Monday 21 March 2011

Servlet


Web Server Intoduction


A web server is the combination of computer and the program installed 
on it.Web server interacts with the client through a web browser. It 
delivers the web pages to the client and to an application by using the 
web browser and  he HTTP protocols respectively. We can also define
 the web server as the package of  large number of programs installed 
on a computer connected to Internet or intranet for downloading the 
requested files using File Transfer Protocol, serving e-mail and building
 and publishing web pages. A web server works on a client server model.
 A computer connected to the Internet or intranet must have a server 
program. While talking about Java language then a web server is a 
server that is used to support the web component like the Servlet and
 JSP. Note that the web server does not support to EJB
 (business logic component) component.

A computer connected to the Internet for providing the services to a 
small  company or a departmental store may contain the HTTP server 
(to access and store the web pages and files), SMTP server 
(to support mail services), FTP server ( for files downloading) and 
NNTP server (for newsgroup). The computer containing all the above
 servers is called the web server. Internet service providers and large
 companies may have all the servers like HTTP server, SMTP server,
 FTP server and many more on separate machines. In case of Java, 
a web server can be defined as the server that only supports to the web 
component like servlet and jsp. Notice that it does not support to the business 
component like EJB.



Servlet Container




A servlet container is nothing but a compiled, executable program. The main function of
 the container is to load, initialize and execute servlets. The servlet container is the official 
Reference Implementation for the Java Servlet and JavaServer Pages technologies. The
 Java Servlet and JavaServer Pages specifications are developed by Sun under the Java
 Community Process.
A container handles large number of requests as it can hold many active servlets, listeners etc. 
It is interesting to note here that the container and the objects in a container are multithreaded. 
So each object must be thread safe in a container as the multiple requests are being handled by 
the container due to the entrance of more than one thread to an object at a time.
Note : A Servlet container may run stand alone i.e. without a web server or even on another host.
We can categorize the servlet containers as:
I. A simple servlet container is not fully functional and therefore it can only run very simple 
servlets and does the following :
  • Wait for HTTP request.
  • Construct a ServletRequest object and a ServletResponse object.
  • If the request is for a static resource, invoke the process method of 
  • theStaticResourceProcessor instance, passing the ServletRequest andServletResponse 
  • objects.
  • If the request is for a servlet, load the servlet class and invoke its service method, passing the 
  • ServletRequest and ServletResponse objects. Note that in this servlet container, the servlet 
  • class is loaded every time the servlet is requested.
II. A fully functional servlet container additionally does the following for each HTTP request for a servlet:
  • When the servlet is called for the first time, load the servlet class and call its init method (once only).
  • For each request, construct an instance of javax.servlet.ServletRequest and an instance of
  •  javax.servlet.ServletResponse.Invoke the servlet's service method, passing the
  •  ServletRequest andServletResponse objects.When the servlet class is shut down, call
  •  the servlet's destroy method and unload the servlet class.
Now lets see what a servlet container does for each HTTP request for a servlet, in general :
  • The servlet container loads the servlet class and calls the init method of the servlet as
  •  soon as the servlet is called for the first time.
  • Then this container makes an instance of javax.servlet.ServletRequest andjavax.servlet.ServletResponse
  •  for each request.
  • Then it passes the ServletRequest and ServletResponse objects by invoking the servlet'sservice method.
  • Finally, it calls the destroy method and unload the servlet class when the servlet class is to be shut down.

What is Java Servlets ?




Servlets are server side components that provide a powerful mechanism for developing server side programs.
 Servlets provide component-based, platform-independent methods for building Web-based applications,
 without the performance limitations of CGI programs. Unlike proprietary server extension mechanisms
 (such as the Netscape Server API or Apache modules), servlets are server as well as platform-independent.
 This leaves you free to select a "best of breed" strategy for your servers, platforms, and tools. Using servlets 
web developers can create fast and efficient server side application which can run on any servlet enabled web 
server. Servlets run entirely inside the Java Virtual Machine. Since the Servlet runs at server side so it does
 not checks the browser for compatibility. Servlets can access the entire family of Java APIs, including the
 JDBC API to access enterprise databases. Servlets can also access a library of HTTP-specific calls, 
receive all the benefits of the mature java language including portability, performance, reusability, and crash
 protection. Today servlets are the popular choice for building interactive web applications. 
Third-party servlet containers are available for Apache Web Server, Microsoft IIS, and others. Servlet 
containers are usually the components of web and application servers, such as BEA WebLogic 
Application Server, IBM WebSphere, Sun Java System Web Server, Sun Java System Application Server 
and others.
Servlets are not designed for a specific protocols. It is different thing that they are most commonly used 
with the HTTP protocols Servlets uses the classes in the java packages javax.servlet and javax.servlet.http.
 Servlets provides a way of creating the sophisticated server side extensions in a server as they follow
 the standard framework and use the highly portable java language.
HTTP Servlet typically used to:
  • Priovide dynamic content like getting the results of a database query and returning to the client.
  • Process and/or store the data submitted by the HTML.
  • Manage information about the state of a stateless HTTP. e.g. an online shopping car manages
  •  request for multiple concurrent customers.

Methods of Servlets



A Generic servlet contains the following five methods:
init()

public void init(ServletConfig config) throws ServletException
The init() method is called only once by the servlet container throughout the life of a servlet. By
 this init() method the servlet get to know that it has been placed into service. 
The servlet cannot be put into the service if
  •  The init() method does not return within a fix time set by the web server. 
  •  It throws a ServletException
Parameters - The init() method takes a ServletConfig object that contains the initialization parameters
 and servlet's configuration and throws a ServletException if an exception has occurred.

service()

public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException

Once the servlet starts getting the requests, the service() method is called by the servlet container
 to respond. The servlet services the client's request with the help of two objects. These two objects 
javax.servlet.ServletRequest and  javax.servlet.ServletResponse are passed by the servlet container.

