chap2 advance java

7/28/2019 chap2 advance java

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Unit-2

Networking

Neha Aggarwal

(Senior Lecturer CSE/IT)

Network

a client, a server, and network

ClientServer

Client machine

Server machine

Client Server Communication

For Writing a Network Program U

Need :

Communication protocols : TCP & UDP

Ports & Internet Addresses.

Sockets URL (Uniform Resource Locator)

Streams.

Classes in java.io and java.netpackages.

TCP (Transmission Control Protocol)

Provides the ability to acknowledge receipt of

IP packets & request of retransmission of lost

packets.

Allows the received packets to be put back

together in the order they were sent.

Supported classes in java.net : URL,URLConnection, Socket & ServerSocket.

UDP (User Datagram Protocol)

Is also an alternative protocol for sending data .

Is an unreliable protocol that does not guaranteethat packets will arrive at their destination.

Does not guarantee that packets are delivered inthe correct order.

Classes supported in java.net : DatagramPacket,DatagramSocket.

DatagramPacket class stuffs bytes of data intoUDP packets calles datagrams.

DatagramSocket sends as well as receive UDPdatagrams.

Port numbers are represented by 16-bit

numbers(0 to 65,535).

Port numbers ranging from 0 to 1023 are

reserved for well known services like :

SMTP – 25 FTP – 21

Internet Addressing

IP address is a unique number for identifying

a device like 137.92.11.13

The host name & IP address is representedby java.net.InetAddress

Its methods include:

InetAddress getByName(String host) String getHostName();

Socket

If u are a client ,u need an API that willallow u to send messages to that service& read replies from it.

if u are a server, u need to be able tocreate a port & listen at it.

u need to be able to read the messages

that comes in & reply to it. The Socket & ServerSocket are client &

server classes to do this.

Socket Communication

A server runs on a specific computer and

has a socket that is bound to a specific

port. The server waits and listens to the

socket for a client to make a connection

request.

server Client

Connection request p or t

Socket Communication

If everything goes well, the server accepts theconnection. Upon acceptance, the server gets a newsocket bounds to a different port. It needs a new socketso that it can continue to listen to the original socket for

connection requests while serving the connected client.

server

Client

Connection

port p o r t

Sockets and Java Socket Classes

A socket is an endpoint of a two-waycommunication link between twoprograms running on the network.

A socket is bound to a port number sothat the TCP layer can identify theapplication that data destined to be sent.

Java’s .net package provides twoclasses:

Socket – for implementing a client

ServerSocket – for implementing a server

Implementing a Client

1. Create a Socket Object:

Socket client = new

Socket(“hostname”, portnumber);

2. Create Output stream that can be used tosend information to the server.

PrintWriter out = new PrintWriter

(client.getOutputStream());

Implementing a Client

3. Create Input Stream to read the

response from server

BufferedReader in = new

BufferedReader(new

InputStreamReader(client.getIn

putStream()));

4. Close the socket when done:

client.close();

Implementing a Server

1. Create a ServerSocket object . ServerSocket listenSocket= new

ServerSocket(portNumber);

2. Create Socket object from

ServerSocket

Socket server=listenSocket.accept();

3. Create Input Stream to read client Input

BufferedReader in = newBufferedReader(new InputStreamReader

(server.getInputStream()));

Implementing a Server

4. Create Output streamused to send info back to

client

PrintWriter out = new PrintWriter

(server.getOutputStream)));

5. Close the Socket

Server.close();

import java.io.*; import java.net.*;

public class cli {

public static void main(String s[] )

Socket server;

String str="";

DataInputStream d=new DataInputStream(System.in);

PrintStream toserver; BufferedReader fromserver;

server=new Socket("192.168.0.66",1096);

InputStreamReader isr=newInputStreamReader(server.getInputStream());

Client Server Program

fromserver= new BufferedReader(isr); toserver=new PrintStream(server.getOutputStream());

while(true)

str=":"+d.readLine();

toserver.println(str); str=fromserver.readLine();

System.out.println(str);

catch(Exception e)

System.out.println(e);

Serve.java import java.io.*;

import java.net.*;

public class serve

public static void main(String s[]) {

ServerSocket sc;

Socket client;

DataInputStream d;

PrintStream toClient;

BufferedReader fromClient;

String str="";

d=new DataInputStream(System.in);

sc=new ServerSocket(1096); System.out.println("ServerStarted");

client=sc.accept();

InputStreamReader isr=new InputStreamReader(client.getInputStream());

fromClient=new BufferedReader(isr);

toClient=new PrintStream(client.getOutputStream());

while(true)

{ str=fromClient.readLine();

System.out.println(str);

str=":"+d.readLine();

toClient.println(str);

} catch(Exception e)

URL is Uniform Resource Locator.

