ICT World9

সাহাবী আনাস রাঃ থেকে বর্নীত অতি মূল্যবান একটি হাদিস

noreply@blogger.com (Md. Al Hasan Juwel) — Sun, 16 Apr 2017 09:03:00 -0700

Java Servlet

noreply@blogger.com (Md. Al Hasan Juwel) — Thu, 7 May 2015 09:49:00 -0700

In the early days, web servers used to deliver
static contents that are indifferent to users' requests. But Now-a-days, Java servlets
are server-side programs (running inside a web server) that handle clients' requests and return a
customized or dynamic response for each request. The dynamic response could be based on user's input (e.g., search, online shopping, online transaction) with data retrieved from databases or other applications, or time-sensitive data (such as news and stock prices).
Java servlets typically run on the HTTP protocol. HTTP is an
asymmetrical request-response protocol.The client sends a request message to the server, and the server returns a response message as illustrated.

Servlet Interface

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 03:48:00 -0700

The javax.servlet.Servlet is the base interface of Java Servlet API. Servlet interface declares the
life cycle methods of servlet. All the servlet classes are required to implement this interface. The methods declared in this interface are:
public abstract void init(ServletConfig paramServletConfig) throws ServletException – This
is the very important method that is invoked by servlet container to initialize the servlet and
unless ServletConfig parameters. The servlet is not ready to process client request until init()
method is finished executing. This method is called only once in servlet life cycle and make
Servlet class different from normal java objects. We can extend this method in our servlet classes
to initialize resources such as DB Connection, Socket c connection etc.

public abstract ServletConfig getServletConfig() – This method returns a servlet config object,
which contains any initialization parameters and startup configuration for this servlet. We can use
servlet this method to get the init parameters of servlet defines in deployment descriptor (web.xml) or
through annotation in Servlet 3. We will look into ServletConfig interface later on.

public abstract void service(ServletRequest req, ServletResponse res) throws
ServletException, IOException – This method is responsible for processing the client request.

Whenever servlet container receives any request, it creates a new thread and execute the

service() method by passing request and response as argument. Servlets usually run in multi-

threaded environment, so it’s developer responsibility to keep shared resources thread-safe using

synchronization.

public abstract String getServletInfo() – This method returns string containing information

about the servlet, such as its author, version, and copyright. The string returned should be plain

text and can’t have markups.

public abstract void destroy() – This method can be called only once in servlet life cycle and

used to close any open resources. This is like finalize method of a java class.

Servlet Class

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 03:43:00 -0700

Servlet is J2EE server driven technology to create web applications in java. The javax.servlet and
javax.servlet.http packages provide interfaces and classes for writing our own servlets.
javax.servlet.Servlet interface, which defines servlet lifecycle
All servlets must implement the javax.servlet.Servlet
methods. When implementing a generic service, we can extend the GenericServlet class provided
with the Java Servlet API. The HttpServlet class provides methods, such as doGet() and doPost(),
specific for handling HTTP-specific services.
Most of the times, web applications are accessed using HTTP protocol and thats why we mostly extend HttpServlet class.

Web Application Directory Structure

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 03:41:00 -0700

Java Web Applications are packaged as Web Archive (WAR) and it has a defined structure. You can
hierarchy. export above dynamic web project as WAR file and unzip it to check the hierarchy. It will be something like below image.

Web Archive Root Directory

Important Work of Web Container

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 03:35:00 -0700

Some of the important work done by web container are:
Communication Support – Container provides easy way of communication between web server
and the servlets and JSPs. Because of container, we don’t need to build a server socket to listen
for any request from web server, parse the request and generate response. All these important and
complex tasks are done by container and all we need to focus is on our business logic for our
applications.
Lifecycle and Resource Management – Container takes care of managing the life cycle of
servlet. Container takes care of loading the servlets into memory, initializing servlets, invoking
servlet methods and destroying them. Container also provides utility like JNDI for resource
pooling and management.

