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Network Programming in Java
Chapter 22
Network Programming
• One of the original purpose for creating
Java
• Network Programming – two or more
computers communicate and share
resources to solve a problem
• Example: Client-Server architecture
Client-Server Architecture
• Server – an application that runs on the host
computer that provides a means of connection
and useful information once a connection is
established.
• Client – application(s) running on different
computer(s) that seek to establish a connection
and request computation/ information from the
server
• Actually, computation can be performed on both
ends of the network connection
Networking Basics
• Computers running on the Internet communicate with each other
using either the Transmission Control Protocol (TCP) or the User
Datagram Protocol (UDP), as this diagram illustrates:
• Java programs communicate over the network at the application
layer, so you typically don't need to concern yourself with the TCP
and UDP layers.
• Instead, you can use the classes in the java.net package to
provide system-independent network communication.
• However, to decide which Java classes your programs should use,
you do need to understand how TCP and UDP differ.
TCP vs. UDP
• TCP (Transmission Control Protocol) is a connection-based protocol
that provides a reliable flow of data between two computers.
– This is analogous to making a telephone call.
– TCP guarantees that data sent from one end of the connection actually
gets to the other end and in the same order it was sent. Otherwise, an
error is reported.
– applications that require reliable communications. The Hypertext
Transfer Protocol (HTTP), File Transfer Protocol (FTP), and Telnet
• UDP (User Datagram Protocol) is a protocol that sends independent
packets of data, called datagrams, from one computer to another
with no guarantees about their order of delivery or arrival at all. UDP
is not connection-based like TCP.
– This is analogous to the sending a series of letters through the postal
service.
– Used for applications that cannot afford or don’t need the slow down of
reliable communication.
– Note: Many firewalls and routers are configured to not allow UDP
packets.
IP Address and Ports
• Data transmitted over the Internet is
accompanied by addressing information that
identifies the computer and the port for which it
is destined.
• Computers usually have a single physical
connection to the Internet whose domain name
address (fienup2.cs.uni.edu) gets mapped to an
IP address (134.161.243.240)
• Ports are identified by a 16-bit number, which
TCP and UDP use to deliver the data to the right
application.
Port Numbers
• port numbers 0 - 1023 are reserved for wellknown services such as HTTP and FTP and
other system services
• IP number and port # are used to create a
socket
Networking Classes in the JDK
• Through the classes in java.net, Java
programs can use TCP or UDP to
communicate over the Internet.
– URL, URLConnection, Socket, and
ServerSocket classes all use TCP
– DatagramPacket, DatagramSocket, and
MulticastSocket classes all use UDP
Socket: Java Network Connection
• Analogous to an electrical socket, the
client can plug into the server
• This creates a connection along which
information can flow
• When the client disconnects, the socket is
free for the next client to use
• Input and output streams can be created
to allow communication over the socket.
Date Server Example
import java.util.Date;
import java.net.*;
import java.io.*;
public class DateServer {
static final public int portNumber = 4291;
public static void main(String [] args) {
try {
DateServer world = new DateServer();
} catch (IOException e) {
System.out.println("IO exception " + e);
} // end try-catch
} // end main
public DateServer ( ) throws IOException {
ServerSocket server = new ServerSocket(portNumber);
for (int i = 0; i < 3; i++) {
System.out.println("Waiting for a client");
Socket sock = server.accept();
System.out.println("Got client #" + i);
OutputStreamWriter out = new
OutputStreamWriter(sock.getOutputStream());
String message = "Current date and time is " + new Date() + "\n";
out.write(message);
out.close();
} // end for
} // end DateServer
} // end class DateServer
Date Server Example
ServerSocket server = new ServerSocket(portNumber);
Socket sock = server.accept();
– A server socket waits for requests to come in over
the network.
– accept() - listens for a connection to be made to this
socket and accepts it.
