Java Networking by Kamalakar Dandu

8/2/2019 Java Networking by Kamalakar Dandu

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Introduction Internet Addresses

Server Sockets and Sockets Datagram Sockets and Packets

Uniform Resource Locators

Java Networking


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The InetAddress Class

The java.net.InetAddress classrepresents an IP address.

It converts numeric addresses tohost names and host names to

numeric addresses. It is used by other network

classes like Socket and

ServerSocket to identify hosts.


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Creating InetAddresses

There are no public

InetAddress() constructors.Arbitrary addresses may not becreated.

All addresses that are created

must be checked with DNS.


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getByName() Method

public static InetAddress getByName(String host)throws UnknownHostException

InetAddress inetadd;

try {

inetadd=InetAddress.getByName("utopia.poly.edu");

// or you could pass an ip address also

// inetadd = InetAddress.getByName("128.238.2.92");}

catch (UnknownHostException e) {

System.err.println(e);


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getByName()

If parameter name is null, then itsearches for the localhost

One can also pass localhost as

a parameter to look for the localhost

One can also find local host bypassing the ip address

127.0.0.1 as a parameter


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class Namepublic

methods

InetAddress Class

InetAddress implements Serializable

//no constructors

String getHostName()

String getHostAddress()

byte getAddress()[]boolean isMulticastAddress()

static InetAddress getByName(String)

static InetAddress[] getAllByName(String)

static InetAddress getLocalHost()

throws UnknownHostException

Eg:testservernames.java


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The java.net.URL class

A URL object represents a URL.

The URL class contains methods

to. create new URLs.

parse the different parts of a URL.

get an input stream from a URL soyou can read data from a server.

get content from the server as a

Java object.


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

URL(URL context, String url)URL(String protocol, String host, int port, String file,

URLStreamHandler handler) /* 1.2 */URL(URL context, String url, URLStreamHandler

handler) /* 1.2 */

All the above constructors are public and all of them

are declared as throws MalformedURLException

URL Constructors


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An example of an absolute URL is

http://www.mypersonal.com:8080/cs1/st

art.html#first"

try {

}

catch (MalformedURLException e) { }URL u = new URL("http", "www.mypersonal.com",8080, "/cs1/start.html#first");

URL u = new

URL("http://www.mypersonal.com:8080/cs1/start.html#first");

Constructing URL Objects


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Relative URLs

Many HTML files contain relativeURLs.

Consider the pagehttp://intranet.satyam.com/home.ht

ml. On this page a link to library refers

to library/default.html.


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Constructing Relative URLs

The fourth constructor creates URLsrelative to a given URL. For example,

try {URL u1 = newURL("http://intranet.satyam.com/home.html");

URL u2 = new URL(u1, "library/default.html");

}

catch (MalformedURLException e) {}


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Info From URLs

The java.net.URL class has fivemethods to split a URL into itscomponent parts. These are:

public String getProtocol()

public String getHost()public int getPort()

public String getFile()

public String getRef()


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Construct a URL by passing the stringhttp://www.mypersonal.com:8080/start.

html#first as a parameter

Display the individual parts of the URLlike

protocol, host, port, file and reference if

any

Construct a relative URL and displaythe info for the newly constructed URL

also

Eg: urlMethodsDemo.java

URLs


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URL - Observations If a port is not explicitly specified in

the URL it's set to -1. This meansthe default port is to be used.

If the ref doesn't exist, it's just null,

so watch out forNullPointerExceptions. Better yet,test to see that it's non-null beforeusing it.

If the file is left off completely, e.G.

Http://java.Sun.Com, then it's set to" "


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Reading Data From a URL

The openStream() method connects to theserver specified in the URL and returns anInputStream object fed by the data from thatconnection.

public final InputStream openStream() throwsIOException.

Any headers that precede the actual dataare stripped off before the stream is opened.

Network connections are less reliable andslower than files. Buffer with aBufferedReader or a BufferedInputStream.


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Start any server (http server) onthe local machine

Write a java program to readinformation sent from a httpserver and display on theconsole (http servers send HTML

files)

Eg: urldemo.java

URL Demo Example


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Can send output as well as readinput

Can post data to CGIs

Can read headers from a

connection

URLConnection Vs URLs


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The java.net.URLConnection class is anabstract class that handlescommunication with different kinds ofservers like ftp servers and web servers.

