The document provides an overview of Java networking, focusing on the java.net package, sockets, and the concept of client-server architecture. It explains how sockets facilitate communication between processes over a network using streams, addresses, and port numbers, and distinguishes between clients and servers. Additionally, it includes examples of TCP client-server applications in Java, detailing socket creation, connection, and data transmission.

1. Networking
Shravan Upadhayay

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Introduction
 Java.net package provides support
for networking.
 What makes Java a good language
for networking are the classes defined
in the java.net package.

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Concept of Networking

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Sockets
 Socket is the name given , in one
particular programming model, to the
end points of a communication link
between processes.
 When processes communicate over a
network, Java Technology uses
streams model.

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Sockets
 Sockets:
 Sockets hold two streams: an input
stream and an output stream.
 Each end of the socket has a pair of
streams.

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Socket
 A process send data to another process
through a network by writing to the
output stream associated with the
Socket.
 A process reads data written by another
process by reading from the input
stream associated with the Socket.

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Setting Up the Connection
 To setup the network connection, one
machine must run a program that is
waiting for a connection, and a
second machine must try to reach the
first.
 Set up of a network connection is
similar to a telephone system: One
end must dial the other end, which
must be listening.

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Networking

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Addressing the Connection
 When you make a network connection,
you need to know the address or the
name of the remote machine.
 In addition, a network connection,
requires a port number, which you can
think of as a telephone extension
number.

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Port Numbers
 Port numbers in TCP/IP systems are
16-bit numbers and the values range
from 0-65535.
 Port numbers below 1024 are reserved
for predefined services.
 Client port numbers are allocated by the
host OS to something not in use, while
server port numbers are specified by the
programmer, and are used to identify a
particular service.

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 Both client and server must agree in
advance on which port to use.
 If the port numbers used by the two
parts of the system do not agree,
communication does not occur.

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Server
 A server is anything that has some
resource that can be shared.
 There are compute servers, which
provide computing power;
 print servers, which manage a collection
of printers;
 disk servers, which provide networked
disk space;
 and web servers, which store web pages.

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Client
 A client is simply any other entity
that wants to gain access to a
particular server.

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 The notion of a socket allows a single computer to
serve many different clients at once, as well as
serving many different types of information.
 This feat is managed by the introduction of a port,
which is a numbered socket on a particular machine.
 A server process is said to "listen" to a port until a
client connects to it. A server is allowed to accept
multiple clients connected to the same port number,
although each session is unique.
 To manage multiple client connections, a server
process must be multithreaded or have some other
means of multiplexing the simultaneous I/O.

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Local Host Info
// Demonstrate InetAddress.
import java.net.*;
class InetAddressTest
{
public static void main(String args[]) throws
UnknownHostException
{
InetAddress Address = InetAddress.getLocalHost();
System.out.println(Address.getHostName());
System.out.println(Address.getHostAddress());
}
}

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TCP/IP
 TCP/IP sockets are used to implement
 reliable,
 bidirectional,
 persistent,
 point-to- point,
 stream-based connections between hosts on the
Internet.
 A socket can be used to connect Java's I/O
system to other programs that may reside
either on the local machine or on any other
machine on the Internet.

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 There are two kinds of TCP sockets in Java.
 One is for servers, and the other is for
clients.
 The ServerSocket class is designed to be
a "listener," which waits for clients to
connect before doing anything.
 The Socket class is designed to connect to
server sockets and initiate protocol
exchanges.

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 The creation of a Socket object
implicitly establishes a connection
between the client and server.
 There are no methods or constructors
that explicitly expose the details of
establishing that connection.

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 Here are two constructors used to create
client sockets:
 Socket(String hostName, int port)
 Creates a socket connecting the local host to the
named host and port; can throw an
UnknownHostException or an IOException.
 Socket(InetAddress ipAddress, int port)
 Creates a socket using a preexisting
InetAddress object and a port; can throw an
IOException.

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 A socket can be examined at any time for
the address and port information
associated with it, by use of the following
methods:
 InetAddress getInetAddress( )
 Returns the InetAddress associated with
the Socket object.
 int getPort( )
 Returns the remote port to which this
Socket object is connected.
 int getLocalPort( )
 Returns the local port to which this Socket
object is connected.

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 Once the Socket object has been created,
it can also be examined to gain access to
the input and output streams associated
with it.
 Each of these methods can throw an
IOException if the sockets have been
invalidated by a loss of connection on the
Net.
 These streams are used exactly like the I/O
streams receive data.
 InputStream getInputStream( )
 Returns the InputStream

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 InputStream getInputStream( )
 Returns the InputStream associated
with the invoking socket.
 OutputStream getOutputStream()
 Returns the OutputStream
associated with the invoking socket.
 void close()
 Closes both the InputStream and
OutputStream.

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Socket functional calls
 socket (): Create a socket
 bind(): bind a socket to a local IP address and port #
 listen(): passively waiting for connections
 connect(): initiating connection to another socket
 accept(): accept a new connection
 Write(): write data to a socket
 Read(): read data from a socket
 close(): close a socket (tear down the connection)

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Socket-programming using TCP
TCP service: reliable byte stream transfer
process
TCP with
buffers,
variables
socket
controlled by
application
developer
controlled by
operating
system
process
TCP with
buffers,
variables
socket
internet
client
serversocket( )
bind( )
connect( )
socket( )
bind( )
listen( )
accept( )
send( )
recv( )
close( ) close( )
recv( )
send( )
TCP conn. request
TCP ACK

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Socket programming with TCP
Example client-server app:
 client reads line from standard
input (inFromUser stream) ,
sends to server via socket
(outToServer stream)
 server reads line from socket
 server converts line to
uppercase, sends back to
client
 client reads, prints modified
line from socket
(inFromServer stream)
outToServer
tonetwork fromnetwork
inFromServer
inFromUser
keyboard monitor
Process
clientSocket
input
stream
input
stream
output
stream
TCP
socket
Input stream:
sequence of bytes
into processoutput stream:
sequence of bytes
out of process
Client
process
client TCP
socket

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Client/server socket interaction: TCP
wait for incoming
connection request
connectionSocket =
welcomeSocket.accept()
create socket,
port=x, for
incoming request:
welcomeSocket =
ServerSocket()
create socket,
connect to hostid, port=x
clientSocket =
Socket()
close
connectionSocket
read reply from
clientSocket
close
clientSocket
Server (running on hostid) Client
send request using
clientSocketread request from
connectionSocket
write reply to
connectionSocket
TCP
connection setup

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TCPClient.java
import java.io.*;
import java.net.*;
class TCPClient {
public static void main(String argv[]) throws Exception
{
String sentence;
String modifiedSentence;
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("hostname", 6789);
DataOutputStream outToServer =
new DataOutputStream(clientSocket.getOutputStream());

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TCPClient.java
BufferedReader inFromServer =
new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
sentence = inFromUser.readLine();
outToServer.writeBytes(sentence + 'n');
modifiedSentence = inFromServer.readLine();
System.out.println("FROM SERVER: " + modifiedSentence);
clientSocket.close();
}
}

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TCPServer.java
import java.io.*;
import java.net.*;
class TCPServer {
public static void main(String argv[]) throws Exception
{
String clientSentence;
String capitalizedSentence;
ServerSocket welcomeSocket = new ServerSocket(6789);
while(true) {
Socket connectionSocket = welcomeSocket.accept();
BufferedReader inFromClient = new BufferedReader(new
InputStreamReader(connectionSocket.getInputStream()));

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TCPServer.java
DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + 'n';
outToClient.writeBytes(capitalizedSentence);
}
}
}