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.

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.

Disadvantages of CGI

There are many problems in CGI technology:

If number of clients increases, it takes more time for sending response.
For each request, it starts a process and Web server is limited to start processes.
It uses platform dependent language e.g. C, C++, perl.

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:

better performance: because it creates a thread for each request not process.
Portability: because it uses java language.
Robust: Servlets are managed by JVM so no need to worry about momory leak, garbage collection etc.
Secure: because it uses java language..

Servlet Terminology

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:

HTTP
HTTP Request Types
Difference between Get and Post method
Container
Server and Difference between web server and application server
Content Type
Introduction of XML
Deployment

HTTP (Hyper Text Transfer Protocol)

Http is the protocol that allows web servers and browsers to exchange data over the web.
It is a request response protocol.
Http uses reliable TCP connections bydefault on TCP port 80.
It is stateless means each request is considered as the new request. In other words, server doesn't recognize the user bydefault.

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 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.

Container

It provides runtime environment for JavaEE (j2ee) applications.

It performs many operations that are given below:

Life Cycle Management
Multithreaded support
Object Pooling
Security etc.

Server

It is a running program or software that provides services.

There are two types of servers:

Web Server
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:

JBoss Open-source server from JBoss community.
Glassfish provided by Sun Microsystem. Now acquired by Oracle.
Weblogic provided by Oracle. It more secured.
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

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:

Servlet
ServletRequest
ServletResponse
RequestDispatcher
ServletConfig
ServletContext
SingleThreadModel
Filter
FilterConfig
FilterChain
ServletRequestListener
ServletRequestAttributeListener
ServletContextListener
ServletContextAttributeListener

Classes in javax.servlet package

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

GenericServlet
ServletInputStream
ServletOutputStream
ServletRequestWrapper
ServletResponseWrapper
ServletRequestEvent
ServletContextEvent
ServletRequestAttributeEvent
ServletContextAttributeEvent
ServletException
UnavailableException

Interfaces in javax.servlet.http package

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

HttpServletRequest
HttpServletResponse
HttpSession
HttpSessionListener
HttpSessionAttributeListener
HttpSessionBindingListener
HttpSessionActivationListener
HttpSessionContext (deprecated now)

Classes in javax.servlet.http package

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

HttpServlet
Cookie
HttpServletRequestWrapper
HttpServletResponseWrapper
HttpSessionEvent
HttpSessionBindingEvent
HttpUtils (deprecated now)

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

Let's see the simple example of servlet by implementing the servlet interface.

It will be better if you learn it after visiting steps to create a servlet.

File: First.java

1. import java.io.*;

2. import javax.servlet.*;

4. public class First implements Servlet{

5. ServletConfig config=null;

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("<html><body>");

19. out.print("<b>hello simple servlet</b>");

20. out.print("</body></html>");

21.

22. }

23. public void destroy(){System.out.println("servlet is destroyed");}

24. public ServletConfig getServletConfig(){return config;}

26.

27. }

download example of servlet by implementing Servlet interface

GenericServlet class

GenericServlet class implements Servlet, ServletConfig and Serializable interfaces. It provides the implementaion of all the methods of these interfaces except the service method.

GenericServlet class can handle any type of request so it is protocol-independent.

You may create a generic servlet by inheriting the GenericServlet class and providing the implementation of the service method.

Methods of GenericServlet class

There are many methods in GenericServlet class. They are as follows:

public void init(ServletConfig config) is used to initialize the servlet.
public abstract void service(ServletRequest request, ServletResponse response) provides service for the incoming request. It is invoked at each time when user requests for a servlet.
public void destroy() is invoked only once throughout the life cycle 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.
public void init() it is a convenient method for the servlet programmers, now there is no need to call super.init(config)
public ServletContext getServletContext() returns the object of ServletContext.
public String getInitParameter(String name) returns the parameter value for the given parameter name.
public Enumeration getInitParameterNames() returns all the parameters defined in the web.xml file.
public String getServletName() returns the name of the servlet object.
public void log(String msg) writes the given message in the servlet log file.
public void log(String msg,Throwable t) writes the explanatory message in the servlet log file and a stack trace.

Servlet Example by inheriting the GenericServlet class

Let's see the simple example of servlet by inheriting the GenericServlet class.

It will be better if you learn it after visiting steps to create a servlet.

File: First.java

1. import java.io.*;

2. import javax.servlet.*;

4. public class First extends GenericServlet{

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

6. throws IOException,ServletException{

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

10. PrintWriter out=res.getWriter();

11. out.print("<html><body>");

12. out.print("<b>hello generic servlet</b>");

13. out.print("</body></html>");

14.