The status code of the response always should be set for a servlet that throws or sends an error.

Parameters -  The service() method takes the ServletRequest object that contains the client's request
 and the object ServletResponse contains the servlet's response. The service() method throws
 ServletException and IOExceptions exception.
 
getServletConfig()

public ServletConfig getServletConfig() 
This method contains parameters for initialization and startup of the servlet and returns aServletConfig 
object. This object is then passed to the init method. When this interface is implemented then it stores
 the ServletConfig object in order to return it. It is done by the generic class which implements this
 inetrface.
Returns -  the ServletConfig object
getServletInfo()

public String getServletInfo() 
The information about the servlet is returned by this method like version, author etc. This method
returns a string which should be in the form of plain text and not any kind of markup. 
 Returns - a string that contains the information about the servlet
destroy()

public void destroy() 
This method is called when we need to close the servlet. That is before removing a servlet instance
 from service, the servlet container calls the destroy() method. Once the servlet container calls the 
destroy() method, no service methods will be then called . That is after the exit of all the threads
 running in the servlet, the destroy() method is called. Hence, the servlet gets a chance to clean up all the resources like memory, threads etc which are being held. 


Life cycle of Servlet




The life cycle of a servlet can be categorized into four parts:
  1. Loading and Inatantiation: The servlet container loads the servlet during startup or when the first request is made. The loading of the servlet depends on the attribute <load-on-startup> of web.xml file. If the attribute <load-on-startup> has a positive value then the servlet is load with loading of the container otherwise it load when the first request comes for service. After loading of the servlet, the container creates the instances of the servlet.
  2. Initialization: After creating the instances, the servlet container calls the init() method and passes the servlet initialization parameters to the init() method. The init() must be called by the servlet container before the servlet can service any request. The initialization parameters persist untill the servlet is destroyed. The init() method is called only once throughout the life cycle of the servlet.

    The servlet will be available for service if it is loaded successfully otherwise the servlet container unloads the servlet.
  3. Servicing the Request: After successfully completing the initialization process, the servlet will be available for service. Servlet creates seperate threads for each request. The sevlet container calls the service() method for servicing any request. The service() method determines the kind of request and calls the appropriate method (doGet() or doPost()) for handling the request and sends response to the client using the methods of the response object.
  4. Destroying the Servlet: If the servlet is no longer needed for servicing any request, the servlet container calls the destroy() method . Like the init() method this method is also called only once throughout the life cycle of the servlet. Calling the destroy() method indicates to the servlet container not to sent the any request for service and the servlet  releases all the resources associated with it. Java Virtual Machine claims for the memory associated with the resources for garbage collection.

                  Life Cycle of a Servlet 





Features of Servlets 2.4




In this tutorial you will learn the new features added in Servlet 2.4.
  1. Upgraded supports for Http, J2SE, and J2EE: Servlet 2.4 depends on Http1.1 and J2SE 1.3.
  2. Additional ServletRequest methods : In Servlet 2.4 four new methods are added in the ServletRequest
    • getRemotePort(): It returns the IP source port of the client.
    • getLocalName(): It returns the host name on which the request was recieved.
    • getLocalAddr(): It returns the IP address on which the request was recieved. 
    • getLocalPort(): It returns the IP port number.
  3. New Support for Internationalization and charset choice: To provide support of internationization, Servlet 2.4 has added two new methods in the ServletResponse interface.
    • setCharacterEncoding(String encoding): The purpose of this method is to set the response's character encoding.  This method helps us to pass a charset parameter tosetContentType(String) or passing a Locale to setLocale(Locale). We can now avoid setting the charset in the setContentType("text/html;charset=UTF-8") assetCharacterEncoding() method pairs with the pre-existinggetCharacterEncoding() method to manipulate and view the response's character encoding.
       
    • getContentType(): It is responsible for returning the response's content type. The content type can be dynamically set with a combination of setContentType(),setLocale(), and setCharacterEncoding() calls, and the method getContentType()provides a way to view the generated type string. 
  4. New features has been added in RequestDispatcher: In Servlet 2.4 five new request attributes has been added for providing extra information during a RequestDispatcherforward() call. This features has been added is Servlet 2.4 to know the true original request URI. The following request attributes are:
    • javax.servlet.forward.request_uri
    • javax.servlet.forward.context_path
    • javax.servlet.forward.servlet_path
    • javax.servlet.forward.path_info
    • javax.servlet.forward.query_string
  5. SingleThreadModel interface has been deprecated: In Servlet 2.4 the SingleThreadModel interface has been deprecated. 
  6. HttpSession details and interaction with logins has been clarified: The new method HttpSession.logout() has been added in Servlet 2.4. Now session allows zero or negative values in the <session-timeout> element to indicate sessions should never time out.
    If the object in the session can't be serialize in a distributed environment then it must throw an IllegalArgumentException. 
  7. Welcome file behavior and Classloading has been clarified: In servlet 2.4 welcome file can be a servlet. 
  8. The web.xml file now uses XML Schema: Version 2.4 servers must still accept the 2.2 and 2.3 deployment descriptor formats, but all new elements are solely specified in Schema

Features of Servlet 2.5




This version has been released onSeptember 26, 2005 by the Sun MicroSystems. It is not necessary that all web servers and application servers support the features of Servlet 2.5. Still most of the popular containers likeTomcat 5.5 and JBoss 4.0 supportServlet 2.4. 
The list of the added features is given below:
  1. Dependency on J2SE 5.0: The minimum platform requirement for Servlet 2.5 is JDK 1.5. Servet 2.5 can't be used in versions below than JDK1.5. All the available features of JDK1.5 like generics, autoboxing, an improved for loop etc are guaranteed available to Servlet 2.5 programmers. 
  2. Support For annotations: Annotations provide a mechanism for decorating java code constructs (classes, methods, fields, etc.) with metadata information. Annotations are mark code in such a way that code processors may alter their behavior based on the metadata information. 
  3. Several web.xml convenience: Servlet 2.5 introduces several small changes to the web.xml file to make it more convenient to use.  For example while writing a <filter-mapping>, we can now use an asterisk in a <servlet-name> which will represent all servlets as well as JSP.