Multithreading Support – Container creates new thread for every request to the servlet and
when it’s processed the thread dies. So servlets are not initialized for each request and saves time
and memory.
JSP Support – JSPs doesn’t look like normal java classes and web container provides support for
JSP. Every JSP in the application is compiled by container and converted to Servlet and then

Web Container

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 03:30:00 -0700

Tomcat is a web container, when a request is made from Client to web server, it passes the request to web container and it’s web container job to find the correct resource to handle the request (servlet or JSP) and then use the response from the resource to generate the response and provide it to web server. Then web server sends the response back to the client.
When web container gets the request and if it’s for servlet then container creates two Objects
HTTPServletRequest and HTTPServletResponse. Then it finds the correct servlet based on the URL and creates a thread for the request. Then it invokes the servlet service() method and based on the HTTP method service() method invokes doGet() or doPost() methods. Servlet methods generate the dynamic page and write it to response. Once servlet thread is complete, container converts the response to HTTP response and send it back to client.

Web.xml

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 03:26:00 -0700

It is called web application deployment descriptor. It provides the configuration
options for that particular web application, such as defining the the mapping between URL and
servlet class.

Form-Data Submission Methods: GET|POST

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 03:17:00 -0700

Two request methods, GET and POST, are available for submitting form data, to be specified in the
<form>'s attribute "method=GET|POST". GET and POST performs the same basic function. That is,
gather the name-value pairs of the selected input elements, URL-encode, and pack them into a query
string. However, in a GET request, the query string is appended behind the URL, separated by a '?'.
Whereas in a POST request, the query string is kept in the request body (and not shown in the URL). The length of query string in a GET request is limited by the maximum length of URL permitted, whereas it is unlimited in a POST request. I recommend POST request for production, as it does not show the strange looking query string in the URL, even if the amount of data is limited. In this tutorial, I use GET method, so that you can inspect the query string on the URL.
To try out the POST request, modify the "form_input.html":

<form method="post" action="echo">
......
</form>
Inside the servlet, GET request is processed by the method doGet(), while POST request is processed by
the method doPost(). Since they often perform identical operations, we re-direct doPost() to
doGet() (or vice versa), as follows:
public class MyServlet extends HttpServlet {
// doGet() handles GET request
@Override
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException {
......
......
}
// doPost() handles POST request

@Override

public void doPost(HttpServletRequest request, HttpServletResponse response)

throws IOException, ServletException {

Configure the Servlet URL Mapping

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 03:14:00 -0700

Configure the Servlet URL mapping in "web.xml"
Our <form>'s action attribute refers to relative URL "echo", which has to be mapped to the
EchoServlet.class in the web application deployment descriptor file "WEB-INF\web.xml":
<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app version="3.0"
xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">

<servlet>
<servlet-name>HelloWorldServlet</servlet-name>
<servlet-class>mypkg.HelloServlet</servlet-class>
</servlet>
<servlet>
<servlet-name>EchoServletExample</servlet-name>
<servlet-class>mypkg.EchoServlet</servlet-class>
</servlet>

<servlet-mapping>
<servlet-name>HelloWorldServlet</servlet-name>
<url-pattern>/sayhello</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>EchoServletExample</servlet-name>
<url-pattern>/echo</url-pattern>
</servlet-mapping>
</web-app>

Cookies

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 03:07:00 -0700

They are small piece of information that is sent by web server in response header
and gets stored in the browser cookies. When client make further request, it adds the cookie to the
request header and we can utilize it to keep track of the session. We can maintain a session with
cookies but if the client disables the cookies, then it won’t work.

URL Rewriting

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 03:05:00 -0700

We can append a session identifier parameter with every request and response
to keep track of the session. This is very tedious because we need to keep track of this parameter
in every response and make sure it’s not clashing with other parameters.

HTML Hidden Field

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 03:03:00 -0700

We can create a unique hidden field in the HTML and when user starts
navigating, we can set its value unique to the user and keep track of the session. This method
can’t be used with links because it needs the form to be submitted every time request is made from client to server with the hidden field. Also it’s not secure because we can get the hidden
field value from the HTML source and use it to hack the session.

Session

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 03:01:00 -0700

HTTP protocol and Web Servers are stateless, what it means is that for web server every request is a new request to process and they can’t identify if it’s coming from client that has been sending request
previously.
But sometimes in web applications, we should know who the client is and process the request
accordingly. For example, a shopping cart application should know who is sending the request to add an item and in which cart the item has to be added or who is sending checkout request so that it can charge the amount to correct client.
Session is a conversional state between client and server and it can consists of multiple request and
response between client and server. Since HTTP and Web Server both are stateless, the only way to
maintain a session is when some unique information about the session (session id) is passed between
server and client in every request and response.
There are several ways through which we can provide unique identifier in request and response.