It is a reference (an address) to a resource on the

web server.

java.net package uses a class called URL torepresent a URL address.

A URL takes the form of a string that describes

how to find a resource on the web server.

It consists of two main components: protocol

needed to access the resource and the location of

the resource.

The protocol identifier indicates the name of

the protocol to be used to fetch the

resource. The resource name is the

complete address to the resource.

The resource name contains components :

1) Host Name: The name of the machine

on which the resource lives.

2) Filename : The pathname to the fileon the machine.

3) Port Number :The port number towhich to connect .

4) Reference : identifies a specificlocation within a file .

Creating a URL

• Absolute URL -An absolute URL contains allof the information necessary to reach theresource.

URL u = new URL ("http://www.gmail.com/");This URL object called an absolute URL.

• Relative URL – A relative URL contains only

enough information to reach the resourcerelative to another URL.

• Eg: suppose u know two URLs at the Gmail

RELATIVE URL

http://www.gmail.com/pages/Gmail.login.html

http://www.gmail.com/pages/Gmail.first.html

U can create URL objects for these pages relative

to their common base URL:http://www.gmail.com/pages/ like this:

URL gmail = newURL("http://www.gmail.com/pages/");

URL gmailLogin = new URL(gmail,"Gmail.login.html");

URL gmailFirst = new URL(gmail,"Gmail.first.html");

Constructors

1) URL(String s)

Creates a URL object from the Stringrepresentation.

2) URL(String protocol, String host, int port,String file)

Creates a URL object from the specified

protocol, host, port number, and file.3) URL(String protocol, String host, String file)

Creates a URL from the specified protocolname, host name, and file name.

Methods

1) getContent()

Gets the contents of this URL

2) getFile()Gets the file name of this URL.

3) getHost()

Gets the host name of this URL4) getPort()

Gets the port number of this URL.

Methods

5) getProtocol()Gets the protocol name of this URL.

6) openConnection()

represents a connection to the remoteobject referred by URL.

7) openStream()

returns an InputStream for reading fromthat connection.

URL CONNECTION

Once you have successfully created aninstance of the URL class, you can begin

to call operations on it.

1) open a connection, so that u can accessthe resource represented by the URL.

2) On successfully connected, it returns an

instance of the URLConnection class.

Opening a Connection

URL url = new

URL("http://www.yahoo.com");URLConnection connection =

url.openConnection();

catch (MalformedURLException e) {System.out.println(e);

Reading From a URL

Once you have a successful connection,ucan retrieve the input stream for the

connection and begin reading.

java.io classes operate on data returnedfrom URLConnection streams.

openStream() method get a stream from

which you can read the contents of the URL. The openStream() method returns a

java.io.InputStream object

Eg: Reading From a URL

import java.net.*;import java.io.*;

public class URLReader {

public static void main(String args[])try

URL url = new

URL("http://www.yahoo.com");URLConnection connection =url.openConnection();

connection.setDoInput(true);

Eg: Reading From a URL

InputStream in =connection.getInputStream();

BufferedReader b = new BufferedReader (new InputStreamReader (in));

String line = "";while ((line = b.readLine()) != null)

System.out.println(line);

b.close();

} catch (Exception e) {System.out.println(e);

Writing to a URL

successful connection, you can retrieve theoutput stream for the connection and begin

writing.

Many HTML pages contain forms-- text fieldsand other GUI objects that let you enter data

to send to the server.

After you type in the required informationand initiate the query by clicking a button,

your Web browser writes the data to the

URL over the network .

Writing to a URL

At the other end the server receives the data,processes it, and then sends you a response.

HTTP use POST method to send data to the

server. The server recognizes the POST requestand reads the data sent from the client.

Before retrieving and writing to a URLConnection

stream, you need to designate the connection as

being write-enabled by setting the Output propertyto true using the setDoOutput() method .