    Previously we used to do

    <filter-mapping>
    <filter-name>FilterName</filter-name>
    <servlet-name>FilterName</servlet-name>
    </filter-mapping>

    Now,

    <filter-mapping>
    <filter-name>FilterName</filter-name>
    <servlet-name>*</servlet-name>
    </filter-mapping>

    Previously in <servlet-mapping> or <filter-mapping> there used to be only one <url-pattern>, but now we can have multiple <url-pattern>, like

    <servlet-mapping>
        <servlet-name>abc</servlet-name>
        <url-pattern>/abc/*</url-pattern>
        <url-pattern>/abc/*</url-pattern>
    </servlet-mapping>

    Apart from these changes, many more facilities added in web.xml.
  4. A Handful of removed restrictions: Servlet 2.5 removed a few restrictions around error handling and session tracking. Now it has removed the restriction that the <error-page> could not call the setStatus() method to alter the error code that triggered them. In session tracking, Servlet 2.5 eased a rule that a servlet called by RequestDispatcher include() couldn't set response headers.
  5. Some edge case clarifications: The servlet 2.4 specification says that before calling request.getReader() we  must call request.setCharacterEncoding().  However there is no such clarification given why it is so.

Advantages of Java Servlets




  1. Portability
  2. Powerful
  3. Efficiency
  4. Safety
  5. Integration
  6. Extensibilty
  7. Inexpensive
Each of the points are defined below:
PortabilityAs we know that the servlets are written in java and follow well known standardized APIs 
so they are highly portable across operating systems and server implementations.  We can develop a
 servlet on Windows machine running the tomcat server or any other server and later we can deploy that 
servlet effortlessly on any other operating system like Unix server running on the iPlanet/Netscape 
Application server. So servlets are write once, run anywhere (WORA)program.
PowerfulWe can do several things with the servlets which were difficult or even impossible to do with
 CGI, for example the servlets can talk directly to the web server while the CGI programs can't do. 
Servlets can share data among each other, they even make the database connection pools easy to 
implement. They can maintain the session by using the session tracking mechanism which helps them
 to maintain information from request to  request. It can do many other things which are difficult to 
implement in the CGI programs.
EfficiencyAs compared to CGI the servlets invocation is highly efficient. When the servlet get loaded 
in the server, it remains in the server's memory as a single object instance. However with servlets there 
are N threads but only a single copy of the servlet class. Multiple concurrent requests are handled by 
separate threads so we can say that the servlets are highly scalable. 
SafetyAs servlets are written in java, servlets inherit the strong type safety of java language. Java's
 automatic garbage collection and a lack of pointers means that servlets are generally safe from memory
 management problems. In servlets we can easily handle the errors due to  Java's exception handling 
mechanism. If any exception occurs then it will throw an exception.
Integration
Servlets are tightly integrated with the server. Servlet can use the server to translate the file paths, 
perform logging, check authorization, and MIME type mapping etc.
ExtensibilityThe servlet API is designed in such a way that it can be easily extensible. As it stands 
today, the servlet API support Http Servlets, but in later date it can be extended for another type of servlets.
InexpensiveThere are number of  free web servers available for personal use or for commercial purpose.
 Web servers are relatively expensive. So by using the free available web servers you can add servlet 
support to it.


Advantages of Servlets over CGI




Servlets are server side components that provides a powerful mechanism for developing server web 
applications for server side. Earlier CGI was developed to provide server side capabilities to the web
 applications. Although CGI played a major role in the explosion of the Internet, its performance, 
scalability and reusability issues make it less than optimal solutions. Java Servlets changes all that.
 Built from ground up using Sun's write once run anywhere technology java servlets provide excellent
 framework for server side processing.
Using servlets web developers can create fast and efficient server side applications and can run it on
 any servlet enabled web server. Servlet runs entirely inside the Java Virtual Machine. Since the servlet 
runs on server side so it does not depend on browser compatibility.
Servlets have a number of advantages over CGI and other API's. They are:
  1. Platform Independence
    Servlets are written entirely in java so these are platform independent. Servlets can run on any 
  2. Servlet enabled web server. For example if you develop an web application in windows machine
  3.  running Java web server, you can easily run the same on apache web server 
  4. (if Apache Serve is installed) without modification or compilation of code. Platform independency 
  5. of servlets provide a great advantages over alternatives of servlets.
  6. Performance
    Due to interpreted nature of java, programs written in java are slow. But the java servlets runs 
  7. very fast. These are due to the way servlets run on web server. For any program initialization 
  8. takes significant amount of time. But in case of servlets initialization takes place first time it 
  9. receives a request and remains in memory till times out or server shut downs. After servlet is 
  10. loaded, to handle a new request it simply creates a new thread and runs service method of 
  11. servlet. In comparison to traditional CGI scripts which creates a new process to serve the request. 
  12. Extensibility
    Java Servlets are developed in java which is robust, well-designed and object oriented language 
  13. which can be extended or polymorphed into new objects. So the java servlets take all these 
  14. advantages and can be extended from existing class to provide the ideal solutions.
  15. Safety
    Java provides very good safety features like memory management, exception handling etc. 
  16. Servlets inherits all these features and emerged as a very powerful web server extension.
  17. Secure
    Servlets are server side components, so it inherits the security provided by the web server. 
  18. Servlets are also benefited with Java Security Manager

JAVA SERVLETS – An Overview


This tutorial covers concepts pertaining to Server side programming in general and Java Servlets 
in particular. In addition, it demonstrates as how to create and compile a simple Servlet and finally
 execute it using a Web Server such as an Apache Tomcat Server. It also guides the student through 
a step by step approach as how to install the above Server, configure it and create the relevant folders
 and files. After completion of this tutorial, the student would not only be familiar with Servlet fundamentals, 
but also get hands-on experience in terms of creating and executing simple Servlet programs.




