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Learning Outcomes
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Networking
To understand basic network terminology
To be able to communicate using Telnet
To be aware of some common network services
To be able to implement client and server
programs in Java using the following classes:
– Sockets
– Server sockets
– URL connections
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CP4044 – Lecture 7
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CP4044 – Lecture 7
TCP/IP Networks
Sockets
• Common, basic communications mechanism.
Clients
– A bi-directional serial communication channel that
associates two network connected computers.
– Acts like a pipe that allows data flow (via a stream) between
the computers.
Network
Server
• As far as the programmer is concerned:
– Data is written to a socket in exactly the same way as it
would be written to a file or display.
– It can then be read from the other end of the socket in
exactly the same way as data is read from a file or keyboard.
• TCP/IP networks are based on a packetswitched, client/server model
• A server runs continuously, listening for
requests from clients
• A client initiates a transaction with the server
• The intricacies of communications are handled within
the socket class.
– As a Java programmer you can get by without knowing
much about networking.
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CP4044 – Lecture 7
TCP/IP
Hostnames and DNS
• IP (Internet Protocol) routes data to the destination
machine
• Each machine connected to the network has a unique
IP address, e.g. 134.220.1.9
• TCP (Transmission Control Protocol) looks after the
transmission of data.
– Provides a reliable, connection-oriented transport
– Delivers the data to a process identified by a port
number
• A socket encapsulates
– the IP address of the destination machine
– the Port number of a process running on that
machine
CP4044 – Lecture 7
Kevan Buckley, Peter Burden, CP4044,
2004
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CP4044 – Lecture 7
• The host name is the name of the server machine.
– In a form that is easily remembered/used by humans
– e.g. www.scit.wlv.ac.uk
• The IP protocol actually needs an Internet address.
– e.g. 134.220.1.9
• Host names are translated into IP addresses by DNS,
the Domain Name System.
• Why have two ways of referring to the same host?
– Allows the host to be moved to a different machine – only
DNS entries need to be changed – users unaffected
– Humans don't like using strings of numbers
– Computers like numbers – allows easy implementation of
hierarchies, subnets etc.
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CP4044 – Lecture 7
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Ports
Some Well-Known Port Numbers
• Ports allow machines to provide multiple
services.
• A server process listens on a specific
(published) port for requests.
Port
Name
RFC Purpose
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Daytime
867
Gets the date and time
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FTP
959
Transfers files
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Telnet
854
Log on to a remote system
• Ports are identified by a 16-bit integer.
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SMTP
821
Internet mail service
70
Gopher
1436 Document archives
80
http
1945 Transfers Web resources
110
Pop3
1939 Download Internet mail
– Ports up to 1024 are reserved for specific services
– Ports above 1025-32767 are free for use.
• Used by clients, they are known as ephemeral ports and
are allocated numbers starting at 32767 working down.
RFCs are obtained from http://www.faqs.org/rfcs/
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CP4044 – Lecture 7
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CP4044 – Lecture 7
Server
Client
• A client process attempts to create a socket
connection to a remote server, usually in response to
some sort of user initialised event.
• A server process runs in a loop, continually
listening for connection requests via its server
socket.
• Once a connection request has been received
the server process can then read data from the
socket and process it.
• It then resumes listening for the next request.
• A common model is for the server process to
create a new process to service the request, so
that it can immediately resume listening for the
next connection request.
• Client/server communication is similar to a telephone
conversation:
– Somebody (the client) picks up a phone and dials a number.
– Another phone rings and somebody (the server) picks it up.
– A bi-directional conversation then continues until somebody
hangs up.
– The phone number called is similar to an IP address.
– Ports are similar to different extension numbers.
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CP4044 – Lecture 7
Telnet
World Wide Web Service
• The Telnet command line tool uses the Telnet protocol
(http://www.faqs.org/rfcs/rfc854.html), based on TCP/IP, to allow
a computer to act as a remote terminal on another
machine anywhere on the Internet.
• It allows us to experiment with client/server systems.
command prompt
server
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CP4044 – Lecture 7
port
C:\> telnet clun.scit.wlv.ac.uk 80
CP4044 – Lecture 7
Kevan Buckley, Peter Burden, CP4044,
2004
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• HTTP – Port 80
– http://www.faqs.org/rfcs/rfc2616.html
– Web servers listen on port 80 for connections
– Once connected the client can send a number of simple
commands. e.g. GET followed by the name of a resource.
