Java Socket Programming

Java Sockets Programming
 The package java.net provides support for
sockets programming (and more).
 Typically you import everything defined in
this package with:
import java.net.*;
Java Socket Programming 2

Classes
InetAddress
Socket
ServerSocket
DatagramSocket
DatagramPacket

InetAddress class
 static methods you can use to create new
InetAddress objects.
 getByName(String host)
 getAllByName(String host)
 getLocalHost()
InetAddress x = InetAddress.getByName(
“cse.unr.edu”);
 Throws UnknownHostException

try {
InetAddress a = InetAddress.getByName(hostname);
System.out.println(hostname + ":" + a.getHostAddress());
} catch (UnknownHostException e) {
System.out.println("No address found for " + hostname);
}
try {
InetAddress a = InetAddress.getByName(hostname);
System.out.println(hostname + ":" + a.getHostAddress());
} catch (UnknownHostException e) {
System.out.println("No address found for " + hostname);
}
Java Socket Programming

Socket class
 Corresponds to active TCP sockets only!
 client sockets
 socket returned by accept();
 Passive sockets are supported by a different
class:
 ServerSocket
 UDP sockets are supported by
 DatagramSocket

JAVA TCP Sockets (Client
Socket)
 java.net.Socket
 Implements client sockets (also called just
“sockets”).
 An endpoint for communication between two
machines.
 Constructor and Methods
• Socket(String host, int port): Creates a stream socket and
connects it to the specified port number on the named host.
• InputStream getInputStream()
• OutputStream getOutputStream()
• close()

ServerSocket
 java.net.ServerSocket
 Implements server sockets.
 Waits for requests to come in over the network.
 Performs some operation based on the request.
 Constructor and Methods
• ServerSocket(int port)
• Socket Accept(): Listens for a connection to be made to this
socket and accepts it. This method blocks until a connection is
made.
Internet 8

Sockets
Client socket, welcoming socket (passive) and connection socket (active)

Socket Constructors
 Constructor creates a TCP connection to a
named TCP server.
 There are a number of constructors:
Socket(InetAddress server, int port);
Socket(InetAddress server, int port,
InetAddress local, int localport);
Socket(String hostname, int port);

Socket Methods
void close();
InetAddress getInetAddress();
InetAddress getLocalAddress();
InputStream getInputStream();
OutputStream getOutputStream();
 Lots more (setting/getting socket options,
partial close, etc.)

Socket I/O
 Socket I/O is based on the Java I/O support
 in the package java.io
 InputStream and OutputStream are abstract
classes
 common operations defined for all kinds of
InputStreams, OutputStreams…

InputStream Basics
// reads some number of bytes and
// puts in buffer array b
int read(byte[] b);
// reads up to len bytes
int read(byte[] b, int off, int len);
Both methods can throw IOException.
Both return –1 on EOF.

OutputStream Basics
// writes b.length bytes
void write(byte[] b);
// writes len bytes starting
// at offset off
void write(byte[] b, int off, int len);
Both methods can throw IOException.

ServerSocket Class
(TCP Passive Socket)
 Constructors:
ServerSocket(int port);
ServerSocket(int port, int backlog);
ServerSocket(int port, int backlog,
InetAddress bindAddr);

ServerSocket Methods
Socket accept();
void close();
InetAddress getInetAddress();
int getLocalPort();
throw IOException, SecurityException

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

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

Sample Echo Server
TCPEchoServer.java
And TCPClient.java
Save both files, compile and run on separate terminal.
First TCPEchoServer and then TCPClient
Based on code from:
TCP/IP Sockets in Java

TCPEchoServer.java
import java.io.*;
import java.net.*;
Public class TCPEchoServer {
public static void main(String argv[]) throws Exception {
String clientSentence;
String capitalizedSentence;
ServerSocket welcomeSocket = new ServerSocket(6789);
// server running and listening
while(true) {
Socket connectionSocket = welcomeSocket.accept();
// new client connected
BufferedReader inFromClient = new BufferedReader
(new InputStreamReader
(connectionSocket.getInputStream()));
DataOutputStream outToClient = new DataOutputStream
(connectionSocket.getOutputStream());

clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase()+ ‘n’;
outToClient.writeBytes(capitalizedSentence);
}
}
}

TCPClient.java
import java.io.*;
import java.net.*;
Public 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());
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();
}
}

UDP Sockets
 DatagramSocket class
 DatagramPacket class needed to specify the
payload
 incoming or outgoing

Socket Programming with UDP
 UDP
 Connectionless and unreliable service.
 There isn’t an initial handshaking phase.
 Transmitted data may be received out of order, or lost.
 Socket Programming with UDP
 No need for a welcoming socket.
 No streams are attached to the sockets.
 The sending hosts creates “packets” by attaching the IP destination
address and port number to each batch of bytes.
 The receiving process must unravel to received packet to obtain the
packet’s information bytes.