Server Side Programming




All of us (or most of us) would have started programming in Java with the ever famous “Hello World!”
 program. If you can recollect, we saved this file with a .java extension and later compiled the program 
using javac and then executed the class file with java. Apart from introducing you to the language basics, 
the point to be noted about this program is that – “It is a client side program”. This means that you write,
compile and also execute the program on a client machine (e.g. Your PC). No doubt, this is the easiest
 and fastest way to write, compile and execute programs. But, it has little practical significance when it 
comes to real world programming.
  1. Why Server Side Programming?
    Though it is technically feasible to implement almost any business logic using client side programs, logically or functionally it carries no ground when it comes to enterprise applications (e.g. banking, air ticketing, e-shopping etc.). To further explain, going by the client side programming logic; a bank having 10,000 customers would mean that each customer should have a copy of the program(s) in his or her PC which translates to 10,000 programs! In addition, there are issues like security, resource pooling, concurrent access and manipulations to the database which simply cannot be handled by client side programs. The answer to most of the issues cited above is – “Server Side Programming”. Figure-1 illustrates Server side architecture in the simplest terms.

  2. Advantages of Server Side Programs
    The list below highlights some of the important advantages of Server Side programs.
    1. All programs reside in one machine called the Server. Any number of remote machines (called clients) can access the server programs.
    2. New functionalities to existing programs can be added at the server side which the clients’ can advantage without having to change anything from their side.
    3. Migrating to newer versions, architectures, design patterns, adding patches, switching to new databases can be done at the server side without having to bother about clients’ hardware or software capabilities.
    4. Issues relating to enterprise applications like resource management, concurrency, session management, security and performance are managed by service side applications.
    5. They are portable and possess the capability to generate dynamic and user-based content (e.g. displaying transaction information of credit card or debit card depending on user’s choice).
    3.
  3. Types of Server Side Programs
    1. Active Server Pages (ASP)
    2. Java Servlets
    3. Java Server Pages (JSPs)
    4. Enterprise Java Beans (EJBs)
    5. PHP
    To summarize, the objective of server side programs is to centrally manage all programs 
    relating to a particular application (e.g. Banking, Insurance, e-shopping, etc). Clients 
    with bare minimum requirement (e.g. Pentium II, Windows XP Professional, MS Internet
     Explorer and an internet connection) can experience the power and performance of a 
    Server (e.g. IBM Mainframe, Unix Server, etc) from a remote location without having to 
    compromise on security or speed. More importantly, server programs are not only 
    portable but also possess the capability to generate dynamic responses based on 
    user’s request.  

    Java Servlets




    Java Servlets are server side Java programs that require either a Web Server or an Application 
    Server for execution. Examples for Web Servers include Apache’s Tomcat Server and Macromedia’s
     JRun. Web Servers include IBM’s Weblogic and BEA’s Websphere server. Examples for other
     Server programs include Java Server Pages (JSPs) and Enterprise Java Beans (EJBs). In 
    the forthcoming sections, we will get acquainted with Servlet fundamentals and other associated information required for creating and executing Java Servlets.
    1. Basic Servlet Structure
               
      As seen earlier, Java servlets are server side programs or to be more specific; web applications that run on servers that comply HTTP protocol. The javax.servlet and javax.servlet.http packages provide the necessary interfaces and classes to work with servlets. Servlets generally extend the HttpServlet class and override the doGet or the doPost methods. In addition, other methods such as init, service and destroy also called as life cycle methods might be used which will be discussed in the following section. The skeleton of a servlet is given in Figure
         





    1. A Servlet’s Life CycleThe first time a servlet is invoked, it is the init method which is called. And remember that this is called only once during the lifetime of a servlet. So, you can put all your initialization code here. This method next calls the service method. The service method in turn calls the doGet or doPost methods (whichever the user has overridden). Finally, the servlet calls the destroy method. It is in a sense equivalent to the finally method. You can reset or close references / connections done earlier in the servlet’s methods (e.g. init, service or doGet /doPost). After this method is called, the servlet ceases to exist for all practical purposes. However, please note that it is not mandatory to override all these methods. More often than not, it is the doGet or doPost method used with one or more of the other life cycle methods.

    2. A Servlet Program


                             



      Output Screens
      To appreciate the execution of the servlet life cycle methods, keep refreshing the browser (F5 in Windows). In the background, what actually happens is – with each refresh, the doGet method is called which increments i’s value and displays the current value. Find below the screen shots (Figures 5 through 7) captured at random intervals. The procedure to run the servlets using a Web Server will be demonstrated in the next section (1.3.).














    Installation, Configuration and running Servlets




    In this section, we will see as how to install a WebServer, configure it and finally run servlets
     using this server. Throughout this tutorial, we will be using Apache’s Tomcat server as the 
    WebServer. Tomcat is not only an open and free server, but also the most preferred WebServer
     across the world. A few reasons we can attribute for its popularity is – Easy to install and configure, 
    very less memory footprint, fast, powerful and portable. It is the ideal server for learning purpose. 



    1. Installation of Tomcat Server and JDK
      As mentioned earlier, Apache’s Tomcat Server is free software available for download @www.apache.org. The current version of Tomcat Server is 6.0 (as of November 2007). This Server supports Java Servlets 2.5 and Java Server Pages (JSPs) 2.1 specifications. In case of doubt or confusion, you can refer to the abundant documentation repository available on this site.

