WEB TECHNOLOGIES AND ADVANCED JAVA VIVA

WEB TECHNOLOGIES AND ADVANCED JAVA VIVA

Question & Answers

Q: After I have edited an HTML file, I cannot view the result in my browser. Why?
A: Make sure that you have saved the file with a proper name and extension like "c:\mypage.htm". Also make sure that you use the same name when you open the file in your browser.

Q: I have edited an HTML file, but the changes don't show in the browser. Why?
A: A browser caches pages so it doesn't have to read the same page twice. When you have modified a page, the browser doesn't know that. Use the browser's refresh/reload button to force the browser to reload the page.

Q: What browser should I use?

A: You can do all the training with all of the well-known browsers, like Internet Explorer, Firefox, Netscape, or Opera. However, some of the examples in our advanced classes require the latest versions of the browsers.

Q: Does my computer have to run Windows? What about a Mac?
A: You can do all your training on a non-Windows computer like a Mac.

1. What is XML?

(ans) XML is the Extensible Markup Language. It improves the functionality of the Web by letting you identify your information in a more accurate, flexible, and adaptable way.

It is extensible because it is not a fixed format like HTML (which is a single, predefined markup language). Instead, XML is actually a metalanguage—a language for describing other languages—which lets you design your own markup languages for limitless different types of documents. XML can do this because it's written in SGML, the international standard metalanguage for text document markup (ISO 8879).

2.What is a markup language?

(ans) A markup language is a set of words and symbols for describing the identity of pieces of a document (for example ‘this is a paragraph’, ‘this is a heading’, ‘this is a list’, ‘this is the caption of this figure’, etc). Programs can use this with a stylesheet to create output for screen, print, audio, video, Braille, etc.

Some markup languages (eg those used in wordprocessors) only describe appearances (‘this is italics’, ‘this is bold’), but this method can only be used for display, and is not normally re-usable for anything else.

XML is sometimes referred to as ‘self-describing data’ because the names of the markup elements should represent the type of content they hold.

3. What is XML for (aka ‘Where should I use XML’)?

(ans) Its goal is to enable generic SGML to be served, received, and processed on the Web in the way that is now possible with HTML. XML has been designed for ease of implementation and for interoperability with both SGML and HTML.

SeeWhere's the spec?

Despite early attempts, browsers never allowed other SGML, only HTML (although there were unknownplugins), and they allowed it (even encouraged it) to be corrupted or broken, which held development back for over a decade by making it impossible to program for it reliably. XML fixes that by making it compulsory to stick to the rules, and by making the rules much simpler than SGML.

But XML is not just for Web pages: in fact it's very rarely used for Web pages on its own because browsers still don't provide reliable support for formatting and transforming it. Common uses for XML include:

Information identification

because you can define your own markup, you can define meaningful names for all your information items.

Information storage

because XML is portable and non-proprietary, it can be used to store textual information across any platform. Because it is backed by an international standard, it will remain accessible and processable as a data format.

Information structure

XML can therefore be used to store and identify any kind of (hierarchical) information structure, especially for long, deep, or complex document sets or data sources, making it ideal for an information-management back-end to serving the Web. This is its most common Web application, with a transformation system to serve it as HTML until such time as browsers are able to handle XML consistently.

Publishing

The original goal of XML as defined in the quotation at the start of this section. Combining the three previous topics (identity, storage, structure) means it is possible to get all the benefits of robust document management and control (with XML) and publish to the Web (as HTML) as well as to paper (as PDF) and to other formats (eg Braille, Audio, etc) from a single source document by using the appropriate stylesheets.

Messaging and data transfer

XML is also very heavily used for enclosing or encapsulating information in order to pass it between different computing systems which would otherwise be unable to communicate. By providing a lingua franca for data identity and structure, it provides a common envelope for inter-process communication (messaging).