15. }

16. }

HttpServlet class

The HttpServlet class extends the GenericServlet class and implements Serializable interface. It provides http specific methods such as doGet, doPost, doHead, doTrace etc.

Methods of HttpServlet class

There are many methods in HttpServlet class. They are as follows:

public void service(ServletRequest req,ServletResponse res) dispatches the request to the protected service method by converting the request and response object into http type.
protected void service(HttpServletRequest req, HttpServletResponse res) receives the request from the service method, and dispatches the request to the doXXX() method depending on the incoming http request type.
protected void doGet(HttpServletRequest req, HttpServletResponse res) handles the GET request. It is invoked by the web container.
protected void doPost(HttpServletRequest req, HttpServletResponse res) handles the POST request. It is invoked by the web container.
protected void doHead(HttpServletRequest req, HttpServletResponse res) handles the HEAD request. It is invoked by the web container.
protected void doOptions(HttpServletRequest req, HttpServletResponse res) handles the OPTIONS request. It is invoked by the web container.
protected void doPut(HttpServletRequest req, HttpServletResponse res) handles the PUT request. It is invoked by the web container.
protected void doTrace(HttpServletRequest req, HttpServletResponse res) handles the TRACE request. It is invoked by the web container.
protected void doDelete(HttpServletRequest req, HttpServletResponse res) handles the DELETE request. It is invoked by the web container.
protected long getLastModified(HttpServletRequest req) returns the time when HttpServletRequest was last modified since midnight January 1, 1970 GMT.

Life Cycle of a Servlet (Servlet Life Cycle)

Life Cycle of a Servlet

1. Servlet class is loaded

2. Servlet instance is created

3. init method is invoked

4. service method is invoked

5. destroy method is invoked

The web container maintains the life cycle of a servlet instance. Let's see the life cycle of the servlet:

Servlet class is loaded.
Servlet instance is created.
init method is invoked.
service method is invoked.
destroy method is invoked.

As displayed in the above diagram, there are three states of a servlet: new, ready and end. The servlet is in new state if servlet instance is created. After invoking the init() method, Servlet comes in the ready state. In the ready state, servlet performs all the tasks. When the web container invokes the destroy() method, it shifts to the end state.

1) Servlet class is loaded

The classloader is responsible to load the servlet class. The servlet class is loaded when the first request for the servlet is received by the web container.

2) Servlet instance is created

The web container creates the instance of a servlet after loading the servlet class. The servlet instance is created only once in the servlet life cycle.

3) init method is invoked

The web container calls the init method only once after creating the servlet instance. The init method is used to initialize the servlet. It is the life cycle method of the javax.servlet.Servlet interface. Syntax of the init method is given below:

1. public void init(ServletConfig config) throws ServletException

4) service method is invoked

The web container calls the service method each time when request for the servlet is received. If servlet is not initialized, it follows the first three steps as described above then calls the service method. If servlet is initialized, it calls the service method. Notice that servlet is initialized only once. The syntax of the service method of the Servlet interface is given below:

1. public void service(ServletRequest request, ServletResponse response)

2. throws ServletException, IOException

5) destroy method is invoked

The web container calls the destroy method before removing the servlet instance from the service. It gives the servlet an opportunity to clean up any resource for example memory, thread etc. The syntax of the destroy method of the Servlet interface is given below:

1. public void destroy()

Steps to create a servlet example

Steps to create the servlet using Tomcat server

1. Create a directory structure

2. Create a Servlet

3. Compile the Servlet

4. Create a deployment descriptor

5. Start the server and deploy the application

There are given 6 steps to create a servlet example. These steps are required for all the servers.

The servlet example can be created by three ways:

By implementing Servlet interface,
By inheriting GenericServlet class, (or)
By inheriting HttpServlet class

The mostly used approach is by extending HttpServlet because it provides http request specific method such as doGet(), doPost(), doHead() etc.

Here, we are going to use apache tomcat server in this example. The steps are as follows:

Create a directory structure
Create a Servlet
Compile the Servlet
Create a deployment descriptor
Start the server and deploy the project
Access the servlet

download this example of servlet
download example of servlet by extending GenericServlet
download example of servlet by implementing Servlet interface

1)Create a directory structures

The directory structure defines that where to put the different types of files so that web container may get the information and respond to the client.

The Sun Microsystem defines a unique standard to be followed by all the server vendors. Let's see the directory structure that must be followed to create the servlet.

As you can see that the servlet class file must be in the classes folder. The web.xml file must be under the WEB-INF folder.