      Important software required for running this server is Sun’s JDK (Java Development Kit) and JRE (Java Runtime Environment). The current version of JDK is 6.0. Like Tomcat, JDK is also free and is available for download at www.java.sun.com.
    2. Configuring Tomcat Server
      • Set JAVA_HOME variable - You have to set this variable which points to the base installation directory of JDK installation. (e.g. c:\program file\java\jdk1.6.0). You can either set this from the command prompt or from My Computer -> Properties -> Advanced -> Environment Variables.
      • Specify the Server Port – You can change the server port from 8080 to 80 (if you wish to) by editing the server.xml file in the conf folder. The path would be something like this – c:\program files\apache software foundation\tomcat6\conf\server.xml
    3. Run Tomcat Server
      Once the above pre-requisites are taken care, you can test as whether the server is successfully installed as follows:

      Step 1

      • Go to C:\Program Files\Apache Software Foundation\Tomcat 6.0\bin and double click on tomcat6

      OR

      • Go to Start->Programs->Apache Tomcat 6.0 -> Monitor Tomcat. You will notice an icon appear on the right side of your Status Bar.   Right click on this icon and click on Start service.

      Step 2

      • Open your Browser (e.g. MS Internet Explorer) and type the following URL :

      http://localhost/ (If you have changed to port # to 80)

      OR

      • Open your Browser (e.g. MS Internet Explorer) and type the following URL :

      http://localhost:8080/ (If you have NOT changed the default port #)

      In either case, you should get a page similar to the one in Figure-8 which signifies that the Tomcat Server is successfully running on your machine.
    4. Compile and Execute your Servlet
      This section through a step by step (and illustration) approach explains as how to compile and then run a servlet using Tomcat Server. Though this explanation is specific to Tomcat, the procedure explained holds true for other Web servers too (e.g. JRun, Caucho’s Resin).

      Step 1 – Compile your servlet program
      The first step is to compile your servlet program. The procedure is no different from that of writing and compiling a java program. But, the point to be noted is that neither the javax.servlet.* nor the javax.servlet.http.* is part of the standard JDK. It has to be exclusively added in the CLASSPATH. The set of classes required for writing servlets is available in a jar file called servlet-api.jar. This jar file can be downloaded from several sources. However, the easiest one is to use this jar file available with the Tomcat server (C:\Program Files\Apache Software Foundation\Tomcat 6.0\lib\servlet-api.jar). You need to include this path in CLASSPATH. Once you have done this, you will be able to successfully compile your servlet program. Ensure that the class file is created successfully.

      Step 2 – Create your Web application folder
      The next step is to create your web application folder. The name of the folder can be any valid and logical name that represents your application (e.g. bank_apps, airline_tickets_booking, shopping_cart,etc). But the most important criterion is that this folder should be created under webapps folder. The path would be similar or close to this - C:\Program Files\Apache Software Foundation\Tomcat 6.0\webapps. For demo purpose, let us create a folder called demo-examples under the webapps folder. 
    5. Step 3 – Create the WEB-INF folder

      The third step is to create the WEB-INF folder. This folder should be created under your web application folder that you created in the previous step. Figure-10 shows the WEB-INF folder being placed under the demo-examples folder.


      Step 4 – Create the web.xml file and the classes folder

      The fourth step is to create the web.xml file and the classes folder. Ensure that the web.xml and classes folder are created under the WEB-INF folder. Figure-11 shows this file and folder being placed under the WEB-INF folder.


      Note – Instead of creating the web.xml file an easy way would be to copy an existing web.xml file (e.g. C:\Program Files\Apache Software Foundation\Tomcat 6.0\webapps\examples\WEB-INF) and paste it into this folder. You can later edit this file and add relevant information to your web application.

      Step 5 – Copy the servlet class to the classes folder

      We need to copy the servlet class file to the classes folder in order to run the servlet that we created. All you need to do is copy the servlet class file (the file we obtained from Step 1) to this folder.

      Step 6 – Edit web.xml to include servlet’s name and url pattern

      This step involves two actions viz. including the servlet’s name and then mentioning the url pattern. Let us first see as how to include the servlet’s name in the web.xml file. Open the web.xml file and include the servlet’s name as shown in Figure-13.


      Figure– Include servlet’s name using the <servlet> </servlet> tag

      Note – The servlet-name need not be the same as that of the class name. You can give a different name (or alias) to the actual servlet. This is one of the main reasons as why this tag is used for.

      Next, include the url pattern using the <servlet-mapping> </servlet-mapping> tag. The url pattern defines as how a user can access the servlet from the browser. Figure-14 shows the url pattern entry for our current servlet.


      Figure – Include url-pattern using the <servlet-mapping> </servlet-mapping> tag

      Note – Please remember that the path given in the url-pattern is a relative path. This means that this path is w.r.t. your web applications folder (demo-examples in this case).

      Step 7 – Run Tomcat server and then execute your Servlet
      This step again involves two actions viz. running the Web Server and then executing the servlet. To run the server, follow the steps explained in Section 1.3.3.

      After ensuring that the web server is running successfully, you can run your servlet. To do this, open your web browser and enter the url as specified in the web.xml file. The complete url that needs to be entered in the browser is:

      http://localhost/demo-examples/servlet_lifecycle


      Figure – Our servlet’s output!

      Eureka! Here’s the output of our first servlet. After a long and pain staking effort, we finally got an output! As mentioned in Section 1.2.3. you can keep refreshing the browser window and see for yourself as how i value is incremented (a proof that the doGet is called every time you re-invoke a servlet).


    Running Servlet



    To run a servlet one should follow the steps illustrated below: 
    • Download and Install the tomcat server:Install the tomcat server in a directory in which you want to install and set the classpath.for the variable JAVA_HOME in the environment variable. To get details about the installation process and setting the classpath click the link Tomcat installation.
    • Set the class for the jar file: Set the classpath of the servlet-api.jar file in the variable CLASSPATH inside the environment variable by using the following steps.