Web services

Building on all of these, as well as its use in browsers, machine-processable data can be exchanged between consenting systems, where before it was only comprehensible by humans (HTML). Weather services, e-commerce sites, blog newsfeeds, AJaX sites, and thousands of other data-exchange services use XML for data management and transmission, and the web browser for display and interaction.

4. What is SGML?

(ans)SGML is the Standard Generalized Markup Language (ISO 8879:1986), the international standard for defining descriptions of the structure of different types of electronic document. There is an SGML FAQ from David Megginson at http://math.albany.edu:8800/hm/sgml/cts-faq.htm

SGML is very large, powerful, and complex. It has been in heavy industrial and commercial use for nearly two decades, and there is a significant body of expertise and software to go with it.

XML is a lightweight cut-down version of SGML which keeps enough of its functionality to make it useful but removes all the optional features which made SGML too complex to program for in a Web environment.

5. What is HTML?

(ANS)HTML is the HyperText Markup Language (RFC 1866), which started as a small application of question A.4, SGML for the Web, originating with Tim Berners-Lee at CERN in 1989–90.

It defines a very simple class of report-style documents, with section headings, paragraphs, lists, tables, and illustrations, with a few informational elements, but very few presentational elements [10], plus some hypertext and multimedia. See the question on extending HTML. The current recommendation is to use the XML version, XHTML. There is a HTML and XHTML FAQ maintained by Steven Pemberton at http://www.w3.org/MarkUp/2004/xhtml-faq

6.Aren't XML, SGML, and HTML all the same thing?

(ans) Not quite; SGML is the mother tongue, and has been used for describing thousands of different document types in many fields of human activity, from transcriptions of ancient Irish manuscripts to the technical documentation for stealth bombers, and from patients' medical and clinical records to musical notation. SGML is very large and complex, however, and probably overkill for most common office desktop applications.

XML is an abbreviated version of SGML, to make it easier to use over the Web, easier for you to define your own document types, and easier for programmers to write programs to handle them. It omits all the complex and less-used options of SGML in return for the benefits of being easier to write applications for, easier to understand, and more suited to delivery and interoperability over the Web. But it is still SGML, and XML files may still be processed in the same way as any other SGML file (see the question on XML software).

HTML is just one of many SGML or XML applications—the one most frequently used on the Web.

Technical readers will find it more useful to think of XML as being SGML−− rather than HTML++.

7.Who is responsible for XML?

(ans) XML is a project of the World Wide Web Consortium (W3C), and the development of the specification is supervised by an XML Working Group. A Special Interest Group of co-opted contributors and experts from various fields contributed comments and reviews by email.

XML is a public format: it is not a proprietary development of any company, although the membership of the WG and the SIG represented companies as well as research and academic institutions. The v1.0 specification was accepted by the W3C as a Recommendation on Feb 10, 1998.

8.Why is XML such an important development?

(Ans) It removes two constraints which were holding back Web developments:

dependence on a single, inflexible document type (HTML) which was being much abused for tasks it was never designed for;

the complexity of full question A.4, SGML, whose syntax allows many powerful but hard-to-program options.

XML allows the flexible development of user-defined document types. It provides a robust, non-proprietary, persistent, and verifiable file format for the storage and transmission of text and data both on and off the Web; and it removes the more complex options of SGML, making it easier to program for.

9.Why not just carry on extending HTML?

(ans) HTML was already overburdened with dozens of interesting but incompatible inventions from different manufacturers, because it provides only one way of describing your information.

XML allows groups of people or organizations to question C.13, create their own customized markup applications for exchanging information in their domain (music, chemistry, electronics, hill-walking, finance, surfing, petroleum geology, linguistics, cooking, knitting, stellar cartography, history, engineering, rabbit-keeping, question C.19, mathematics, genealogy, etc).

HTML is now well beyond the limit of its usefulness as a way of describing information, and while it will continue to play an important role for the content it currently represents, many new applications require a more robust and flexible infrastructure.

10.Why should I use XML? (aka ‘What is XML for?)