2)Create a Servlet

There are three ways to create the servlet.

1. By implementing the Servlet interface

2. By inheriting the GenericServlet class

3. By inheriting the HttpServlet class

The HttpServlet class is widely used to create the servlet because it provides methods to handle http requests such as doGet(), doPost, doHead() etc.

In this example we are going to create a servlet that extends the HttpServlet class. In this example, we are inheriting the HttpServlet class and providing the implementation of the doGet() method. Notice that get request is the default request.

DemoServlet.java

1. import javax.servlet.http.*;

2. import javax.servlet.*;

3. import java.io.*;

4. public class DemoServlet extends HttpServlet{

5. public void doGet(HttpServletRequest req,HttpServletResponse res)

6. throws ServletException,IOException

7. {

8. res.setContentType("text/html");//setting the content type

9. PrintWriter pw=res.getWriter();//get the stream to write the data

10.

11. //writing html in the stream

12. pw.println("<html><body>");

13. pw.println("Welcome to servlet");

14. pw.println("</body></html>");

15.

16. pw.close();//closing the stream

17. }}

3)Compile the servlet

For compiling the Servlet, jar file is required to be loaded. Different Servers provide different jar files:

Jar file	Server
1) servlet-api.jar	Apache Tomcat
2) weblogic.jar	Weblogic
3) javaee.jar	Glassfish
4) javaee.jar	JBoss

Two ways to load the jar file

set classpath
paste the jar file in JRE/lib/ext folder

Put the java file in any folder. After compiling the java file, paste the class file of servlet in WEB-INF/classes directory.

4)Create the deployment descriptor (web.xml file)

The deployment descriptor is an xml file, from which Web Container gets the information about the servet to be invoked.

The web container uses the Parser to get the information from the web.xml file. There are many xml parsers such as SAX, DOM and Pull.

There are many elements in the web.xml file. Here is given some necessary elements to run the simple servlet program.

web.xml file

1. <web-app>

3. <servlet>

4. <servlet-name>sonoojaiswal</servlet-name>

5. <servlet-class>DemoServlet</servlet-class>

6. </servlet>

8. <servlet-mapping>

9. <servlet-name>sonoojaiswal</servlet-name>

10. <url-pattern>/welcome</url-pattern>

11. </servlet-mapping>

12.

13. </web-app>

Description of the elements of web.xml file

There are too many elements in the web.xml file. Here is the illustration of some elements that is used in the above web.xml file. The elements are as follows:

<web-app> represents the whole application.

<servlet> is sub element of <web-app> and represents the servlet.

<servlet-name> is sub element of <servlet> represents the name of the servlet.

<servlet-class> is sub element of <servlet> represents the class of the servlet.

<servlet-mapping> is sub element of <web-app>. It is used to map the servlet.

<url-pattern> is sub element of <servlet-mapping>. This pattern is used at client side to invoke the servlet.

5)Start the Server and deploy the project

To start Apache Tomcat server, double click on the startup.bat file under apache-tomcat/bin directory.

One Time Configuration for Apache Tomcat Server

You need to perform 2 tasks:

set JAVA_HOME or JRE_HOME in environment variable (It is required to start server).
Change the port number of tomcat (optional). It is required if another server is running on same port (8080).

1) How to set JAVA_HOME in environment variable?

To start Apache Tomcat server JAVA_HOME and JRE_HOME must be set in Environment variables.

Go to My Computer properties -> Click on advanced tab then environment variables -> Click on the new tab of user variable -> Write JAVA_HOME in variable name and paste the path of jdk folder in variable value -> ok -> ok -> ok.

Go to My Computer properties:

Click on advanced system settings tab then environment variables:

Click on the new tab of user variable or system variable:

Write JAVA_HOME in variable name and paste the path of jdk folder in variable value:

There must not be semicolon (;) at the end of the path.

After setting the JAVA_HOME double click on the startup.bat file in apache tomcat/bin.

Note: There are two types of tomcat available:

1. Apache tomcat that needs to extract only (no need to install)

2. Apache tomcat that needs to install

It is the example of apache tomcat that needs to extract only.

Now server is started successfully.

2) How to change port number of apache tomcat

Changing the port number is required if there is another server running on the same system with same port number.Suppose you have installed oracle, you need to change the port number of apache tomcat because both have the default port number 8080.

Open server.xml file in notepad. It is located inside the apache-tomcat/conf directory . Change the Connector port = 8080 and replace 8080 by any four digit number instead of 8080. Let us replace it by 9999 and save this file.

5) How to deploy the servlet project

Copy the project and paste it in the webapps folder under apache tomcat.