– We can retrieve raw HTML using a Telnet client
C:\> telnet www.scit.wlv.ac.uk 80
Trying 134.220.4.130...
Connected to clun.scit.wlv.ac.uk.
Escape character is '^]'.
GET
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.or
g/TR/html4/loose.dtd">
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>School of Computing and IT</title>
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CP4044 – Lecture 7
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File Transfer Protocol Service
Network Programming with Java
• FTP – Port 21
• Java provides many classes for programming in
a TCP/IP network environment
• These are in the java.net package
• Writing client/server programs which implement
sockets, use these classes:
– http ://www.faqs.org/rfcs/rfc959.html
– FTP servers listen on port 21 for connections
– USER and PASS commands are required before other
commands can be issued.
C:\> telnet ftp.scit.wlv.ac.uk 21
Trying 134.220.4.130...
Connected to clun.scit.wlv.ac.uk.
Escape character is '^]'.
220 clun FTP server (Version 4.143 Mon Oct 16 15:20:07 BST 2000) ready.
USER anonymous
331 Guest login ok, send e-mail address as password.
PASS [email protected]
... Output omitted ...
QUIT
230- Goodbye
Connection to host lost
– java.net.ServerSocket
– java.net.Socket
• A number of higher-level classes are available.
We look at just one:
– java.net.URL
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ServerSocket Class
Socket Class
• import java.net.*;
• Constructor:
• Constructor:
– Socket (String host, int port)
– ServerSocket(int port)
• Some methods:
• Methods include:
– InputStream getInputStream()
– Socket accept()
– OutputStream getOutputStream()
– void close()
– void setSoTimeout(int timeout)
Use this class only for Server programs.
– void close()
As with all I/O, sockets can throw a variety
of exceptions.
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CP4044 – Lecture 7
01 import java.io.*;
02 import java.net.*;
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04 public class Server {
05 public static void main(String[] args) {
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String messageIn, messageOut;
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try {
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ServerSocket ssock = new ServerSocket(6789);
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while (true) {
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Socket connsock = ssock.accept();
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BufferedReader inNet =
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new BufferedReader(
new InputStreamReader(
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connsock.getInputStream() ));
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DataOutputStream outNet =
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new DataOutputStream(connsock.getOutputStream());
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messageIn = inNet.readLine();
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messageOut = messageIn.toUpperCase() + "\n";
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outNet.writeBytes(messageOut);
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}
22 }}catch(IOException e) { e.getMessage() }}
A Simple Server
CP4044 – Lecture 7
Kevan Buckley, Peter Burden, CP4044,
2004
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CP4044 – Lecture 7
Simple Server
• Line 7 – The main method may throw exceptions. It
requires an exception handler.
• Line 8 – Constructs a ServerSocket object to listen on
port 6789.
• Lines 9-21 – Infinite loop to continually process
requests.
• Line 10 – Waits (by blocking the thread) for a
connection to be made to the ServerSocket. Once
connected, the connsock socket can be used to
communicate with the client.
CP4044 – Lecture 7
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Simple Server
• Lines 11-14 – Constructs a BuffererReader to
read text data line-by-line from the client
• Lines 15-16 – Constructs an OutputDataStream
to send data to the client.
• Lines 18-20 – Read a line of text from the client,
convert it to upper case and send it back to the
client.
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CP4044 – Lecture 7
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Simple Client
• Line 5 – Exceptions may be thrown, so use exception handlers.
• Lines 6-7 – Construct a BufferedReader to read lines of text
from the keyboard.
• Line 15 – A line of text is read in through the socket.
• Lines 11-12 – Construct a BufferedReader to read in lines of
text through the socket. If binary data was required
DataInputStream could have been used (to process primitive
data types) or bytes could be read directly from the
InputStream.
• Line 16 – The text that was read from the socket is
printed out.
• Line 17 – The socket is closed, making it unavailable
and freeing up resources.