      For Windows XP, 
      Go to Start->Control Panel->System->Advanced->Environment Variables->New button and Set the values as
      Variable Name:  CLASSPATH 
      Variable Value:  C:\Program Files\Java\Tomcat 6.0\lib\servlet-api.jar

      For Windows 2000 and NT
      Go to Start->Settings->Control Panel->System->Environment Variables->New button and Set the values as
      Variable Name:  CLASSPATH
      Variable Value:        C:\Program Files\Java\Tomcat 6.0\lib\servlet-api.jar
    • Create a java source file and a web.xml file in a directory structure. 
    • Compile the java source file, put the compiled file (.class file) in the classes folder of your application and deploy the directory of your application in the webapps folder inside the tomcat directory.
    • Start the tomcat server, open a browser window and type the URLhttp://localhost:8080/directory (folder name of your application) name/servler nameand press enter.

      If everything is correct your servlet will run.


    Let Starts With Hello World



    We should start understanding the servlets from the beginning. Lets start by making one program
     which will just print the "Hello World" on the browser. Each time the user visits this page it will 
    display "Hello World" to the user.
    As we know that the our servlet extends the HttpServlet and overrides thedoGet() method which
     it inherits from the HttpServlet class. The server invokes doGet() method  whenever web
     server recieves the GET request from the servlet. The doGet() method takes two arguments
     first is HttpServletRequest object and the second o
    ne is HttpServletResponse object and this method throws the ServletException.
    Whenever the user sends the request to the server then server generates two obects, first is
     HttpServletRequest object and the second one is HttpServletResponse object.
     HttpServletRequest object represents the client's request and the HttpServletResponse represents
     the servlet's response. 
    Inside the doGet(() method our servlet has first used the setContentType() method of the response 
    object which sets the content type of the response to text/html. It is the standard MIME content 
    type for the Html pages. After that it has used the method getWriter() of the response object
     to retrieve a PrintWriter object.  To display the output on the browser we use the println()
     method of the PrintWriter class. 
    The code the program is given below:
    import java.io.*;
    import javax.servlet.*;
    import javax.servlet.http.*;

    public class HelloWorld extends HttpServlet{ 
      public void doGet(HttpServletRequest request, HttpServletResponse response)
                                       throws ServletException,IOException{
        response.setContentType("text/html");
        PrintWriter pw = response.getWriter();
        pw.println("<html>");
        pw.println("<head><title>Hello World</title></title>");
        pw.println("<body>");
        pw.println("<h1>Hello World</h1>");
        pw.println("</body></html>");
      }
    }
    web.xml file for this program:
    <?xml version="1.0" encoding="ISO-8859-1"?>
    <!--<!DOCTYPE web-app
     PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
     "http://java.sun.com/dtd/web-app_2_3.dtd"> -->

    <web-app>
     <servlet>
      <servlet-name>Hello</servlet-name>
      <servlet-class>HelloWorld</servlet-class>
     </servlet>
     <servlet-mapping>
     <servlet-name>Hello</servlet-name>
     <url-pattern>/HelloWorld</url-pattern>
     </servlet-mapping>
    </web-app>
    The output of the program is given below:







    Displaying Date in Servlet




    In this example we are going to show how we can display a current date and time on our browser.
     It is very easy to display it on our browser by using the Date class of thejava.util package. 
    As we know that the our servlet extends the HttpServlet and overrides thedoGet() method which 
    it inherits from the HttpServlet class. The server invokes doGet() method  whenever web server
     recieves the GET request from the servlet. The doGet() method takes two arguments first is
     HttpServletRequest object and the second one is HttpServletResponse object and this method 
    throws the ServletException.
    Whenever the user sends the request to the server then server generates two obects, first is 
    HttpServletRequest object and the second one is HttpServletResponse object. 
    HttpServletRequest object represents the client's request and the HttpServletResponse represents 
    the servlet's response. 
    Inside the doGet(() method our servlet has first used the setContentType() method of the response
     object which sets the content type of the response to text/html. It is the standard MIME 
    content type for the Html pages. The MIME type tells the browser what kind of data the browser 
    is about to receive. After that it has used the method getWriter() of the response object to retrieve 
    a PrintWriter object.  To display the output on the browser we use the println()method of the 
    PrintWriter class. 
    The code the program is given below:
    import java.io.*;
    import java.util.*;
    import javax.servlet.*;
    import javax.servlet.http.*;

    public class DisplayingDate extends HttpServlet{         public void doGet(HttpServletRequest request, HttpServletResponse
                        response    ) throws ServletException, IOException{
        PrintWriter pw = response.getWriter();
        Date today = new Date();
        pw.println("<html>"+"<body><h1>Today Date is</h1>");
        pw.println("<b>"+ today+"</b></body>"+ "</html>");
      }
    }
    XML File for this program
    <?xml version="1.0" encoding="ISO-8859-1"?>
    <!--<!DOCTYPE web-app
     PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
     "http://java.sun.com/dtd/web-app_2_3.dtd"> -->

    <web-app>
     <servlet>
      <servlet-name>Hello</servlet-name>
      <servlet-class>DateDisplay</servlet-class>
     </servlet>
     <servlet-mapping>
     <servlet-name>Hello</servlet-name>
     <url-pattern>/DateDisplay</url-pattern>
     </servlet-mapping>
    </web-app>
     The output of the program is given below:



