CSC 308 2.0

System Development with Java

Budditha Hettige

Department of Statistics and Computer Science

JAVA Networking

Networking Basics

• Computers running on the Internet communicate to

each other using either

– Transmission Control Protocol (TCP)

– User Datagram Protocol (UDP)

• Java programs that communicate over the network,

you are programming at the application layer

• use the classes in the java.net package

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TCP (Transmission Control

Protocol)

• is a connection-based protocol that provides a

reliable flow of data between two computers

• provides a point-to-point channel for applications that

require reliable communications

– Hypertext Transfer Protocol (HTTP)

– File Transfer Protocol (FTP)

– Telnet

• To communicate over the network (Through java.net)

– URL

– URLConnection

– Socket

– ServerSocket

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UDP (User Datagram Protocol)

• is a protocol that sends independent packets of data,

called datagrams, from one computer to another

• UDP is not connection-based

• Many firewalls and routers have been configured not

to allow UDP packets

• The TCP and UDP protocols use ports to map

incoming data to a particular process running on a

computer

• To communicate over the network (Through java.net)

– DatagramPacket

– DatagramSocket

– MulticastSocket

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URL- Universal Resource Locator • A string that describes how to find a resource on the Internet

• Is a specific character string that constitutes a reference to an

Internet resource.

• Syntax

– scheme://domain:port/path?query_string#fragment_id

– http://vnc.example.com:5800

• URL class

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Create a URL object

Using URL Class

URL myURL = new URL("http://example.com/");

Sample Code try {

// With components.

URL url = new URL("http", "hostname", 80, "index.html");

// With a single string.

url = new URL("http://hostname:80/index.html");

} catch (MalformedURLException e) {

}

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Parsing a URL

can use these getXXX methods to get information about the

URL

Sample Code

try {

URL url = new URL("http://hostname:80/index.html#_top_");

String protocol = url.getProtocol(); // http

String host = url.getHost(); // hostname

int port = url.getPort(); // 80

String file = url.getFile(); // index.html

String ref = url.getRef(); // _top_

} catch (MalformedURLException e) {

}

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Reading Directly from a URL

Sample Code

import java.net.*;

import java.io.*;

public class URLReader {

public static void main(String[] args) throws Exception {

URL oracle = new URL("http://www.dscs.sjp.ac.lk/");

BufferedReader in = new BufferedReader(

new InputStreamReader(

oracle.openStream()));

String inputLine;

while ((inputLine = in.readLine()) != null)

System.out.println(inputLine);

in.close();

}

}

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Network

a client, a server, and network

Client Server

Client machine Server machine

Elements of Client-Server Computing

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Client-Server Paradigm

• Server waits for client to request

a connection.

• Client contacts server to

establish a connection.

• Client sends request.

• Server sends reply.

• Client and/or server terminate

connection.

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Sockets

• A socket is one end-point of a two-way

communication link between two programs

running on the network

• Represent the connection between a client

program and a server program

• The java.net package

– Socket

– ServerSocket

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Socket Connection

Client socket, welcoming socket and connection socket

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Socket Connection

server

Client

Connection request po

rt

server

Client

Connection

po

rt

port po

rt

a client making a connection request to the server

A session established with temporary ports used for two way communication.

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More on Computer ports

• The port numbers in the range from 0 to 1023 are the well-known ports – Ports 20/21 File Transfer Protocol

– Port 23 Telnet

– Port 25 Simple Mail Transport Proto.

– Port 79 Finger

– Port 80 HTTP

– Port 110 POP3 (Post Office Protocol)

– All well known ports in the range 1..1023

• Registered ports are those from 1024 through 49151

• Dynamic, private or ephemeral ports

– The range 49152–65535 - above the registered ports

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Socket Class

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ServerSocket

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Socket and ServerSocket

• Establish a connection with peer process.

• Socket provides an IO stream associated with the

connection.

– all Java IO methods/Objects can be used.

• New exceptions to deal with.

• Great it what you really need is to communicate

• using nothing more than a stream.

• Other endpoint could be written in any language!

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Example

Resolving a Hostname

try {

InetAddress addr = InetAddress.getByName(

"java.sun.com");

} catch (UnknownHostException e) {

}

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Example

Creating a Client Socket

try {

InetAddress addr = InetAddress.getByName(

"java.sun.com");

int port = 80;

Socket sock = new Socket(addr, port);

} catch (IOException e) {

}

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Example

Creating a Server Socket

try {

int port = 2000;

ServerSocket srv = new ServerSocket(port);

// Wait for connection from client.

Socket socket = srv.accept();

} catch (IOException e) {

}

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Example

Reading Text from a Socket

try {

BufferedReader rd = new BufferedReader(

new InputStreamReader(socket.getInputStream()));

String str;

while ((str = rd.readLine()) != null) {

System.out.println(str);

}

rd.close();

} catch (IOException e) {

}

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Example

Writing Text to a Socket

try {

BufferedWriter wr = new BufferedWriter(

new OutputStreamWriter(socket.getOutputStream()));

wr.write("aString");

wr.flush();

} catch (IOException e) {

}

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Example

• Client-Server Networking

– TCPClient.java

– TCPServer.java

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What is datagram

• A datagram is an independent, self-

contained message sent over the

network whose arrival, arrival time, and

content are not guaranteed

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UDP Sockets Programming

• Sending/Receiving data.

– java.net.DatagramPacket class

• Type of UDP classes

– DatagramSocket

– DatagramPacket

– MultiCastSocket

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DatagramPacket

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MulticastSocket

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Sending a Datagram

public static void send(InetAddress dst,

int port, byte[] outbuf, int len)

{

try { DatagramPacket request =

new DatagramPacket(outbuf, len, dst, port);

DatagramSocket socket =

new DatagramSocket();

socket.send(request);

} catch (SocketException e) {

} catch (IOException e) {

}

}

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Receiving a Datagram

try

{ byte[] inbuf = new byte[256]; // default size

DatagramSocket socket = new DatagramSocket();

// Wait for packet

DatagramPacket packet = new DatagramPacket

(inbuf, inbuf.length);

socket.receive(packet);

// Data is now in inbuf

int numBytesReceived = packet.getLength();

} catch (SocketException e)

{ }

catch (IOException e)

{ }

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Sending a big file

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Building a Simple Web Server

• Handles only one HTTP request

• Accepts and parses the HTTP request

• Gets the required file from the server’s file system.

• Creates an HTTP response message consisting of the requested file preceded by header lines

• Sends the response directly to the client

• Example – WebBrowser.java

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