Server-Side Technologies

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 02:57:00 -0700

There are many (competing) server-side technologies available: Java-based (servlet, JSP, JSF, Struts,
Spring, Hibernate), ASP, PHP, CGI Script, and many others.
Java servlet is the foundation of the Java server-side technology, JSP (JavaServer Pages), JSF (JavaServer Faces), Struts, Spring, Hibernate, and others, are extensions of the servlet technology.

Importance of Servlet and JSPs

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 02:53:00 -0700

Web servers are good for static contents HTML pages but they don’t know how to generate dynamic
content or how to save data into databases, so we need another tool that we can use to generate dynamic content. There are several programming languages for dynamic content like PHP, Python, Ruby on Rails, Java Servlets and JSPs.
Java Servlet and JSPs are server side technologies to extend the capability of web servers by providing support for dynamic response and data persistence.

Mime Type

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 02:50:00 -0700

If you see above sample HTTP response header, it contains tag “Content-Type”. It’s also called MIME type and server sends it to client to let them know the kind of data it’s sending. It helps client in rendering the data for user. Some of the mostly used mime types are text/html, text/xml, application/xml etc.

HTTP Response

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 02:47:00 -0700

Some of the important parts of HTTP Response are:
Status Code – an integer to indicate whether the request was success or not. Some of the well
known status codes are 200 for success, 404 for Not Found and 403 for Access Forbidden.
Content Type – text, html, image, pdf etc. Also known as MIME type
Content – actual data that is rendered by client and shown to user.

HTTP Request

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 02:46:00 -0700

Some of the important parts of HTTP Request are:
HTTP Method – action to be performed, usually GET, POST, PUT etc.
URL – Page to access
Form Parameters – similar to arguments in a java method, for example user, password details
from login page.

HTTP

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 02:44:00 -0700

Web server and client needs a common communication protocol, HTTP (HyperText Transfer Protocol) is the communication protocol between server and client. HTTP runs on top of TCP/IP communication protocol.

HTML

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 02:42:00 -0700

Web Server and Web Client are two separate softwares, so there should be some common language for communication. HTML is the common language between server and client and stands for HyperText Markup Language.

Web Client

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 02:40:00 -0700

A web client is a software that helps in communicating with the server. Some of the most widely used
web clients are Firefox, Google Chrome, Safari etc. When we request something from server (through URL), web client takes care of creating a request and sending it to server and then parsing the server response and present it to the user.

Web Server

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 02:37:00 -0700

Web Server is a software that can process the client request and send the response back to the client. For example, Apache is one of the most widely used web server. Web Server runs on some physical machine and listens to client request on specific port.

Web Application

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 02:35:00 -0700

Web Applications are used to create dynamic websites. Java provides support for web application
through Servlets and JSPs. We can create a website with static HTML pages but when we want
information to be dynamic, we need web application.

New Webapp for the JSF Hello-world

noreply@blogger.com (Md. Al Hasan Juwel) — Mon, 4 May 2015 02:30:00 -0700

Let's define a new web context (web application) called "hellojsf" in Tomcat for our JSF Hello-world

application. First of all, we need to create the standard directory structure for the web context (as shown

in the figure below). Create a directory called "hellojsf", under the $CATALINA_HOME\webapps

directory (where $CATALINA_HOME is the Tomcat's installed directory). Create a sub-directory "WEB-

INF" under "hellojsf". Create sub-sub-directories: "classes", "lib" and "src" under "WEB-INF".

Keep the files in the appropriate directories:

"$CATALINA_HOME\webapps\hellojsf": This directory is known as context root, and keeps

the "jsp", "html" and resources available to web users.

"$CATALINA_HOME\webapps\hellojsf\WEB-INF": Keeps the webapp configuration files,

such as "web.xml", "faces-config.xml".

"$CATALINA_HOME\webapps\hellojsf\WEB-INF\src": Keeps the java program source files

(optional).