    Simple Counter in Servlet




    In this example we are going to know how we can make a program on counter which will keep
     track how many times the servlet has been accessed. 
    To make this program firstly we have to make one classSimpleCounterInServlet. The name of 
    the class should follow the  naming convention. Remember to keep the name of the class in such a 
    way that it becomes easy to understand what the program is going to do just by seeing the class 
    name. After making a class define one variable counter which will keep record for how many times
    the servlet has been accessed. Now use method either doGet() or doPost() to write a logic of the 
    program. Our program logic is simple. We have to just increment the value of the counter by 1. To
     display the output use the method getWriter() method of the response object which will in turn 
    return the object  of the PrintWriter class. Now display the value of the counter. 
    The code of the program is given below:
    import java.io.*;
    import javax.servlet.*;
    import javax.servlet.http.*;

    public class SimpleCounter extends HttpServlet{
      int counter = 0;
      public void doGet(HttpServletRequest request, HttpServletResponse response)
                                throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter pw = response.getWriter();
        counter++;
        pw.println("At present the value of the counter is " + counter);
      }
    }
    The output of the program is given below:






    A Holistic counter in Servlet




    In this program we are going to make a such a servlet which will count the number it has been 
    accessed and the number of threads created by the server.
    In this example firstly we are going to create one class named as HolisticCounterInServlet. Now 
    declare a variable counter of int with initial value 0, the value of this counter will be different for
     each servlet and create a Hashtableobject. This object will be shared by all the threads in the 
    container. Inside the doGet() method use the method getWriter() method of the response object
     which will return the PrintWriter object. 
    The code of the program is given below:
    import java.io.*;
    import java.io.IOException;
    import java.util.*;
    import javax.servlet.*;
    import javax.servlet.http.*;

    public class HolisticCounter extends HttpServlet{
    int counter = 0; //separate For Each Servlet
    static Hashtable hashTable = new Hashtable(); //Shared by all the threads 

    public void doGet(HttpServletRequest request, HttpServletResponse response)
                                       throws ServletException, IOException {
      response.setContentType("text/html");
      PrintWriter pw = response.getWriter();
      counter++;
      pw.println("This servlet has been accessed" + counter + "times<br>");
      hashTable.put(this,this);
      pw.println("There are currently" + hashTable.size() + "threads<br>");
      }
    }
    The output of the program is given below:




    Counter in Init() Method




    In this program we are going to make a such a servlet which will count and displays the number of
     times it has been accessed and by reading the init parameter to know from where the counting will
     begin.
    In this program we are going to make use of the init method of the Servlet interface which takes 
    one argument of ServletConfig. Firstly declare a variable counter which will have the initial value 
    of the counter. The init() method accepts an object which implements ServletConfig interface. It
     uses the method getInitParameter() method of  the ServletConfig interface to the value of the init 
    parameter initial which we have defined in the deployment descriptor file. You need to parse the 
    String value which you will get from the getInitParameter() method to a Integer. 
    The code of the program is given below: 
    import java.io.*;
    import javax.servlet.*;
    import javax.servlet.http.*;

    public class CounterInInit extends HttpServlet {
      int counter;
      public void init(ServletConfig config) throws ServletException{
        super.init(config);
      String initValue = config.getInitParameter("initial");
      try{
        counter = Integer.parseInt(initValue);
      }
      catch(NumberFormatException e){
        counter = 0;
      }
      }
      protected void doGet(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {response.setContentType("text/html");
      PrintWriter pw = response.getWriter();
      counter++;
      pw.println("Since loading this servlet has been accessed" + counter + "times");
      }
    }
    web.xml file for this program:
    <?xml version="1.0" encoding="ISO-8859-1"?>
    <!--<!DOCTYPE web-app
     PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
     "http://java.sun.com/dtd/web-app_2_3.dtd"> -->

    <web-app>
     <servlet>
      <servlet-name>Hello</servlet-name>
      <servlet-class>CounterInInit</servlet-class>
     </servlet>
     <servlet-mapping>
     <servlet-name>Hello</servlet-name>
     <url-pattern>/CounterInInit</url-pattern>
     </servlet-mapping>
    </web-app>
    The output of the program is given below:




    Snooping the server




    In this program we are going to tell you how can a use servlet to display information about its server. 
    Firstly we will create a class in which there will be doGet() method which takes two objects as
     arguments, first is request object and the second one is of response. 
    To display the name of the server you are using use the method getServerName() of the
    ServletRequest interface. To display the server port number use the method getServerPort(). 
    You can also use other methods of theServletRequest interface like getProtocol() to display 
    the protocol you are using and many more methods depending on your needs. 
    The code of the program is given below: 
    import java.io.*;
    import javax.servlet.*;
    import javax.servlet.http.*;

    public class SnoopingServerServlet extends HttpServlet{
      protected void doGet(HttpServletRequest request, HttpServletResponse response) 
                                      throws     ServletException, IOException {
        PrintWriter pw = response.getWriter();
        pw.println("The server name is " + request.getServerName() + "<br>");
        pw.println("The server port number is " + request.getServerPort()+ "<br>");
        pw.println("The protocol is " + request.getProtocol()+ "<br>");
        pw.println("The scheme used is " + request.getScheme());
      }
    }
    web.xml file for this program:
    <?xml version="1.0" encoding="ISO-8859-1"?>
    <!--<!DOCTYPE web-app
     PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
     "http://java.sun.com/dtd/web-app_2_3.dtd"> -->

    <web-app>
     <servlet>
      <servlet-name>Hello</servlet-name>
      <servlet-class>SnoopingServerServlet</servlet-class>
     </servlet>
     <servlet-mapping>
     <servlet-name>Hello</servlet-name>
     <url-pattern>/SnoopingServerServlet</url-pattern>
     </servlet-mapping>
    </web-app>
    The output of the program is given below:



