Prepared by: Eng/ Basma Elshoky

Section 3: Socket
Part I: socket for clients

Java sockets are essential for network communication, acting as endpoints for
sending and receiving data. A socket is created on a client device and attempts to
connect to a server socket at a specified IP address and port number. Java programs
normally use client sockets in the following fashion:

• The program creates a new socket with a constructor.

• The socket attempts to connect to the remote host.

Socket class: Socket class represents a client-side socket, used to establish

connections to a server. Constructors & Methods:

Constructor Description
Socket() Creates an unconnected socket.
Socket(String host, int port) Creates a socket and connects it to the specified remote
host on the specified port.
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Socket(String host, int port, Creates a socket and connects it to the specified remote
InetAddress localAddr, int host on the specified port, using the specified local address
localPort) and port to bind the socket.
Socket(InetAddress address, int Creates a socket and connects it to the specified remote
port) address on the specified port.
Socket(InetAddress address, int Creates a socket and connects it to the specified remote
port, InetAddress localAddr, int address on the specified port, using the specified local
localPort) address and port to bind the socket.

Method Description
connect(SocketAddress endpoint) Connects this socket to the server with the specified
address and port number.
close(): Closes the socket.
getInputStream() Returns an input stream for this socket.
getOutputStream() Returns an output stream for this socket.
getInetAddress() Returns the remote address to which this socket is
connected.
getLocalAddress(): Returns the local address to which this socket is bound.
getPort() Returns the remote port to which this socket is connected.
getLocalPort() Returns the local port to which this socket is bound.
setSoTimeout(int timeout) Sets the timeout in milliseconds for blocking socket
operations. operations.
getSoTimeout() Returns the timeout in milliseconds for blocking socket
operations.
isClosed() Returns true if the socket has been closed.
isConnected() Returns true if the socket is connected to a remote host.
isBound() Returns true if the socket is bound to a local address and
port.
getChannel() Returns the socket's SocketChannel if available.
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Example 1: retrieve data programmatically using sockets (website on port 13).

import java.io.*;
import java.net.*;
import java.util.*;
public class App {
public static void main(String[] args) throws Exception {

// String hostname = "localhost"; //java.net.ConnectException: Connection

refused: connect

String hostname = "https://translate.google.com.eg/";

Socket socket = null;
try {
socket = new Socket(hostname, 13);
// read from the socket...
}
catch (IOException ex) {
System.out.println("Error2:");
System.err.println(ex);
}
finally {
if (socket != null) {
try {
socket.close();
}
catch (IOException ex) {
System.out.println("Error2:");
System.err.println(ex);}
}
}
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Lab 1: Check a connection with a website then apply some methods.

import java.io.*;
import java.net.*;
import java.util.*;
public class App {
public static void main(String[] args) throws Exception {
try {
Socket toOReilly = new Socket("www.oreilly.com", 80);
// send and receive data...
System.out.println("IP:"+ toOReilly.getInetAddress()) ;
System.out.println("local port:"+ toOReilly.getLocalPort()) ;
System.out.println("Port:"+ toOReilly.getPort()) ;
System.out.println("InputStream:"+ toOReilly.getInputStream()) ;
System.out.println("OutStream:"+ toOReilly.getOutputStream()) ;
} catch (UnknownHostException ex) {
System.err.println(ex);
} catch (IOException ex) {
System.err.println(ex);
}
} }
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Example 2: Find out which of the first 1024 ports seem to be hosting TCP servers
on a specified host
import java.io.*;
import java.net.*;
import java.util.*;
public class App {
public static void main(String[] args) throws Exception {

String host = args.length > 0 ? args[0] : "localhost";

for (int i = 1; i < 1024; i++) {
try {
Socket s = new Socket(host, i);
System.out.println("There is a server on port " + i + " of "
+ host);
s.close();
} catch (UnknownHostException ex) {
System.err.println(ex);
break;
} catch (IOException ex) { }
} } }
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Lab 2:
Create a connection with btu website “Constructing Without Connecting “ then
apply other methods in the table.
Some help: this mean you need using connection method and SocketAddress
class.
Socket socket = new Socket();
SocketAddress address = new InetSocketAddress("domain", “port”);
socket.connect(address);

Next Section: Socket for servers.

Reference: Harold, Elliotte Rusty. "Java Network Programming", Chapter 9:

Sockets for Clients. O'Reilly.