(ans) Here are a few reasons for using XML (in no particular order). Not all of these will apply to your own requirements, and you may have additional reasons not mentioned here (if so, please let the editor of the FAQ know!).

·         XML can be used to describe and identify information accurately and unambiguously, in a way that computers can be programmed to ‘understand’ your information (well, at least manipulate as if they could understand it).

·         XML allows documents which are all the same type to be created and handled consistently and without structural errors, because it provides a standardised way of describing, controlling, or allowing/disallowing particular types of document structure. [Note that this has absolutely nothing whatever to do with formatting, appearance, or the actual text or data content of your documents, only the structure of them. If you want styling or formatting, see question C.24, ‘How do I control formatting and appearance?’.]

·         XML provides a robust and durable format for information storage and transmission. Robust because it is based on a proven standard, and can thus be tested and verified; durable (persistent) because it uses plain-text file formats which will outlast proprietary binary ones.

·         XML provides a common syntax for messaging systems for the exchange of information between applications. Previously, each messaging system had its own format and all were different, which made inter-system messaging unnecessarily messy, complex, and expensive. If everyone uses the same syntax it makes writing these systems much faster and more reliable.

·         XML is free. Not just free of charge (free as in beer) but free of legal encumbrances (free as in speech). It doesn't belong to anyone, so it can't be hijacked or pirated. And you don't have to pay a fee to use it (you can of course choose to use commercial software to deal with it, for lots of good reasons, but you don't pay for XML itself).

·         XML information can be manipulated programmatically (under machine control), so XML documents can be pieced together from disparate sources, or taken apart and re-used in different ways. They can be converted into any other format with no loss of information.

·         XML lets you separate form (appearance) from content. Your XML file contains your document information (text, data) and identifies its structure: your formatting and other processing needs are identified separately in a stylesheet or processing system. The two are combined at output time to apply the required formatting to the text or data identified by its structure (location, position, rank, order, or whatever).

·         Any of the Design Goals listed in the XML Specification.

11. Where do I find more information about XML?

(ans) Online, there's the XML Specification and the ancillary documentation available from the W3C; Robin Cover's SGML/XML Web pages with an extensive list of online reference material and links to software; and a summary and condensed FAQFPRIVATE Google or other search engine.

For offline resources, see the lists of books, articles, and software for XML in Robin Cover's SGML and XML Web pages. That site should always be your first port of call.

The events listed below are the ones I have been told about. Please mail me if you come across others: there are many other XML events around the world, and most of them are announced on the mailing lists and newsgroups.

12.Where can I discuss implementation and development of XML?

(Ans) The two principal online support media are Usenet newsgroups and mailing lists. The IRC network is also used to some extent, and most individual projects and programs have their own topic-specific bulletin-boards on their web sites.

For off-line support, see question A.11, ‘Where do I find more information about XML?’ for details of conferences and summerschools.

·         The Usenet newsgroups are comp.text.xml and to a certain extent comp.text.sgml. Ask your Internet Provider for access to these, or use a Web interface like Google Groups. If your browser or mailer doesn't provide newsreading facilities, install one that does, or (better) use a standalone newsreader.

·         The general-purpose mailing list for public discussion is XML-L: to subscribe, visit the Web site and click on the link to join.

·         For those developing software components for XML there is the xml-dev mailing list. You can subscribe by sending a 1–line mail message to xml-dev-request@lists.xml.org saying just SUBSCRIBE. Note that this list is for those people actively involved in developing resources for XML. It is not for general information about XML (use the XML-L list above for that).

·         The XSL-List is for for discussing XSL (both XSLT and XSL:FO). For details of how to subscribe, see http://www.mulberrytech.com/xsl/xsl-list.