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Testing Client/Server Progs
CP4044 – Lecture 7
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Running Programs across the Network
• When program testing, it is convenient to run
both the server and the client on the same
machine
• The local machine can be addressed with this
reserved address:
– IP address:
127.0.0.1
– Hostname:
localhost
• Run the Server first in one command prompt
• Then run the Client in a second command
prompt
Kevan Buckley, Peter Burden, CP4044,
2004
• Line 13 – A line of text is read from the keyboard into
the message variable.
• Line 14 – The text stored in the message variable is
output through the socket.
• Lines 9-10 – Construct a DataOutputStream to send data
through the socket. (Note it is necessary to write raw bytes to
the socket).
CP4044 – Lecture 7
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CP4044 – Lecture 7
Simple Client
CP4044 – Lecture 7
A Simple Client
import java.io.*;
import java.net.*;
ublic class Client {
public static void main(String[] args) {
try {
BufferedReader kbd = new BufferedReader(
new InputStreamReader(System.in));
Socket csock = new Socket("localhost", 6789);
DataOutputStream outNet =
new DataOutputStream(csock.getOutputStream());
BufferedReader inNet = new BufferedReader(
new InputStreamReader(csock.getInputStream()));
String message = kbd.readLine();
outNet.writeBytes(message + "\n");
message = inNet.readLine();
System.out.println("Server sent: " + message);
csock.close();
}}
}
• Run the Server and Client programs on
separate networked machines
• You will need to know the IP address of the
machine that the Server is running on:
ipconfig
(for Windows)
ifconfig
(for Linux)
• Use this IP address when creating the socket in
the Client program
• Will not be able to connect if the machine is
running a firewall
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Universal Resource Locator (URL)
The Java URL class
• Identifies a resource on the internet
• The URL class has a number of uses, including:
– Validating a url string entered by the user
http://www.wlv.ac.uk:80/~in6659/clientserver/index.html
protocol
host
port
path
If not specified
uses the default for
the given protocol,
e.g. 80 for http
The location
of the
resource
being
requested
• Will throw MalformedURLException if the string
does not have valid syntax
– Parsing a url string
• Class provides methods to return the different
parts of the url string
• getProtocol(), getHost(), getPath(), getPort(), etc.
– Provides high level means of communication
URLs may contain other information such as user
names, passwords and locations of resources within
resources.
• Do not need to create sockets
• openStream()
See http://www.faqs.org/rfcs/rfc1738.html
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CP4044 – Lecture 7
Using the URL Class
01 import java.io.*;
02 import java.net.*;
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URL Class Example
The URL class allows simplified
communication for common protocols.
• Line 7 – The main method may throw exceptions.
03 public class GetURL {
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public static void main(String[] args) {
try {
URL url = new URL(args[0]);
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InputStreamReader inStr = new InputStreamReader(in);
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BufferedReader inNet = new BufferedReader(inStr);
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while ((line = inNet.readLine()) != null) {
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• Lines 13-16 – Continually reads data from the
BufferedReader until it fails to read any more (null is
received).
lineCount++;
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System.out.println(line);
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}
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• Lines 9-12 – Opens up an InputStream to read from
the URL and attaches it to a BufferedReader to allow
line-by-line data retrieval.
InputStream in = url.openStream();
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• Line 8 – Uses the text that was passed as a parameter
to this program to construct a URL object.
String line;
int lineCount = 0;
System.out.println("Lines read = " + lineCount);
}
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Validating and Parsing a URL
Summary
01 import java.net.*;
• Introduced some TCP/IP services
02 public class DisplayURL {
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public static void main(String[] args) {
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URL url = null;
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try { url = new URL(args[0]);
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} catch(MalformedURLException e) {
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System.out.println("Bad url");
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– Telnet - remote connection
– WWW and FTP services
• New Java classes introduced:
System.exit(1);
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}
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System.out.println(url);
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System.out.println("Protocol: "+url.getProtocol());
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System.out.println("Path:
"+url.getPath());
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System.out.println("Host:
"+url.getHost());
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System.out.println("Port:
"+url.getPort());
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System.out.println("Query: "+url.getQuery());
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System.out.println("File:
– java.net.ServerSocket
– java.net.Socket
– java.net.URL
"+url.getFile());
18 } }
CP4044 – Lecture 7
Kevan Buckley, Peter Burden, CP4044,
2004
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CP4044 – Lecture 7
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