Introduction to the Web
The term "web" refers to the Internet service that allows users to browse through webpages. The protocol used for this type of communication is HTTP protocol (HyperText Transfer Protocol), which explains why the URLs that you type into a browser begin with http://.
Version 1.1 of HTTP protocol is currently being used (specified by the W3C consortium). More and more, however, HTTPS protocol (secure HTTP protocol) is being used along with electronic commerce systems because it is a secure protocol that enables encrypted requests to be exchanged between the browser and the server.
Today, the WAP protocol (Wireless Application Protocol) allows WAP compatible mobile phones to communicate with the mobile telephony operator's WAP gateway. Then the communications are converted according to HTTP protocol by the gateway. The latest version of the WAP protocol specified by the WAP Forum (the authority in charge of defining the WAP standard) is version 1.1 (beginning of 2001).
HTML and Static Pages
For now, the standard language established for the distribution of documents on the web is HTML (HyperText Markup Language). HTML is the language used to specify a document's presentation and hypertext links to other documents through the use of formatting tags.
The W3C, an organization in charge of standardized Internet rules, specified HTML version 4.01 at the beginning of 2001.
The often talked about DHTML (Dynamic HyperText Markup Language) is not a true markup language. In truth, it is a group of languages that work with HTML that are used to make a webpage more dynamic. These languages are:
- HTML 4.0 or higher
- DOM (Document Object Model), which defines an object tree structure that represents a document's configuration. DOMs are used to modify the objects included in the document.
- CSS (Cascading Style Sheet), which groups together in one document the definition of each tag's style for an entire webpage or website
A "static webpage" is a webpage made up of a text file containing HTML code and possibly some images and links to other documents. A website made up of static webpages is thus defined as a "static website". A static website will work fine if it contains less than a couple dozen pages. However, operating and updating it may run up against the following limitations:
- difficult to maintain because each page must be manually modified (especially when all the pages have the same menu)
- impossible to display a page personalized for each visitor
- impossible to dynamically create a page depending on database entries
This is why solutions have been perfected that allow servers to automatically generate webpages. Many solutions exist that allow scripting languages to be used on web servers. The most widely used are:
- The first solution, called CGI (Common Gateway Interface), interpreted programs (generally written in perl
or C programming language) and then had them send back a content that was compatible with the HTTP protocol
- Microsoft's ASP language (Active Server Pages) made writing these scripts easier through the manipulation of objects in VBScript
- The language PHP (Hypertext preprocessor) uses its own language (a derivative of C++
and Perl) and enables many functionalities (equivalent to those in ASP technology)
- The JSP language (Java Server Pages) is the most recent among these technologies. JSP allows web designers to use all the power of Java to create dynamic webpages.
Java and the Web
For a long time, Java was mostly used to write
applets, i.e. small programmes run on the client's browser within a webpage that make the page much more interactive (but much slower, as well).
Now Java is mainly used on web servers for the following reasons:
- Java is portable (it can be run on every kind of platform)
- Java is safe (it was designed to not cause any runtime errors that can create security weaknesses for the server)
The different Java objects are:
- Servlets, which allow user requests to dictate processing (access to databases with JDBC, communication with other servlets thanks to RMI technology, accessing LDAP, etc.) ;
- JSP pages representing HTML code in which the Java code is called
- EJBs (Enterprise JavaBeans), server-side components written in Java used to access their methods
Java technologies are based on use of these three objects within an application server, i.e. a specific server using a virtual Java machine. The main application servers are:
HTML quickly showed its limitations not only because of its limited number of tags but especially because it is impossible to separate content and presentation, which leads to problems when making even minor adjustments to presentation. Thus, XML
was introduced as a meta language, i.e. a language used to define a new language (comprised of new tags).
Moreover, given that XML separates the presentation from the data, it is possible to exchange documents in XML format without influencing their form. This makes adapting content to a browser and other display devices simple (in that way, WML
(Wireless Markup Language) is an XML implementation option for displaying webpages on mobile terminals).
XSL (eXtensible StyleSheet Language) is used to format an XML document with the help of formatting rules in order to compose a document in another format (HTML, WML, PDF, LaTeX, etc.)
XML organizes information according to a tree structure defined by the DOM
(Document Object Model). Thus, it is possible to read an XML document by using a parser (also called a syntactic analyzer), i.e. a programme that uses a function library (API, for Application Programmable Interface) to read and modify an XML document.