Technology is used to create web application (resides at server side and generates dynamic web page)


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Servlet Tutorial

java servlet tutorial

Servlet technology is used to create web application (resides at server side and generates dynamic web page).

Servlet technology is robust and scalable because of java language. Before Servlet, CGI (Common Gateway Interface) scripting language was popular as a server-side programming language. But there was many disadvantages of this technology. We have discussed these disadvantages below.

There are many interfaces and classes in the servlet API such as Servlet, GenericServlet, HttpServlet, ServletRequest, ServletResponse etc.

What is a Servlet?

Servlet can be described in many ways, depending on the context.

  • Servlet is a technology i.e. used to create web application.

  • Servlet is an API that provides many interfaces and classes including documentations.

  • Servlet is an interface that must be implemented for creating any servlet.

  • Servlet is a class that extend the capabilities of the servers and respond to the incoming request. It can respond to any type of requests.

  • Servlet is a web component that is deployed on the server to create dynamic web page.

servlet

Do You Know?

  • What is the web application and what is the difference between Get and Post request ?

  • What information is received by the web server if we request for a servlet ?

  • How to run servlet in Eclipse, MyEclipse and Netbeans IDE ?

  • What are the ways for servlet collaboration and what is the difference between RequestDispatcher and sendRedirect() method ?

  • What is the difference between ServletConfig and ServletContext interface?

  • How many ways we can maintain state of an user ? Which approach is mostly used in web development ?

  • How to count total number of visitors and total response time for a request using Filter ?

  • How to run servlet with annotation ?

  • How to create registration form using Servlet and Oracle database ?

  • How can we upload and download file from the server ?

What is web application?

A web application is an application accessible from the web. A web application is composed of web components like Servlet, JSP, Filter etc. and other components such as HTML. The web components typically execute in Web Server and respond to HTTP request.
CGI(Commmon Gateway Interface)

CGI technology enables the web server to call an external program and pass HTTP request information to the external program to process the request. For each request, it starts a new process.

problem in cgi and how servlet is better

Disadvantages of CGI

There are many problems in CGI technology:

  1. If number of clients increases, it takes more time for sending response.

  2. For each request, it starts a process and Web server is limited to start processes.

  3. It uses platform dependent language e.g. C, C++, perl.


Advantage of Servlet

advantage of servlet

There are many advantages of servlet over CGI. The web container creates threads for handling the multiple requests to the servlet. Threads have a lot of benefits over the Processes such as they share a common memory area, lightweight, cost of communication between the threads are low. The basic benefits of servlet are as follows:

  1. better performance: because it creates a thread for each request not process.

  2. Portability: because it uses java language.

  3. Robust: Servlets are managed by JVM so we don't need to worry about memory leak, garbage collection etc.

Secure: because it uses java language..

Servlet Terminology

  1. Basics of Servlet

  2. HTTP

  3. Http Request Methods

  4. Difference between Get and Post

  5. Anatomy of Get Request

  6. Anatomy of Post Request

  7. Content Type

There are some key points that must be known by the servlet programmer like server, container, get request, post request etc. Let's first discuss these points before starting the servlet technology.

The basic terminology used in servlet are given below:

  1. HTTP

  2. HTTP Request Types

  3. Difference between Get and Post method

  4. Container

  5. Server and Difference between web server and application server

  6. Content Type

  7. Introduction of XML

  8. Deployment


HTTP (Hyper Text Transfer Protocol)

  1. Http is the protocol that allows web servers and browsers to exchange data over the web.

  2. It is a request response protocol.

  3. Http uses reliable TCP connections bydefault on TCP port 80.

  4. It is stateless means each request is considered as the new request. In other words, server doesn't recognize the user bydefault.

http protocol used in servlet
Http Request Methods

Every request has a header that tells the status of the client. There are many request methods. Get and Post requests are mostly used.

The http request methods are:

  • GET

  • POST

  • HEAD

  • PUT

  • DELETE

  • OPTIONS

  • TRACE

HTTP Request

Description

GET

Asks to get the resource at the requested URL.

POST

Asks the server to accept the body info attached. It is like GET request with extra info sent with the request.

HEAD

Asks for only the header part of whatever a GET would return. Just like GET but with no body.

TRACE

Asks for the loopback of the request message, for testing or troubleshooting.

PUT

Says to put the enclosed info (the body) at the requested URL.

DELETE

Says to delete the resource at the requested URL.

OPTIONS

Asks for a list of the HTTP methods to which the thing at the request URL can respond


What is the difference between Get and Post?

There are many differences between the Get and Post request. Let's see these differences:

GET

POST

1) In case of Get request, only limited amount of data can be sent because data is sent in header.

In case of post request, large amount of data can be sent because data is sent in body.

2) Get request is not secured because data is exposed in URL bar.

Post request is secured because data is not exposed in URL bar.

3) Get request can be bookmarked

Post request cannot be bookmarked

4) Get request is idempotent. It means second request will be ignored until response of first request is delivered.

Post request is non-idempotent

5) Get request is more efficient and used more than Post

Post request is less efficient and used less than get.