Protocol specific subclasses ofURLConnection handle different kinds ofservers.

By default, connections to HTTP URLsuse the GET method(class:HTTPURLConnection).

URL Connections


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1. The URL is constructed.

2. The URLs openConnection() methodcreates the URLConnection object.

3. The parameters for the connection andthe request properties that the clientsends to the server are set up.

4. The connect() method makes theconnection to the server.

5. The response header information is read

using getHeaderField().

Steps to URLConn..


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URLConnection//no constructorsObject getContent()String getHeaderField(String)/ (int)

String getHeaderFieldKey(int)InputStream getInputStream()OututStream getOutputStream()String getContentEncoding()

String getContentLength()/ getContentType()long getDate()/ getLastModified()long getExpiration()// expiration date of resource; 0 if unknown

class Namepublic

methods

Class URL Connection


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Write a java program that opensa URLConnection to the server,reads and displays the header

information (content type/length.)

Eg: urlconndemo.java

URLConn. Example


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Data may be read from theconnection in one of two ways

raw by using the input stream

returned bygetInputStream()

through a content handler with

getContent(). Data can be sent to the server

using the output stream

provided by getOutputStream().

I / OURLConn.


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Connect to a the server, readdata from the server usinggetContent() methods (shows

how one can read header andother info throughURLConnection)

Eg: UrlConnDemoReadFromServer.java

Read - Example


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Connect to a the server, send arequest in the http protocolformat. After sending the request

read from the server theinformation sent from the server,and display the information on

the screen.

Eg: UrlConnDemoReadWriteServer.java

Read Write Example


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HTTP Protocol

server

client

http://www.mcp.com/index.html

GET /index.html HTTP/1.0

HTTP/1.0 200 OKContent-type: text/html

Content-Length: 128

[ followed by the content

information ]

Protocol handlers implement the corresponding

protocols


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The exact protocols that Javasupports vary from implementationto implementation though http and

file are supported pretty mucheverywhere.

file

ftp gopher

http

mailto

telnet

doc

netdoc

verbatim

jar

Java Supported Protocol


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Data is sent across the Internetfrom one host to another is split

into independent units of finitesize called packets / datagrams.

They contain enough information to

travel from source to destination They vary in size from a few

dozen bytes to about 60,000

bytes.

Packets Vs Datagram


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TCP/IP protocol allows for errorcorrection

If one packet is lost or has error,it can be retransmitted withoutrequiring redelivery of all other

packets. If packets arrive out of order they

can be reordered at the receiving

end of the connection.

Packets Allow Error Correction


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Sockets : Intro

Working at different layers of thenetworking architecture (OSI Model)is time consuming and would wastethe effort of the programmer.

Instead programmers are presentedwith an abstraction layer (socket) to

communicate with servers/applications through the network.


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What is a socket?

Socket is a abstraction layer (aclass/object) that allows users(programmers) to communicate(read/write) through the network toremote hosts (servers)

A socket can talk to a single porton the host(server)

Sockets


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Sockets

A socket is a reliable connection for thetransmission of data between two hosts.

Sockets isolate programmers from the

details of packet encodings, lost andretransmitted packets, and packets thatarrive out of order.

There are limits. Sockets are more likely tothrow IOExceptions than files, for example.


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Socket Operations

There are four fundamental operations a socketperforms. These are:

1. Connect to a remote machine.

2. Send data.3. Receive data.

4. Close the connection.

A socket may not be connected to more than one

host at a time.

A socket may not reconnect after it's closed.


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class Name

public

methods

Socket Class

Socket

//constructorsSocket(String,int);

Socket(InetAddress,int);

Socket(String,int,InetAddress,int);

Socket(InetAddress,int,InetAddress,int);

Socket(String,int,boolean);// deprecated

Socket(InetAddress,int,boolean); //deprecated

Requirements for Socket constructors:

Specify host as

String or InetAddressSpecify port

an integer between 1 and 65535

All these constructors could throw IOException

onnec on e ween en an


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server

128

0.

Client

Connection

request

Client

server 12

8

0.

1 2 80 .

1280.