But there are several ways to deploy the project. They are as follows:

By copying the context(project) folder into the webapps directory
By copying the war folder into the webapps directory
By selecting the folder path from the server
By selecting the war file from the server

Here, we are using the first approach.

You can also create war file, and paste it inside the webapps directory. To do so, you need to use jar tool to create the war file. Go inside the project directory (before the WEB-INF), then write:

1. projectfolder> jar cvf myproject.war *

Creating war file has an advantage that moving the project from one location to another takes less time.

6) How to access the servlet

Open broser and write http://hostname:portno/contextroot/urlpatternofservlet. For example:

1. http://localhost:9999/demo/welcome

How Servlet works?

It is important to learn how servlet works for understanding the servlet well. Here, we are going to get the internal detail about the first servlet program.

The server checks if the servlet is requested for the first time.

If yes, web container does the following tasks:

loads the servlet class.
instantiates the servlet class.
calls the init method passing the ServletConfig object

else

calls the service method passing request and response objects

The web container calls the destroy method when it needs to remove the servlet such as at time of stopping server or undeploying the project.

How web container handles the servlet request?

The web container is responsible to handle the request. Let's see how it handles the request.

maps the request with the servlet in the web.xml file.
creates request and response objects for this request
calls the service method on the thread
The public service method internally calls the protected service method
The protected service method calls the doGet method depending on the type of request.
The doGet method generates the response and it is passed to the client.
After sending the response, the web container deletes the request and response objects. The thread is contained in the thread pool or deleted depends on the server implementation.

What is written inside the public service method?

The public service method converts the ServletRequest object into the HttpServletRequest type and ServletResponse object into the HttpServletResponse type. Then, calls the service method passing these objects. Let's see the internal code:

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

2. throws ServletException, IOException

3. {

4. HttpServletRequest request;

5. HttpServletResponse response;

6. try

7. {

8. request = (HttpServletRequest)req;

9. response = (HttpServletResponse)res;

10. }

11. catch(ClassCastException e)

12. {

13. throw new ServletException("non-HTTP request or response");

14. }

15. service(request, response);

16. }

What is written inside the protected service method?

The protected service method checks the type of request, if request type is get, it calls doGet method, if request type is post, it calls doPost method, so on. Let's see the internal code:

1. protected void service(HttpServletRequest req, HttpServletResponse resp)

2. throws ServletException, IOException

3. {

4. String method = req.getMethod();

5. if(method.equals("GET"))

6. {

7. long lastModified = getLastModified(req);

8. if(lastModified == -1L)

9. {

10. doGet(req, resp);

11. }

12. ....

13. //rest of the code

14. }

15. }

War File

A war (web archive) File contains files of a web project. It may have servlet, xml, jsp, image, html, css, js etc. files.

Here, we will discuss what is war file, how to create war file, how to deploy war file and how to extract war file.

What is war file?

web archive (war) file contains all the contents of a web application. It reduces the time duration for transferring file.

Advantage of war file

saves time: The war file combines all the files into a single unit. So it takes less time while transferring file from client to server.

How to create war file?

To create war file, you need to use jar tool of JDK. You need to use -c switch of jar, to create the war file.

Go inside the project directory of your project (outside the WEB-INF), then write the following command:

1. jar -cvf projectname.war *

Here, -c is used to create file, -v to generate the verbose output and -f to specify the arhive file name.

The * (asterisk) symbol signifies that all the files of this directory (including sub directory).

How to deploy the war file?

There are two ways to deploy the war file.

By server console panel
By manually having the war file in specific folder of server.

If you want to deploy the war file in apache tomcat server manually, go to the webapps directory of apache tomcat and paste the war file here.

Now, you are able to access the web project through browser.

Note: server will extract the war file internally.

How to extract war file manually?

To extract the war file, you need to use -x switch of jar tool of JDK. Let's see the command to extract the war file.

1. jar -xvf projectname.war

Gurrala Ajay Kumar

Monday, 23 March 2015

Steps to create a servlet example

1)Create a directory structures

2)Create a Servlet

3)Compile the servlet

Two ways to load the jar file

4)Create the deployment descriptor (web.xml file)

Description of the elements of web.xml file

5)Start the Server and deploy the project

One Time Configuration for Apache Tomcat Server

1) How to set JAVA_HOME in environment variable?

2) How to change port number of apache tomcat

5) How to deploy the servlet project

6) How to access the servlet

War File

What is war file?

Advantage of war file

How to create war file?

How to deploy the war file?

Note: server will extract the war file internally.

How to extract war file manually?

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