J2EE-self-study

Multi Tiered J2ee architecture

1.client tier

2.webtier

3.business tier

4.EIS tier (enterprise information system) like DB ,ERP., etc.,


The client tier comprosises of programs executed on the client side.

The html pages, xml pages,applets etc are executed on the cleint browser..

web clients(html xml etc) are generally markup language based pages.

application clients are applets( small application clients embedded
in a markup page like html ) and other GUI based..

The application clients directly communicate withthe business tieror
via web tier(http connections)

coming to web clients and applets,
who gives the browser the required programs?

none other than web tier components..

web components are jsp,servlets .. (J2ee specification defines these as
j2ee components)

But the components executed in the client side
are given by the web components..s
so the webcomponents' code has embedded code for giving the client
the required client components like html ,applet etc.,

the embedded client components are not treated as j2ee components..

eg:
You open a website home page from your browser,
the displayed html page or applet is actually delivered by a web component
and executed by the browser..,


application client:

eg: Toad is a tool which requires oracle client..,
and the tool is meant to directly connect with EIS and
manipulate the data bases..
which means the application cleints are basically used
by the application developers or testers or mainteners..

the web clients are generally used by the end-user (customer)..

XML-points

1.xml tags are case sensitive
2.the declaration tag is not an xml element
and it doesnt have a close tag
3.all xml elements must have a close tag..
4,xml attributes' values must be quoted
5.
In Windows applications, a new line is normally stored as a pair of characters: carriage return (CR) and line feed (LF). The character pair bears some resemblance to the typewriter actions of setting a new line. In Unix applications, a new line is normally stored as a LF character. Macintosh applications use only a CR character to store a new line.

With XML, a new line is always stored as LF.
With XML, CR / LF is Converted to LF

XML Elements Must be Properly Nested

XML Documents Must Have a Root Element

With XML, White Space is Preserved

UnixExplained

This is in Draft Version: it will be modified and beautified later : thanks for patience:
***********************************************
There are two kinds of Pattern Matching.
1.File Name matching pattern
2.File content Matching pattern(Regular expressions)

File matching patterns are different from
regular expression patterns.
while the former is interpreted by shell to search for specific files
the latter is interpreted by the command to search for the lines inside
a file.,


When you press the enter button after typing the command with the arguments etc.,
the shell has something to do before calling the command finally..
That is it has to do Shell interpreting & expansion

Shell interpeting & expansion:

The power of shell lies in interpreting certain meta characters.
When the shell starts parsing the command line,
and if it encounters any meta characters ,it searches for the files
with the specific patterns in the pwd, and if any files are found
then the list of matched files(space separated) is substitued
in the command line..

Now Shell will call the final command..

---------------------------------

Now not only shell there are certain built in commands
which can do special character interpretation..
This is the power of unix, but this is the foundation stone for confusion..

We have to pass the special characters via arguments to the command..
and the special characters to be interpreted by shell also should be
passed to the shell via the command line..

Whats the confusion here?

the shell and command may have the same meta characters...
for eg: * is interpreted differently by shell and command?

Then wats the problem?

if you intended for a meta character like * to be interpreted by
the command ,there is a chance that it might be interpreted by shell
and substituted by the shell if any matches occur.

Why?
because shell interpretation precedes the command interpretation.,
that is the regular expression pattern that you type on the command line
is parsed by the shell
so there is a danger because the regular expression meta
characters are interpreted by shell to do its file matching..

So we have to be extra cautious for such things?

Solution:
Escaping the meta characters,ie., instruct the shell not to interpret
such characters..
Way1: back slash
way2: double quotes
way3:single quotes