13. What is the difference between XML and C or C++ or Java?

(ans) C and C++ (and other languages like FORTRAN, or Pascal, or Visual Basic, or Java or hundreds more) are programming languages with which you specify calculations, actions, and decisions to be carried out in order:

mod curconfig[if left(date,6) = "01-Apr",

    t.put "April Fool!",

    f.put days('31102005','DDMMYYYY') -

          days(sdate,'DDMMYYYY')

    " more shopping days to Samhain"];

         

XML is a markup specification language with which you can design ways of describing information (text or data), usually for storage, transmission, or processing by a program. It says nothing about what you should do with the data (although your choice of element names may hint at what they are for):

<part num="DA42" models="LS AR DF HG KJ"

 update="2001-11-22">

  <name>Camshaft end bearing retention circlip</name>

  <image drawing="RR98-dh37" type="SVG" x="476"

   y="226"/>

  <maker id="RQ778">Ringtown Fasteners Ltd</maker>

  <notes>Angle-nosed insertion tool <tool

         id="GH25"/> is required for the removal

         and replacement of this part.</notes>

</part>

         

On its own, an SGML or XML file (including HTML) doesn't do anything. It's a data format which just sits there until you run a program which does something with it. See also the question about how to run or execute XML files.

: Are sessions created on the server side?

A: A servlet session is created and stored on the server side. The servlet container keeps track of all the sessions it manages and fulfils servlet API requests to get HttpSessions, manipulate object data stored with them and trigger event callbacks.

To the maintain the session, Web clients must pass back a valid session identifier as a cookie header value or dynamically generated URL parameter. In this sense, the session is also stored by the client, but only as a token reference.

Q: Can I create a session with GenericServlet?

A: There are no protocol-specific features in GenericServlet, which is an implementation of the basic, general purpose Servlet interface. Servlet-based sessions are designed only for interactions using the HTTP protocol, which has two key features necessary for a servlet container to simulate continuous user sessions: cookies and URL-based navigation, which supports URL-rewriting. The servlet API therefore places the HttpSession interface in the javax.servlet.http package, and session references are only available through classes in this package.

Q: How can I assemble data from multiple input forms?

A: First, it is best to use a single servlet to handle each form submission. A single servlet for all input would be too complicated. Give each servlet responsibility to validate a single form input, and pass the error cases on to JSP documents that explain the problem and allow users to amend the input.

Servlet concepts

Q: What's the difference between applets and servlets?

A: There are many fundamental differences between Applet and Servlet classes, the Java API documentation for the two types will show you they have little in common.

Applets are essentially graphical user interface (GUI) applications that run on the client side in a network environment, typically embedded in an HTML page. Applets are normally based on Abstract Windowing Toolkit components to maintain backward-compatibility with the widest range of browsers' Java implementations. The application classes are downloaded to the client and run in a Java Virtual Machine provided by the browser, in a restrictive security environment called a "sandbox".

Servlets are used to dynamically generate HTTP responses and return HTML content to Web browsers on the server side. Servlets are often used to validate and process HTML form submissions and control a series of user interactions in what is known as a Web application. Servlets can be used to control all aspects of the request and response exchange between a Web browser and the server, called a servlet container.

Q: How can I write a servlet using Javascript?

A: Java servlets is a server side technology that delivers dynamic content to Web browsers and other clients. Javascript is also delivered by a Web server, but the code is only interpreted and executed after it has been downloaded by the Web browser. This means that it is not possible to write servlet code in Javascript.

It is possible to include Javascript in the output of servlets and Java Server Pages, just like standard Web pages. It is also possible to dynamically generate Javascript using a servlet and use it as the source for a script tag, though this is only advisable in rare cases.

Q: How does the JVM execute a servlet compared with a regular Java class?

A: Servlets are standard Java classes and are executed by the Java Virtual Machine in exactly the same way as any other. However, the environment or context in which servlets are executed is different. A servlet is not invoked directly through a main method, the class is loaded and run by a servlet container.

Can I include normal Java classes in servlets?

A: Any Java class can be used in a Web application, provided you make the classes available to the servlet container at runtime. The Java API classes can be used directly by adding import statements to your servlet class. Other supporting classes can also be imported, but these classes must be added to the classes or lib directory of your application.

