1. COMP1681 / SE15 Introduction to Programming
Fast Track Session 2
Java Network Programming

2. Today’s Learning Objectives
For you to learn the various ways in which Java supports the development of networked applications
For you to see simple examples of client and server programs written in Java
SE15 Fast-Track: Java Network Programming

3. SE15 Fast-Track: Java Network Programming
Lecture Outline
Overview
Accessing resources by URL
Low-level protocol support
Address handling
Sockets
Writing servers
Non-standard libraries
SE15 Fast-Track: Java Network Programming

4. SE15 Fast-Track: Java Network Programming
Java & Networking
Java was marketed as a programming language for ‘network-centric computing’…
…so it provides good support in standard library for writing networked applications
HTTP
TCP & UDP
Relevant packages are
java.net
java.io, java.nio
javax.net, javax.net.ssl
SE15 Fast-Track: Java Network Programming

5. Treating URLs Like Filenames
Create a java.net.URL object, given the URL of a resource expressed as a string
Call its openStream method to get an InputStream from which we can read data…
…or, for more control, call openConnection to get a URLConnection object
getContentLength gives document size
getContentType gives doc type (HTML, GIF image…)
getInputStream gives stream that we can read
SE15 Fast-Track: Java Network Programming

6. SE15 Fast-Track: Java Network Programming
Example
URL url = new URL(urlString); URLConnection conn = new URLConnection(url); if (conn.getContentType().equals("text/html")) { BufferedReader in = new BufferedReader( new InputStreamReader(conn.getInputStream())); String line = in.readLine(); while (line != null) { System.out.println(line); line = in.readLine(); } in.close(); }
SE15 Fast-Track: Java Network Programming

7. SE15 Fast-Track: Java Network Programming
IP, TCP & UDP
Data travel over the network as IP datagrams
Datagram payload conforms to TCP, UDP or ICMP
Transmission Control Protocol (TCP)
Provides a reliable connection between endpoints
Endpoints defined by port numbers
Uses sequence numbers to order datagrams
Retransmits datagrams if necessary
User Datagram Protocol (UDP)
Simpler than TCP
No promise of reliable delivery
SE15 Fast-Track: Java Network Programming

8. Hostnames & IP Addresses
Networked machines are identified by IP address
IPv4 address is a 32-bit integer, typically expressed in ‘dotted quad’ notation
Example:
Human-readable hostname is resolved to an IP address using the domain name system (DNS)
Example: cslin-gps.leeds.ac.uk →
Java encapsulates hostnames and IP addresses in the InetAddress class
SE15 Fast-Track: Java Network Programming

9. SE15 Fast-Track: Java Network Programming
InetAddress Class
Provides static factory methods for obtaining InetAddress objects
getByName
getByAddress
getAllByName (all available addresses for host)
getLocalHost (address of local machine)
InetAddress objects support
getHostName (→ hostname as string)
getAddress (→ IP address as array of bytes)
getHostAddress (→ IP address as string)
SE15 Fast-Track: Java Network Programming

10. SE15 Fast-Track: Java Network Programming
Sockets
Provide an abstraction of a TCP connection
Much easier to use in Java than in C
Much less set-up code
No platform variation (e.g. Unix vs. Windows)
Object-oriented abstraction
Socket must be bound to a local endpoint and connected to a remote endpoint
Done automatically by Socket constructor
Usually we specify remote address and port and let the system choose an ephemeral port on local machine
SE15 Fast-Track: Java Network Programming

11. SE15 Fast-Track: Java Network Programming
Example of Socket Code
Socket socket = new Socket(address, port); BufferedReader input = new BufferedReader( new InputStreamReader(socket.getInputStream())); String line = input.readLine(); while (line != null) { System.out.println(line); line = input.readLine(); } input.close(); socket.close();
SE15 Fast-Track: Java Network Programming

12. An HTTP Client Using Sockets
Socket socket = new Socket(address, port); BufferedReader input = new BufferedReader( new InputStreamReader(socket.getInputStream())); PrintWriter output = new PrintWriter( new OutputStreamWriter(socket.getOutputStream())); output.print("GET " + document + " HTTP/1.0\n\n"); output.flush(); String line = input.readLine(); while (line != null) { System.out.println(line); line = input.readLine(); } ...
SE15 Fast-Track: Java Network Programming

13. SE15 Fast-Track: Java Network Programming
Writing Servers
Socket alone isn’t sufficient to write a server
We need something to ‘sit by the phone, waiting for incoming calls’…
Java provides ServerSocket class to
Listen on a particular port
Negotiate connection with client
Open a Socket connection between hosts
SE15 Fast-Track: Java Network Programming

14. SE15 Fast-Track: Java Network Programming
Using ServerSocket
Create ServerSocket object
Call accept to listen for incoming connection attempts
Blocks until client attempts to connect
Returns Socket object when connection succeeds
Call getInputStream and/or getOutputStream on Socket to get streams that communicate with client
Interact with client according to agreed protocol
Server, client or both close connection
Return to Step 2
SE15 Fast-Track: Java Network Programming

15. Beyond The Standard Library
JavaMail API
Jakarta Commons, jakarta.apache.org/commons/
Commons Net
Support for FTP, NNTP, SMTP, POP3, Telnet…
Commons HttpClient
Authentication, HTTPS, cookie handling…
Commons
Simplified interface to JavaMail
SE15 Fast-Track: Java Network Programming

16. SE15 Fast-Track: Java Network Programming
Summary
We have
Seen how easy it is to read data from the web, given the URL of a resource
Examined how sockets can be used in Java to created network clients of various kinds
Considered how a simple server can be written in Java
Looked at some of the advanced applications that are possible using non-standard libraries
SE15 Fast-Track: Java Network Programming

17. SE15 Fast-Track: Java Network Programming
Exercise
Implement a very simple game (e.g., number guessing, Hangman or Pontoon) as a networked application
Devise a protocol for server and client to follow
Write a class to act as a server + one to act as a client
Get help at
SE15 Fast-Track: Java Network Programming