Network Programming

NETWORK PROGRAMMING
Network programming refers
to writing programs that execute

across multiple devices (computers), in which the devices are all connected
to each other using a network.
The java.net package of the J2SE APIs contains a collection of classes and
interfaces that provide the low-level communication details, allowing you to
write programs that focus on solving the problem at hand.
The java.net package provides support for the two common network
protocols:
TCP: TCP stands for Transmission Control Protocol, which allows for
reliable communication between two applications. TCP is typically used over
the Internet Protocol, which is referred to as TCP/IP.
UDP: UDP stands for User Datagram Protocol, a connection-less

protocol that allows for packets of data to be transmitted between
applications.
We build an application with a client and a server in which both interact

using socket programming which is a widely used concept in Networking.
Socket Programming:
Sockets provide the communication mechanism between two computers

using TCP. A client program creates a socket on its end of the communication
and attempts to connect that socket to a server.
When the connection is made, the server creates a socket object on its end
of the communication. The client and server can now communicate by
writing to and reading from the socket.
The
java.net.Socket
class
represents
a
socket,
and
the
java.net.ServerSocket class provides a mechanism for the server program to
listen for clients and establish connections with them.
The following steps occur when establishing a TCP connection between two
computers using sockets:
The server instantiates a ServerSocket object, denoting which port

number communication is to occur on.
The server invokes the accept() method of the ServerSocket class. This
method waits until a client connects to the server on the given port.
After the server is waiting, a client instantiates a Socket object,

specifying the server name and port number to connect to.
The constructor of the Socket class attempts to connect the client to

the specified server and port number. If communication is established, the
client now has a Socket object capable of communicating with the server.
On the server side, the accept() method returns a reference to a new

socket on the server that is connected to the client's socket.
After the connections are established, communication can occur using I/O
streams. Each socket has both an Output Stream and an Input Stream. The
client's Output Stream is connected to the server's Input Stream, and the
client's Input Stream is connected to the server's Output Stream.
TCP is a two-way communication protocol, so data can be sent across both
streams at the same time. There are following useful classes providing
complete set of methods to implement sockets.
Socket Client:
GreetingClient is a client program that connects to a server by using a socket
and sends a greeting, and then waits for a response.
//Client Socket Program
import java.net.*;
import java.io.*;
import java.lang.*;
public class GreetingClient
{
public static void main(String [] args)
{
String serverName = args[0];
int port = Integer.parseInt(args[1]);
try
{
System.out.println("Connecting to " + serverName+ " on
port " + port);
Socket client = new Socket(serverName, port);
System.out.println("Just connected
to"+client.getRemoteSocketAddress());
OutputStream outToServer = client.getOutputStream();
DataOutputStream out = new
DataOutputStream(outToServer);
out.writeUTF("Hello from+
client.getLocalSocketAddress());
InputStream inFromServer = client.getInputStream();
DataInputStream in = new
DataInputStream(inFromServer);
System.out.println("Server says " + in.readUTF());
client.close();
}
catch(IOException e)
{
e.printStackTrace();
}
}
}
Socket Server:
GreetingServer program is an example of a server application that uses the
Socket class to listen for clients on a port number specified by a commandline argument:
//Server Socket Program
import java.net.*;
import java.io.*;
public class GreetingServer extends Thread
{
private ServerSocket serverSocket;
public GreetingServer(int port) throws IOException
{
serverSocket = new ServerSocket(port);
serverSocket.setSoTimeout(10000);
}
public void run()

{
while(true)
{
try
{
System.out.println("Waiting for client on port "
+
serverSocket.getLocalPort() + "...");
Socket server = serverSocket.accept();

System.out.println("Just connected to "
+ server.getRemoteSocketAddress());
DataInputStream in = new
DataInputStream(server.getInputStream());
System.out.println(in.readUTF());
DataOutputStream out =new
DataOutputStream(server.getOutputStream());
out.writeUTF("Thank you for connecting to "
+ server.getLocalSocketAddress() +
"\nGoodbye!");
server.close();
}
catch(SocketTimeoutException s)
{
System.out.println("Socket timed out!");
break;
}
{
break;
}
}
}
public static void main(String [] args)
{
int port = Integer.parseInt(args[0]);
try
{
Thread t = new GreetingServer(port);
t.start();
}
{
}
}
}
METHOD DESCRIPTION
The java.net.ServerSocket class is used by server applications to obtain a
port and listen for client requests
Public ServerSocket(int port) throws IOException- Attempts to

create a server socket bound to the specified port. An exception occurs
if the port is already bound by another application.
If the ServerSocket constructor does not throw an exception, it means that

our application has successfully bound to the specified port and is ready for
client requests.
Here are some of the common methods of the ServerSocket class:
public int getLocalPort()- Returns the port that the server socket is
listening on. This method is useful if you passed in 0 as the port
number in a constructor and let the server find a port for you.
public Socket accept() throws IOException- Waits for an incoming

client. This method blocks until either a client connects to the server
on the specified port or the socket times out, assuming that the timeout value has been set using the setSoTimeout() method. Otherwise,
this method blocks indefinitely.
public void setSoTimeout(int timeout)- Sets the time-out value for

how long the server socket waits for a client during the accept().
When the ServerSocket invokes accept(), the method does not return until a
client connects. After a client does connect, the ServerSocket creates a new
Socket on an unspecified port and returns a reference to this new Socket. A
TCP connection now exists between the client and server, and
communication can begin.

Network Programming

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NETWORK PROGRAMMING

Network programming refers

to writing programs that execute

UDP: UDP stands for User Datagram Protocol, a connection-less

We build an application with a client and a server in which both interact

Sockets provide the communication mechanism between two computers

The server instantiates a ServerSocket object, denoting which port

After the server is waiting, a client instantiates a Socket object,

The constructor of the Socket class attempts to connect the client to

On the server side, the accept() method returns a reference to a new

public void run()

Socket server = serverSocket.accept();

Public ServerSocket(int port) throws IOException- Attempts to

If the ServerSocket constructor does not throw an exception, it means that

Here are some of the common methods of the ServerSocket class:

public Socket accept() throws IOException- Waits for an incoming

public void setSoTimeout(int timeout)- Sets the time-out value for

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