Eg: Writing to a URL

import java.net.*;import java.io.*;

public static void main(String args[]) {

URL url = new URL("http://www.yahoo.com");

URLConnection connection =

url.openConnection();connection.setDoOutput(true);

OutputStream outStream =connection.getOutputStream();

Eg: Writing to a URL

ObjectOutputStream objectStream = newObjectOutputStream(outStream);

objectStream.writeInt(54367);

objectStream.writeObject("Hello there");}

catch (Exception e) {

Advanced Socket Programming

Socket Timeouts

Internet Addresses

Interruptible sockets Half Close

Socket Timeouts

• We use socket timeouts to deal withconnection errors.

If the host machine is unreachable then our

application waits for a long time & your operating system times out u eventually.

So to avoid this, u should decide whattimeout value is reasonable for your application.

Call the setSoTimeout() method to set atimeout value (in milliseconds).

Socket Timeouts

Socket s=new Socket (“host name”, port

number);

s.setSoTimeout(10000);

It will throw SocketTimeoutException, whenthe timeout has been reached before the

operation has completed its work.

Internet Addresses

Each computer is identified by a unique

number called an Internet address or an IP

address.

IP addresses are four bytes long, these are

referred to as IPv4 addresses.

Bytes are separated by periods.For

example,address for www.yahoo.com is

152.2.21.2.

InetAddress Class

U can use this class, if u need to convert

between host names & Internet addresses.

getByName() method returns an

InetAddress object of a host.

InetAddress

address=InetAddress.getByName(“www.yah

oo.com”);

this returns an IP address of this host name.

Interruptible Sockets

When u connect to a socket, current thread

blocks until the connection has been

established or timeout has elapsed.

If u want to give users an option to cancel a

socket connection that doesnot producing a

result, u will call interrupt.

To interrupt a socket operation, u will use a

SocketChannel, a feature of java.nio

package.

1) Open the SocketChannel like this :

SocketChannel channel =

SocketChannel.open(new

InetSocketAddress(host, port)); SocketChannel has read & write methods that are

declared in interfaces ReadableByteChannel &

WriteableByteChannel.

We will use Scanner class to read a

SocketChannel .Scanner has a constructor with a

ReadableByteChannel parameter.

2) Scanner in =new Scanner(channel);

3) To turn a channel into an output stream,use Channel.newOutputStream method.

OutputStream out=Channels.newOutputStream(channel);

Now whenever a thread is interrupted

during an open, read & write, theoperation doesnot block but is terminatedwith an exception.

Half Close

When a client program sends a request to aserver, the server needs to be able to determinewhen the end of the request occurs (ends of writing of data to server).

Indicating the end of the request data is harder than writing data to a file.

If u close a socket, then u immediatelydisconnect from a server.

The Half Close overcomes this problem, byclosing the output stream of a socket, indicating tothe server the end of request data.

Half Close

Socket s = new Socket (“host name”, port

number);

Scanner in = new Scanner

(socket.getInputStream());

PrintWriter writer= new

PrintWriter(socket.getOutputStream());

writer.print(“end of data”);

socket.shutdownOutput();

Sending Email

What is SMTP ?

Simple Mail Transport Protocol (SMTP) is

the network protocol used to send email

across the Internet.

SMTP is generally used to send messagesfrom a mail client to a mail server.

Steps for Sending Email

1) Importing the necessary packages:

import com.jscape.inet.smtp.*;

Import com.jscape.inet.email.*;

The com.jscape.inet.smtp package contains

the necessary classes for communicating

with an SMTP server.

The com.jscape.inet.email packagecontains the necessary classes for

generating an email message.

2) Establishing a Connection :To send email you must first establish a

network connection to your SMTP server.

create a new Smtp instance using SMTPhostname as argument.

Smtp smtp = new Smtp("smtp.yahoo.com");

establish connection

smtp.connect();

3) Composing the Email :

Create a new EmailMessage instance

EmailMessage message = new

EmailMessage();

set From address

message.setFrom(“abc@yahoo.com");

set To address

message.setTo(“xyz@yahoo.com");

set subject of messagemessage.setSubject("Hello!");

set body of message

message.setBody("Have a nice day");

set carbon-copy recipients

message.setCc(“one@yahoo.com");

4) Sending the Email :

send the email message

smtp.send(message);

5) Releasing the Connection :

release SMTP server connection

smtp.disconnect();

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