236369 Networking Part II

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Java Networking

CS 236369, Spring 2010


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Todays Menu

Networking Basics

TCP, UDP, Ports, DNS, Client-Server Model

TCP/IP in Java

Sockets

URL

The java classes: URL, URLEncoder,

URLConnection, HTTPURLConnection

Datagrams

Networking in JDBC


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URL - Uniform Resource Locator

Protocol

Host

Name

Port

Number

Path &

File

Name

Reference

URL is a reference (an address) to a resourceon the Internet. A resource can be a file, a database query and more.

URLs are just a subsetofthe more general conceptofUniformResource Identifiers (URIs) which are meanttodescribe all

points in the information space

http://www.javapassion.com:80/javaintro/index.html#Networking_API


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Class URL

Class URL represents a Uniform Resource

Locator, a pointerto a "resource" on the World

Wide Web.

Wedistinguish between: Absolute URL - contains all ofthe information

necessary to reach the resource.

Relative URL - contains only enough information to

reach the resource relativeto (or in the contextof)another URL.


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Class URL Cont.

Constructing URLs: URL w3c1 = new URL("http://www.w3.org/TR/");

URL w3c2 = new

URL("http","www.w3.org",80,"TR/"); URL w3c3 = new URL(w3c2, "xhtml1/");

Ifthe string is not an absolute URL, then it is

considered relativetothe URL

More constructors can befound in theURLAPI


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URL Encoding

URL Encoding is the process ofconverting string

into valid URL format.

V

alid

URLfo

rmat

me

anst

hat

t

he

URL co

nt

ainsonly what is termed "alpha | digit | safe | extra |

escape" characters.

You can read more aboutthewhat andthewhys ofthese

terms on the World Wide Web Consortiumsite: http://www.w3.org/Addressing/URL/url-spec.html

URL Encoding Reference


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URL EncodingCont.

Among the rest, URL encoding is performedtoconvertdata passed via html forms, because suchdata may contain special character, such as "/",".", "#", and soon, which couldeither:

Have special meanings

Is not a valid characterfor an URL

For instance, the "#" character needs to beencoded because it has a special meaning ofthatofan html anchor. The character alsoneeds to beencoded because is not allowedon avalid URL format.


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URL EncodingCont.

Example: The URL encoding ofThis is a

simple & short test is

This+is+a+simple+%26+short+test

Notethat becausethe character is very

commonly used, a special code ( the "+" sign) hasbeen reserved as its URL encoding


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URL addresses with Special characters

Some URL addresses also contain these special characters, for

examplethe space character.

Likethis: http://foo.com/helloworld/

To maketheses characters legal they needto beencoded before

passing them tothe URL constructor.URL url = new URL("http://foo.com/hello%20world");

One class that can help us with this is theURI class :

URI uri = new URI("http", "foo.com", "/hello world/", "");

URL url = uri.toURL();

Anotherone is theURLEncoder class


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URLEncoder

Contains a utility methodencodefor converting a string

into an encodedformat (used in URLs)

To convert a string, each character is examined in turn:

Space is converted into a plus sign +

a-z, A-Z, 0-9, ., -, * and_remain the same.

The bytes ofall special characters are replaced by hexadecimal

numbers, precededwith %

Todecode an encoded string, use decode() ofthe classURLDecoder

The URLEncoder API.


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MalformedURLException

URL constructors throws aMalformedURLException ifthe arguments to

the constructor referto a null orunknown protocol.

Typically, youwantto catch and handlethisexception by embedding your URL constructor

statements in a try/catch pair, likethis:

try

{ URL myURL = new URL(. . .) }

catch(MalformedURLException e)

{ ... // exception handler code here ... }


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ImFeelingLucky Example Thefollowing program sends a requesttothe Google server andextracts the result.

Google search engine accepts GET requests ofa specific format. Its reply alwayscontain a Location header line ifthe search is successful.


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ImFeelingLucky Example Cont.

Finally wouldbethe right

place


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UTF-8 In Short Unicode

is a comp

uting in

dustry s

tan

dard

allow

ing comp

uters

toconsistently represent and manipulatetextexpressed in mostofthe

world's writing systems.

Unicode provides a unique numberforevery character,no matter what the platform,no matter what the program,

no matter what the language. UTF-8 (8-bit UCS/Unicode Transformation Format) is a variable-

length characterencoding for Unicode. It is ableto represent anycharacter in the Unicode standard, yet is backwards compatiblewithASCII. Forthese reasons, it is steadily becoming the preferredencoding fore-mail, web pages, andother places where characters arestoredor streamed.

UTF-8 encodes each character in onetofouroctets (8-bit bytes), withthe 1-byteencoding usedonly forthe 128 US-ASCII characters.


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Parsing a URL

Thefollowing methods ofURL can beusedfor

parsing URLs:

getProtocol(), getHost(),

getPort(), getPath(), getFile(),

getQuery(), getRef()


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import java.net.*;

import java.io.*;

public class ParseURL {

public static voidmain(String[] args) throws Exception {URL aURL = new URL("http://java.sun.com:80/docs/books/tutorial"

+ "/index.html?name=networking#DOWNLOADING");

System.out.println("protocol = " + aURL.getProtocol());

System.out.println("authority = " + aURL.getAuthority());

System.out.println("host = " + aURL.getHost());

System.out.println("port = " + aURL.getPort());

System.out.println("path = " + aURL.getPath());

System.out.println("query = " + aURL.getQuery());System.out.println("filename = " + aURL.getFile());

System.out.println("ref = " + aURL.getRef());

}

}

The Outputprotocol = http

authority = java.sun.com:80

host = java.sun.com

port = 80

path = /docs/books/tutorial/index.html

query = name=networking

filename = /docs/books/tutorial/index.html?name=networking

ref = DOWNLOADING


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URLConnection

Represent a communications link between the application and aURL. Instances ofthis class can beused both to readfrom andtowritetothe resource referenced by the URL.

Creating a connection to a URL:

The connection object is created by invoking theopenConnectionmethodon a URL. Ifthe protocol ofthe URL is HTTP, the returned

object is ofclass HttpURLConnection.

The setup parameters and general request properties are manipulated.

The actual connection tothe remoteobject is made, using theconnectmethod.

The remoteobject becomes available. The headerfields andthe

contents ofthe remoteobject can be accessed. SeetheURLConnection class API for more information


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URLConnection Example

Whatwill happened ifyouwill try torun this codewhile not

connectedtotheweb?


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Class HttpURLConnection

A URLConnection with supportfor HTTP-

specific features.

responseMessage field

getRequestMethod()

usingProxy()

There is no needto create HTTP parsers

The HttpURLConnection API


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Datagrams

A datagram is an independent, self-contained

message sentoverthe networkwhose arrival,

arrival time, and content are not guaranteed.

Thejava.netpackage contains three classes to

help youwrite Java programs thatusedatagrams

to send and receive packets overthe network:

DatagramSocket, DatagramPacket, andMulticastSocket


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Networking in JDBC

You actually already met Sockets.


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Can be anywhere! (not

necessarily localhost

usually remote)


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Resources

The Java Tutorials Sun Microsystems

An Introduction to XML and Web Technologies /

Anders Mller andMichael I. Schwartzbach

course literature

http://www.cs.huji.ac.il/~dbi

Wikipedia

http://www.permadi.com/tutorial/urlEncoding/

236369 Networking Part II

Documents