OutputStreamWriter out =
new OutputStreamWriter(sock.getOutputStream());
out.write(message);
out.close();
– Output stream created to allow communication over the socket
Date Client Example
import java.net.*;
import java.io.*;
public class DateClient {
static final public int portNumber = 4291;
public static void main(String [] args) {
try {
DateClient world = new DateClient();
} catch (IOException e) {
System.out.println("Received an IO exception " + e);
} // end try-catch
} // end main
public DateClient ( ) throws IOException {
Socket sock = new Socket(InetAddress.getLocalHost(),
portNumber);
Reader isread = new InputStreamReader(sock.getInputStream());
BufferedReader input = new BufferedReader(isread);
System.out.println("message is " + input.readLine());
} // end DateClient
} // end class DateClient
Date Client/Server Limitations
• Only one-way communication from the
server to the client was utilized
• Only one client can be handled by the server
at any time
Therapist Client/Server Example
Eliza-like (Weizenbaum ’76) simulation of a
Gestalt psychotherapist conducting a
question-and-answer session with a user
• Multiple clients are possible by creating a
new server thread to handle each client
Therapist Server Example
import java.net.*;
import java.io.*;
class Therapist {
static public void main (String [ ] args) {
try {
Therapist world = new Therapist();
} catch (IOException e) {
System.out.println("Received an IO Exception" + e);
} // end try-catch
} // end main
static final public int portNumber = 5321;
public Therapist () throws IOException {
ServerSocket server = new ServerSocket(portNumber);
while (true) {
Socket sock = server.accept();
Thread session = new TherapySession
(sock.getInputStream(),
sock.getOutputStream());
session.start();
} // end while
} // end Therapist constructor
} // end class Therapist
• Spawns a new server thread to handle each new client
Therapist Server Example
import java.io.*;
import java.util.Vector;
import java.util.StringTokenizer;
class TherapySession extends Thread {
public TherapySession (InputStream ins, OutputStream outs) {
Reader isread = new InputStreamReader(ins);
in = new BufferedReader(isread);
out = new OutputStreamWriter(outs);
} // end TherapySession constructor
private String name = "";
private BufferedReader in;
private Writer out;
private String response(String text) {
// answer a question with a question
if (text.endsWith("?"))
return "Why do you want to know?";
// break up line
Vector words = new Vector();
StringTokenizer breaker = new StringTokenizer(text, " .,?!");
while (breaker.hasMoreElements())
words.addElement(breaker.nextElement());
// look for ``I feel''
if ((words.size() > 1) &&
words.elementAt(0).equals("i") &&
words.elementAt(1).equals("feel"))
return "Why do you feel that way?";
// look for relatives
for (int i = 0; i < words.size(); i++) {
String relative = (String) words.elementAt(i);
if (isRelative(relative))
return "Tell me more about your " + relative;
} // end for
// nothing else, generic response
return "Tell me more";
} // end response
Therapist Server Example
private boolean isRelative(String name) {
return name.equals("mother") || name.equals("father")
|| name.equals("brother") || name.equals("sister")
|| name.equals("uncle");
} // end isRelative
public void run() {
try {
// get name
out.write("Hello. Welcome to therapy. What is your name?\n");
out.flush();
name = in.readLine();
out.write("Well " + name + " what can we do for you today?\n");
out.flush();
// now read and respond
while (true) {
String text = in.readLine();
out.write(response(text) + "\n");
out.flush();
} // end while
} catch (IOException e) { stop(); }
} // end run
} // end class TherapySession
Therapist Client Example
import java.io.*;
import java.net.*;
class TherapyClient {
public static void main (String [ ] args) {
try {
TherapyClient world = new TherapyClient();
} catch (IOException e) {
System.out.println("got IO exception " + e);
} // end try-catch
} // end main
static final public int portNumber = 5321;
private BufferedReader input, term;
private Writer output;
public TherapyClient () throws IOException {
// open standard input as buffered reader
term = new BufferedReader(new InputStreamReader(System.in));
// open socket as a reader and a writer
Socket sock = new Socket(InetAddress.getLocalHost(), portNumber);
Reader isread = new InputStreamReader(sock.getInputStream());
input = new BufferedReader(isread);
output = new OutputStreamWriter(sock.getOutputStream());
// now read and print
while (true) {
// read and print something from therapist
String line = input.readLine();
System.out.println(line);
// get our response
line = term.readLine();
if (line.equals("Quit"))
break;
output.write(line + "\n");
output.flush();
} // end while
} // end TherapyClient constructor
} // end class TherapyClient
Transmitting Objects over the Network
• Objects to be transmitted over the network via object
serialization
• The object to be transmitted implements the Serializable
interface
• Classes that do not implement this interface will not have any
of their state serialized or deserialized.
• All subtypes of a serializable class are themselves
serializable.
• The serialization interface has no methods or fields and
serves only to identify the semantics of being serializable.
• To allow subtypes of non-serializable classes to be serialized,
the subtype may assume responsibility for saving and
restoring the state of the supertype's public, protected, and (if
accessible) package fields. The subtype may assume this
responsibility only if the class it extends has an accessible noarg constructor to initialize the class's state. It is an error to
declare a class Serializable if this is not the case. The error
will be detected at runtime.