Connection

onnec on e ween en anServer


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Choosing a Local Port

You can also specify a local portnumber,

Setting the local port to 0 (last int) tellsthe system to randomly choose anavailable port.

If you need to know the port you're

connecting from, you can always get itwith getLocalPort().

Socket s = new Socket(mysite.com", 80,

mynewsite.com", 0);


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You cannot just connect to anyport on any host. The remote host

must actually be listening forconnections on that port.

You can use the constructors to

determine which ports on a hostare listening for connections.

Notes

Eg:ListOfPorts.java


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clientserver

Execute java applicationCreate a socket-returns if host found

get handle to Outputstream

write request to stream get handle for InputStream

read data sent fromserver through stream


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Start any server (http server) on thelocal machine

write a java program that opens a

socket to the http server. Get anoutput stream and write inforequesting some html page (GET

). Then, open an inputstream andread the info sent from server intothe application.Eg: SocketReadWriteServer.java

Socket s = new Socket(.);

// s.getInputStream() returns handle to

input stream and s.getOutputStream()

returns handle to an output stream

Socket Read Write Example


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Class Socket

Socket//socket optionsvoid setTcpNoDelay(boolean on)boolean getTcpNoDelay()

void setSoLinger(boolean on, int val)int getSoLinger()void setSoTimeout(int timeout)int getSoTimeout()InetAddress getInetAddress()

InetAddress getLocalInetAddress()int getPort()int getLocalPort()String toString()

class Namepublic

methods

throws SocketException


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Servers

Servers are hosts that supply clients withinformation when requested

Hence, they must be waiting for a clientrequest to arrive . The server then servesinfo to the client

Many users (clients) may be reaching the

server using the same protocol (port). Itmust be able to process and service them

Java provides Server side sockets to

enable programmers to write servers


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Server Sockets

A server socket binds to a particularport on the local machine.

Once it has successfully bound to a

port, it listens for incomingconnection attempts.

When a server detects a connection

attempt, it accepts the connection.This creates a socket between theclient and the server over which theclient and the server communicate.


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Class ServerSocket

The ServerSocket class represents aserver socket.

A ServerSocket object is constructed on a

particular local port. Then it calls accept()to listen for incoming connections.

accept() blocks until a connection is

detected. Then accept() returns a Socketobject that performs the actualcommunication with the client.


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Class ServerSocket

class Name

public

methods

Socket

ServerSocket(int);ServerSocket(int,int);ServerSocket(int,int, InetAddress);InetAddress getInetAddress();int getLocalPort();Socket accept();void close();

void setSoTimeout(int);int getSoTimeout();

new ServerSocket(5000)

will attempt to bind to the port 5000 on the

machine on which the application is running


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Example

Write a simple java server.

Create a server socket on a port

wait for connections in an infiniteloop

if request comes, service the requestthat comes and continue waiting

Eg:myServer.java


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Example

Write a client.

Create a socket connecting to themachine and port

read the data from the server

display data on the console

Eg:myClient.java


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Notes ...

No more than one process/ thread canlisten to a port at a time (including non-

java processes / threads). If an http server is already running on port

80, you cannot bind ServerSocket to port80.

If such an attempt is made BindExceptionis generated.

If the port parameter is 0. It tells java to

pick an available port.


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Adding Threading to a Server

It's better to make your server multi-threaded.

There should be a loop whichcontinually accepts new connections.

Rather than handling the connection

directly the socket should be passedto a Thread object that handles theconnection.


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Multi-threading is a good thing but it's still not aperfect solution.

Look at this accept loop:

while (true) {try {

Socket s = ss.accept();

ThreadedServer ts = new ThreadedServer(s) ;

ts.start(); // run method of Thread sends data

}

catch (IOException e) {}


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Every time you pass through this loop, a newthread gets created. Every time a connection isfinished the thread is disposed of.

Spawning a new thread for each connectiontakes a non-trivial amount of time, especiallyon a heavily loaded server. It would be betternot to spawn so many threads.

One solution is to create a thread pool, anduse them from a thread


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UDP

User Datagram protocol

Packet oriented, not stream oriented likeTCP/IP

Much faster but no error correction (hencesome times called as unreliable protocol)

NFS, TFTP, and FSP use UDP/IP

Must fit data into packets of about 8K or lessas the protocol unlike the TCP will not do itfor you


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The UDP Classes

Java's support for UDP iscontained in two classes:

java.net.DatagramSocket.

java.net.DatagramPacket.