    Snooping Headers




    In this program we are going to going to make a servlet which will retrieve all the Http request header. 
    To make a program over this firstly we need to make one class namedGettingSnoopingHeader. 
    In HttpRequest there are too many headers. To retrieve all the headers firstly we need to call the 
    getWriter() which returns PrintWriter object and helps us to display all the headers. To get a
     header names call the method getHeaderNames() of the request object which will return the 
    Enumeration of the headers. Now to retrieve all the headers from the Enumeration use the method 
    hasMoreElements(). This method checks whether there are more headers or not. To display the 
    output on your browser use the PrintWriter object.
    The code of the program is given below:
    import java.io.*;
    import java.util.*;
    import javax.servlet.*;
    import javax.servlet.http.*;


    public class HeaderSnoopServlet extends HttpServlet{
      protected void doGet(HttpServletRequest request, HttpServletResponse response)
                                       throws ServletException, IOException {
        PrintWriter pw = response.getWriter();
        pw.println("Request Headers are");
        Enumeration enumeration = request.getHeaderNames();
        while(enumeration.hasMoreElements()){
          String headerName = (String)enumeration.nextElement();
          Enumeration headerValues = request.getHeaders(headerName);
          if (headerValues != null){
            while (headerValues.hasMoreElements()){
              String values = (String) headerValues.nextElement();
              pw.println(headerName + ": " + values);
            }
          }
        }
      }
    }
    web.xml file for this program:
    <?xml version="1.0" encoding="ISO-8859-1"?>
    <!--<!DOCTYPE web-app
     PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
     "http://java.sun.com/dtd/web-app_2_3.dtd"> -->

    <web-app>
     <servlet>
      <servlet-name>Hello</servlet-name>
      <servlet-class>HeaderSnoopServlet</servlet-class>
     </servlet>
     <servlet-mapping>
     <servlet-name>Hello</servlet-name>
     <url-pattern>/HeaderSnoopServlet</url-pattern>
     </servlet-mapping>
    </web-app>
    The output of the program is given below:




       

      Dice Roller




We are going to make one program on the dice roller in which the number in the dice will be selected
 randomly. 
To make a program over this firstly we need to make a class DiceRoller in which we will have a doGet()
 method in which we will have our application logic. To make the dice working randomly use the random() 
method of the classjava.lang.Math. To print the number on the browser call the method getWriter() 
of the response object which will return thePrintWriter object. Now by the object of the PrintWriter 
class print the values of the dice on the browser. 
The code of the program is given below:
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class DiceRollerServlet extends HttpServlet{
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException{
  PrintWriter pw = response.getWriter();
  String dice1 = Integer.toString((int)(Math.random()*6)+1);
  String dice2 = Integer.toString((int)(Math.random()*6)+1);
  pw.println("<html><body>");
  pw.println("dice roller<br>");
  pw.println("dice1 value is " + dice1 + " and <br>dice2 value is " +dice2);
}
}
XML File for this program:
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE web-app
 PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>
 <servlet>
 <servlet-name>Zulfiqar</servlet-name>
 <servlet-class>DiceRollerServlet</servlet-class>
 </servlet>
 <servlet-mapping>
 <servlet-name>Zulfiqar</servlet-name>
 <url-pattern>/DiceRollerServlet</url-pattern>
 </servlet-mapping>
</web-app>
The output of the program is given below:


Getting Init Parameter Names







Passing Parameter Using Html Form




This is a very simple example in which we are going to display the name on the browser which we have 
entered from the Html page. 
To get the desired result firstly we have to make one html form which will have only one field named as 
name in which we will enter the name. And we will also have one submit button, on pressing the submit 
button the request will go to the server and the result will be displayed to us. 
In the servlet which will work as a controller here picks the value from the html page by using the methodgetParameter().  The output will be displayed to you by the object of the PrintWriter class. 
The code of the program is given below: 
<html>

<head>
<title>New Page 1</title>
</head>

<body>

<h2>Login</h2>
<p>Please enter your username and password</p>
<form method="GET" action="/htmlform/LoginServlet">
  <p> Username  <input type="text" name="username" size="20"></p>
  <p> Password  <input type="text" name="password" size="20"></p>
  <p><input type="submit" value="Submit" name="B1"></p>
</form>
<p>&nbsp;</p>

</body>

</html>
LoginServlet.java
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class LoginServlet extends HttpServlet{
  public void doGet(HttpServletRequest request, HttpServletResponse response)
                                   throws ServletException, IOException {
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    String name = request.getParameter("username");
    String pass = request.getParameter("password");
    out.println("<html>");
    out.println("<body>");
    out.println("Thanks  Mr." + "  " + name + "  " + "for visiting roseindia<br>" );
    out.println("Now you can see your password : " + "  " + pass + "<br>");
    out.println("</body></html>");
  }
}
 web.xml file for this program:
<?xml version="1.0" encoding="ISO-8859-1"?>
<!--<!DOCTYPE web-app
 PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd"> -->

<web-app>
 <servlet>
  <servlet-name>Hello</servlet-name>
  <servlet-class>LoginServlet</servlet-class>
 </servlet>
 <servlet-mapping>
 <servlet-name>Hello</servlet-name>
 <url-pattern>/LoginServlet</url-pattern>
 </servlet-mapping>
</web-app>
 The output of the program is given below:





Time Updater in Servlet




In this program we are going to make one program on servlet which will keep on updating the time in every second and the result will be displayed to you. 
To make this servlet firstly we need to make a class named TimeUpdater.  The name of the class should
 be such that it becomes easy to understand what the program is going to do. Call the methodgetWriter() method of the responseobject which will return a PrintWriter object. Use the method getHeader() of the response object to add a new header. We can also use setHeader() in place of getHeader(). The
 setHeader() method overrides the previous set header. Now by using the PrintWriter object display
 the result on the browser.
The code of the program is given below:

import java.io.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class TimeUpdater extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
PrintWriter pw = response.getWriter();

response.addHeader("Refresh", "1");
pw.println(new Date().toString());
}
}








Posted by Prasobh.K at 05:43

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