If you need to configure the supporting classes, this can be done with standard servlet configuration features using the ServletConfig and ServletContext objects available to the init(ServletConfig) method.

Q: How can I tell when a servlet is instantiated?

A: A servlet must be instantiated before it is brought into service by the servlet container, so one way to check is to make a request to the servlet and check the response. If you need to check indirectly, you can override the init(ServletConfig) method and add log(String) statements to it. This method is called after the servlet container has instantiated the servlet before it is brought into service.

Q: What is URL-rewriting?

A: URL-rewriting is a way of maintaining a session between an HTTP client and a servlet container which does not use cookies. Rather than exchange a session ID in a cookie, the servlet container includes it in the hyperlink URLs it generates for servlets and JSP.

: Are servlets multi-threaded?

A: Yes, servlets are normally multi-threaded. The servlet container allocates a thread for each new request for a single servlet without any special programming. Each thread of your servlet runs as if a single user were accessing it alone, but you can use static variables to store and present information that is common to all threads, like a hit counter for instance.

How can I pass values between JSPs without using sessions?

A: There are four main alternatives to full session-based transfer of data values, two of them use features of the session tracking API: cookies and URL-rewriting, detailed below.

What is JavaServer Pages technology?

JavaServer Pages (JSP) technology provides a simplified, fast way to create web pages that display dynamically-generated content. The JSP specification, developed through an industry-wide initiative led by Sun Microsystems, defines the interaction between the server and the JSP page, and describes the format and syntax of the page.

How does the JavaServer Pages technology work?

JSP pages use XML tags and scriptlets written in the Java programming language to encapsulate the logic that generates the content for the page. It passes any formatting (HTML or XML) tags directly back to the response page. In this way, JSP pages separate the page logic from its design and display.

JSP technology is part of the Java technology family. JSP pages are compiled into servlets and may call JavaBeans components (beans) or Enterprise JavaBeans components (enterprise beans) to perform processing on the server. As such, JSP technology is a key component in a highly scalable architecture for web-based applications.

JSP pages are not restricted to any specific platform or web server. The JSP specification represents a broad spectrum of industry input.

What is a servlet?

A servlet is a program written in the Java programming language that runs on the server, as opposed to the browser (applets). Detailed information can be found at http://java.sun.com/products/servlet.

Why do I need JSP technology if I already have servlets?

JSP pages are compiled into servlets, so theoretically you could write servlets to support your web-based applications. However, JSP technology was designed to simplify the process of creating pages by separating web presentation from web content. In many applications, the response sent to the client is a combination of template data and dynamically-generated data. In this situation, it is much easier to work with JSP pages than to do everything with servlets.

Where can I get the most current version of the JSP specification?

The JavaServer Pages 2.1 specification is available for download from here.

How does the JSP specification relate to the Java Enterprise Edition 5 Platform?

The JSP 2.1 specification is an important part of the Java EE 5 Platform. Using JSP and Enterprise JavaBeans technologies together is a great way to implement distributed enterprise applications with web-based front ends.

Which web servers support JSP technology?

There are a number of JSP technology implementations for different web servers. The latest information on officially-announced support can be found at http://java.sun.com/products/jsp/industry.html.

Is Sun providing a reference implementation for the JSP specification?

The GlassFish project is Sun's free, open-source Java EE 5 implementation.  It includes an implementation of JSP technology version 2.1.  You can download GlassFish builds from https://glassfish.dev.java.net/. 

How is JSP technology different from other products?

JSP technology is the result of industry collaboration and is designed to be an open, industry-standard method supporting numerous servers, browsers and tools. JSP technology speeds development with reusable components and tags, instead of relying heavily on scripting within the page itself. All JSP implementations support a Java programming language-based scripting language, which provides inherent scalability and support for complex operations.

Where do I get more information on JSP technology?

The first place to check for information on JSP technology is http://java.sun.com/products/jsp/. This site includes numerous resources, as well as pointers to mailing lists and discussion groups for JSP technology-related topics.  

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