Anatomy of Get Request

As we know that data is sent in request header in case of get request. It is the default request type. Let's see what informations are sent to the server. anatomy of get request
Anatomy of Post Request

As we know, in case of post request original data is sent in message body. Let's see how informations are passed to the server in case of post request. anatomy of post request
Container

It provides runtime environment for JavaEE (j2ee) applications.

It performs many operations that are given below:

  1. Life Cycle Management

  2. Multithreaded support

  3. Object Pooling

  4. Security etc.


Server

It is a running program or software that provides services.

There are two types of servers:

  1. Web Server

  2. Application Server


Web Server

Web server contains only web or servlet container. It can be used for servlet, jsp, struts, jsf etc. It can't be used for EJB.

Example of Web Servers are: Apache Tomcat and Resin.
Application Server

Application server contains Web and EJB containers. It can be used for servlet, jsp, struts, jsf, ejb etc.

Example of Application Servers are:

  1. JBoss Open-source server from JBoss community.

  2. Glassfish provided by Sun Microsystem. Now acquired by Oracle.

  3. Weblogic provided by Oracle. It more secured.

  4. Websphere provided by IBM.


Content Type

Content Type is also known as MIME (Multipurpose internet Mail Extension) Type. It is a HTTP header that provides the description about what are you sending to the browser.

There are many content types:

  • text/html

  • text/plain

  • application/msword

  • application/vnd.ms-excel

  • application/jar

  • application/pdf

  • application/octet-stream

  • application/x-zip

  • images/jpeg

video/quicktime etc.

Servlet API

  1. Servlet API

  2. Interfaces in javax.servlet package

  3. Classes in javax.servlet package

  4. Interfaces in javax.servlet.http package

  5. Classes in javax.servlet.http package

The javax.servlet and javax.servlet.http packages represent interfaces and classes for servlet api.

The javax.servlet package contains many interfaces and classes that are used by the servlet or web container. These are not specific to any protocol.

The javax.servlet.http package contains interfaces and classes that are responsible for http requests only.

Let's see what are the interfaces of javax.servlet package.

Interfaces in javax.servlet package

There are many interfaces in javax.servlet package. They are as follows:

  1. Servlet

  2. ServletRequest

  3. ServletResponse

  4. RequestDispatcher

  5. ServletConfig

  6. ServletContext

  7. SingleThreadModel

  8. Filter

  9. FilterConfig

  10. FilterChain

  11. ServletRequestListener

  12. ServletRequestAttributeListener

  13. ServletContextListener

  14. ServletContextAttributeListener

Classes in javax.servlet package

There are many classes in javax.servlet package. They are as follows:

  1. GenericServlet

  2. ServletInputStream

  3. ServletOutputStream

  4. ServletRequestWrapper

  5. ServletResponseWrapper

  6. ServletRequestEvent

  7. ServletContextEvent

  8. ServletRequestAttributeEvent

  9. ServletContextAttributeEvent

  10. ServletException

  11. UnavailableException


Interfaces in javax.servlet.http package

There are many interfaces in javax.servlet.http package. They are as follows:

  1. HttpServletRequest

  2. HttpServletResponse

  3. HttpSession

  4. HttpSessionListener

  5. HttpSessionAttributeListener

  6. HttpSessionBindingListener

  7. HttpSessionActivationListener

  8. HttpSessionContext (deprecated now)

Classes in javax.servlet.http package

There are many classes in javax.servlet.http package. They are as follows:

  1. HttpServlet

  2. Cookie

  3. HttpServletRequestWrapper

  4. HttpServletResponseWrapper

  5. HttpSessionEvent

  6. HttpSessionBindingEvent

HttpUtils (deprecated now)

Servlet Interface

  1. Servlet Interface

  2. Methods of Servlet interface

Servlet interface provides common behaviour to all the servlets.

Servlet interface needs to be implemented for creating any servlet (either directly or indirectly). It provides 3 life cycle methods that are used to initialize the servlet, to service the requests, and to destroy the servlet and 2 non-life cycle methods.

Methods of Servlet interface

There are 5 methods in Servlet interface. The init, service and destroy are the life cycle methods of servlet. These are invoked by the web container.

Method

Description

public void init(ServletConfig config)

initializes the servlet. It is the life cycle method of servlet and invoked by the web container only once.

public void service(ServletRequest request,ServletResponse response)

provides response for the incoming request. It is invoked at each request by the web container.

public void destroy()

is invoked only once and indicates that servlet is being destroyed.

public ServletConfig getServletConfig()

returns the object of ServletConfig.

public String getServletInfo()

returns information about servlet such as writer, copyright, version etc.


Servlet Example by implementing Servlet interface

File: First.java

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  1. import java.io.*;  

  2. import javax.servlet.*;  

  3.   

  4. public class First implements Servlet{  

  5. ServletConfig config=null;  

  6.   

  7. public void init(ServletConfig config){  

  8. this.config=config;  

  9. System.out.println("servlet is initialized");  

  10. }  

  11.   

  12. public void service(ServletRequest req,ServletResponse res)  

  13. throws IOException,ServletException{  

  14.   

  15. res.setContentType("text/html");  

  16.   

  17. PrintWriter out=res.getWriter();  

  18. out.print("");  

  19. out.print("hello simple servlet");  

  20. out.print("

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