagramPacket(byte buf, int length,
  InetAddress, int port) - sending
• DatagramPacket(byte buf, int offset, int
  length) - receiving
• DatagramPacket(byte buf, int offset, int
  length, InetAddress address, int port) - sending

11
Connection-based

• What does a URL consist of?
• Differs based on protocol
• http
• protocol // hostname port / file
• http//a1.cs.ua.edu80/cs424/MP0
• ftp
• protocol // user password _at_ hostname
  port / path type typecode
• ftp//jonessecret_at_a1.cs.ua.edu21/cs424/Lecture01
  .ppt

12
What about unsupported protocols?

• Socket

13
Further Reading

• Java Tutorial on Networking
• http//java.sun.com/docs/books/tutorial/networking
  /index.html
• Textbook, Chapter 16