Application Protocols:HTTP

CSNB534

Semester 2, 2007/2008

Asma Shakil

What is HTTP?

HTTP (HyperText Transfer Protocol) is an application-level protocol that uses the TCP as a transport mechanism.

HTTP originated as a means of sharing documents across the Internet. By placing a hyperlink over a word, users would be able to

jump instantly from one document to another, even though the document could reside on servers located in another country.

Published as RFC 1954, pioneered by Tim Berners-Lee.

How does HTTP work?

When an HTTP client, such as a Web browser or a search engine, needs to access a file, it establishes a TCP connection to the Web server. Which by default uses TCP port 80 for

connection. The client sends a request for a particular file

and receives an HTTP response Which will often include the contents of the file.

How does HTTP work? The response includes

A status code (success/failure of a request) HTTP header information

Content-Length Size of item in octets Content-Type Type of item Content-Encoding Encoding used for item Content-Language Language(s) used in item

Example Of Header Content-Length: 34 Content-Type : text/html Content-Language: english Content-Encoding: ascii

The file contents (if appropriate)

Web Clients

Under HTTP/1.0, there are three types of requests that a client application can issue to a web server:

GET HEAD POST

GET Request Method

Used when a client needs to retrieve a resource from a web server.

Takes three parameters Pathname of the resource CGI Parameters (for CGI scripts) : to support interactive

pages rather than just static pages. Version of HTTP being used

For example GET /cgi-bin/birthday.pl?month=august&date=24 HTTP/1.0

HEAD Request Method

If the client is interested in information about a resource but not the resource itself.

Example HEAD /files/bigfile.zip /HTTP/1.0

Returns only the header fileds.

Example

Code for WebServer Demo.

HTTP and Java

While developers are free to write, their own HTTP implementations using TCP, the java.net package provides several in-built classes that offer HTTP functionality Java.net.URL Java.net.URLConnection Java.net.HttpURLConnection

URL Class URLs can point to

Files Web sites ftp sites Newsgroups E-mail addresses

Examples ftp://metalab.uniten.edu.my/~asma/csnb334/ telnet://localhost:8000/ mailto:president@whitehouse.gov?subject=My%20Opinion

Components of a URL Protocol :// hostname (:port) – optional Path (can include the CGI parameters also after a ?) (#reference) – optional - fragment identifier

Creating a URL Constructors Six constructors

URL(String url_str) Creates a URL object based on the string parameter.

URL(String protocol, String host, String file) Creates a URL object based with the specified protocol, host and file path.

URL(String protocol, String host, int port, String file) Creates a URL object based with the specified protocol, host, port and file path.

URL(String protocol, String host, int port, String file, URLStreamHandler handler) Creates a URL object based with the specified protocol, host, port and file path

and stream handler. URL(URL context, String relative)

Creates a URL object using the context of an existing URL and relative URL E.g. URL = http://somewebsite.com; relative URL = /images/icon.gif Resulting URL http://somewebsite.com/images/icon.gif

URL(URL context, String relative, URLStreamHandler handler) All of them throw java.net.MalformedURLException if the URL cannot be

correctly parsed.

Using a URL The URL class provides the following methods to parse a URL and

extract individual components as well as to open an HTTP connection to the resource that it specifies.

Methods Object getContent()

Retrieves the contents of the resource located at the URL. String getProtocol()

Returns the protocol component of a URL String getHost()

Returns the host component of a URL. String getPort()

Returns the port component of a URL. String getFile()

Returns the pathname component of a URL. String getRef()

Returns the reference component of a URL. URLConnection openConnection()

Returns a URLConnection object which can be used to establish a connection to the remote resource.

InputStream openStream() Establishes a connection to the remote server where the resource is located

and provides an input stream to read the resource’s contents

A Simple Example

Parsing with the URL Class

Running URL parser

Valid URLS http://thisisnotarealmachine/norisithisarealpath http://www.uniten.edu.my:80/#top ftp://ftp.metelab.uniten.edu.my/~asma

Invalid URLS ttp://www.uniten.edu.my:80/#top abcdef://abcdef.com/absoup/

Retrieving a Resource with the URL Class

To retrieve a resource using the URL class InputStream URL.openStream() URLConnection URL.openConnection()

A simple example to fetch a URL.

URLConnection Class

The URLConnection Class is used to send HTTP requests and read HTTP responses.

URLConnection allows you to Connect to a web server. Set request header fields Read response header fields Read the contents of the resource.

Creating a URLConnection

No public constructors for the URLConnection class.

Instead, call the URL.openConnection() method which returns a URLConnection instance. URL myURL = new URL (_some URL_); URLConnection connection =

myURL.openConnection();

Using a URLConnection Methods Void connect();

The act of creating a URLConnection object does not establish a connection with the remote host until this method is called.

Object getContent(); Process the contents of a resource and return it as an Object.

String getContentEncoding(); Returns the value of the “Content-encoding” header field.

int getContentLength(); Returns the value of the “Content-length” header field.

String getContentType(); Returns the value of the “Content-type” header field.

Content-type Type application: Multipurpose files

application/javascript: Javascript application/octet-stream: Arbitrary byte stream. This is thought of as the "default"

media type used by several operating systems, often used to identify executable files. application/x-shockwave-flash: Adobe Flash files;

Type audio: Audio audio/mpeg: MP3 audio/x-ms-wma: Windows Media Audio; audio/x-wav: WAV audio

Type text: Human-readable text and source code text/html: HTML; text/plain: Textual data; text/xml: Extensible Markup Language;

Type video: Video video/mpeg: MPEG-1 video/mp4: MP4 video; video/quicktime: QuickTime video; video/x-ms-wmv: Windows Media Video;

Using a URLConnection

Methods InputStream getInputStream();

Returns an input stream object that reads the resource’s contents pointed to by the URLConnection.

OutputStream getOutputStream(); Returns an output stream object that writes to the

remote connection pointed to by the URLConnection. If the connection does not support writing

UnknownServiceException.

A Simple Example

Retrieving a resource with the URLConnection Class