A Datagram socket is used tosend and receive Datagrampackets.


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java.net.DatagramPacket

A wrapper for an array of bytesfrom which data will be sent or

into which data will be received.

Also contains the address andport to which the packet will besent.


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java.net.DatagramSocket

A DatagramSocket object is a localconnection to a port that does the sendingand receiving.

There is no distinction between a UDP socket

and a UDP server socket. Also unlike TCP sockets, a DatagramSocket

can send to multiple, different addresses.

The address to which data goes isstored inthe packet, not in the socket.


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UDP Ports

Separate from TCP ports.

Each computer has 65,536 UDPports as well as its 65,536 TCPports.

A server socket can be bound toTCP port 20 at the same time as adatagram socket is bound to UDP

port 20.

Class DatagramPacket


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Class DatagramPacket

DatagramPacket

DatagramPacket(byte[], int)DatagramPacket(byte[], int, InetAddress, int)void setAddress(InetAddress)

void setPort(int )void setData(byte [])void setLength(int )InetAddress getAddress()

int getPort()byte[] getData()int getLength()

class Name

public

methods

First constructor for receiving data, and

second for sending to a specific address


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Example

String s = "My first UDP Packet"

byte[] b = s.getBytes();

DatagramPacket dp = newDatagramPacket

(b, b.length);

Wi h D i i


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With a Destination

try {InetAddress inetadd = new InetAddess(localhost");

int chargen = 19;

String s = "My second UDP Packet"

byte[] b = s.getBytes();

DatagramPacket dp = new DatagramPacket(b,b.length, inetadd, chargen);

}

catch (UnknownHostException e) {

System.out.println(e);

}

Class DatagramSocket


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Class DatagramSocket

DatagramSocket

DatagramSocket()DatagramSocket(int)DatagramSocket(int, InetAddress )

void send(DatagramPacket);void receive(DatagramPacket);InetAddress getLocalAddress();getLocalPort();

void setSoTimeout(int);int getSoTimeout();void close();

class Name

public

methods

First constructor for clients, and second two

are for servers specifying a port / host address


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Sending UDP Datagrams

To send data to a particular server.

Convert the data into byte array.

Pass this byte array, the length of the datain the array (most of the time this will be thelength of the array) and the InetAddress andport to which you wish to send it into the

DatagramPacket() constructor. Next create a DatagramSocket and pass the

packet to its send() method.


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Receiving UDP Datagrams

Construct a DatagramSocket object onthe port on which you want to listen.

Pass an empty DatagramPacket objectto the DatagramSocketsreceive()method.

public synchronized void receive(DatagramPacket

dp) throws IOException.

The calling thread blocks until aDatagram is received.


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DatagramPacket dp is filled with the datafrom that datagram.

Use getPort() and getAddress() to tell where

the packet came from, use getData() to retrieve the data, and

use getLength() to see how many bytes werein the data.

If the received packet was too long for thebuffer, it's truncated to the length of thebuffer.

Example


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Exampletry {byte buffer = new byte[65536];DatagramPacket incoming = newDatagramPacket(buffer, buffer.length);DatagramSocket ds = newDatagramSocket(2134);

ds.receive(incoming);byte[] data = incoming.getData();String s = new String(data, 0,data.getLength());System.out.println("Port " +

incoming.getPort() + " on " +incoming.getAddress() + " sent thismessage:");System.out.println(s);

}

catch (IOException e) { System.err.println(e); }

Example


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Example

Write a simple java UDP SERVER

Create a server socket on a port

wait for connections in an infiniteloop

if request comes, read the information,and send it back to the client.

Eg: UDPServer.java

Example


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Example

Write a UDP client.

Create a socket connecting to the

machine and port Send data to the server

Read the info sent back from the

serverEg: UDPClient.java


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Exercise (contd)

18. This works fine on the same localmachine but the data is needed to beaccessed across the network so that the

data/information can be shared. Developthe appropriate socket interfaces andmake the information available across the

network.

Documents

Java Networking by Kamalakar Dandu