JAVA UDP Sockets
 In Package java.net
 java.net.DatagramSocket
• A socket for sending and receiving datagram packets.
• Constructor and Methods
– DatagramSocket(int port): Constructs a datagram
socket and binds it to the specified port on the local
host machine.
– void receive (DatagramPacket p)
– void send (DatagramPacket p)
– void close()

DatagramSocket Constructors
DatagramSocket();
DatagramSocket(int port);
DatagramSocket(int port, InetAddress a);
All can throw SocketException or SecurityException

Datagram Methods
void connect(InetAddress, int port);
void close();
void receive(DatagramPacket p);
void send(DatagramPacket p);
Lots more!

DatagramPacket
 Contain the payload
 (a byte array, length of byte array,
InetAddress, port)
 Can also be used to specify the destination
address
 when not using connected mode UDP

DatagramPacket Constructors
For receiving:
DatagramPacket( byte[] buf, int len);
For sending:
DatagramPacket( byte[] buf, int len
InetAddress a, int port);

DatagramPacket methods
byte[] getData();
void setData(byte[] buf);
void setAddress(InetAddress a);
void setPort(int port);
InetAddress getAddress();
int getPort();

Example: Java client (UDP)
sendPacket
tonetwork fromnetwork
receivePacket
inFromUser
keyboard monitor
Process
clientSocket
UDP
packet
input
stream
UDP
packet
UDP
socket
Output: sends
packet (TCP sent
“byte stream”)
Input: receives
packet (TCP
received “byte
stream”)
Client
process
client UDP
socket

Client/server socket interaction: UDP
close
clientSocket
Server (running on hostid)
read reply from
clientSocket
create socket,
clientSocket = DatagramSocket()
Client
Create, address (hostid, port=x,
send datagram request
using clientSocket
create socket,
port=x, for
incoming request:
serverSocket =
DatagramSocket()
read request from
serverSocket
write reply to
serverSocket
specifying client
host address,
port umber

Sample UDP code
UDPEchoServer.java
Simple UDP Echo server.
Test using nc as the client (netcat):
> nc –u hostname port

UDPEchoServer.java
import java.io.*;
import java.net.*;
class UDPEchoServer {
public static void main(String args[]) throws Exception {
int port = 9876;
DatagramSocket serverSocket = new DatagramSocket(port);
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true) {
DatagramPacket receivePacket =
new DatagramPacket (receiveData, receiveData.length);
serverSocket.receive(receivePacket);
String sentence = new String(receivePacket.getData());
InetAddress IPAddress = receivePacket.getAddress();

int clientPort = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
DatagramPacket sendPacket = new DatagramPacket
(sendData, sendData.length, IPAddress, clientPort);
serverSocket.send(sendPacket);
}
}
}

UDPClient.java
import java.io.*;
import java.net.*;
public class UDPClient {
public static void main(String args[]) throws Exception {
BufferedReader inFromUser = new BufferedReader
(new InputStreamReader (System.in));
int port = 9876;
DatagramSocket clientSocket = new DatagramSocket();
InetAddress IPAddress = InetAddress.getByName("hostname");
byte[] sendData = new byte[1024];
byte[] receiveData = new byte[1024];
String sentence = inFromUser.readLine();
sendData = sentence.getBytes();

DatagramPacket sendPacket = new DatagramPacket (sendData,
sendData.length, IPAddress, port);
clientSocket.send(sendPacket);
DatagramPacket receivePacket = new DatagramPacket
(receiveData, receiveData.length);
clientSocket.receive(receivePacket);
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
clientSocket.close();
}
}

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
 sendto(): send a datagram to another UDP socket
 recvfrom(): read a datagram from a UDP socket
 close(): close a socket (tear down the connection)

Java URL Class
 Represents a Uniform Resource Locator
 scheme (protocol)
 hostname
 port
 path
 query string

Parsing
 You can use a URL object as a parser:
URL u = new URL(“http://www.cs.unr.edu/”);
System.out.println(“Proto:” + u.getProtocol());
System.out.println(“File:” + u.getFile());

URL construction
 You can also build a URL by setting each part individually:
URL u = new URL(“http”,
www.cs.unr.edu,80,”/~mgunes/”);
System.out.println(“URL:” + u.toExternalForm());
System.out.println(“URL: “ + u);

Retrieving URL contents
 URL objects can retrieve the documents they refer
to!
 actually this depends on the protocol part of the URL.
 HTTP is supported
 File is supported (“file://c:foo.html”)
 You can get “Protocol Handlers” for other protocols.
 There are a number of ways to do this:
Object getContent();
InputStream openStream();
URLConnection openConnection();

Getting Header Information
 There are methods that return information extracted
from response headers:
String getContentType();
String getContentLength();
long getLastModified();

URLConnection
 Represents the connection (not the URL itself).
 More control than URL
 can write to the connection (send POST data).
 can set request headers.
 Closely tied to HTTP

Java Socket Programming

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