2. • Servlets are programs that run on a Web server and build
Web pages. Building Web pages on the fly is useful (and
commonly done) for a number of reasons:
• The Web page is based on data submitted by the
user. For example the results pages from search engines
are generated this way, and programs that process
orders for e-commerce sites do this as well.
3. • The data changes frequently. For example, a weather-
report or news headlines page might build the page
dynamically, perhaps returning a previously built page if it
is still up to date.
• The Web page uses information from corporate
databases or other such sources. For example, you
would use this for making a Web page at an on-line store
that lists current prices and number of items in stock.
4. • Java Servlets technology provides an HTTP-
based request and response paradigm on web
servers.
• Java Servlets can handle generic service
requests and respond to the client’s requests.
• Java Servlets are used in embedded systems,
wireless communication, and any other generic
request/response application.
• The Common Gateway Interface (CGI) was a
popular technology used to generate dynamic
HTTP web contents in the mid-1990s.
5.
6. Efficient :With traditional CGI, a new process is started
for each HTTP request. If the CGI program does a
relatively fast operation, the overhead of starting the
process can dominate the execution time. With servlets,
the Java Virtual Machine stays up, and each request is
handled by a lightweight Java thread, not a heavyweight
operating system process. Similarly, in traditional CGI, if
there are N simultaneous request to the same CGI
program, then the code for the CGI program is loaded
into memory N times. With servlets, however, there
are N threads but only a single copy of the servlet class.
7. • Convenient. Hey, you already know Java. Why learn
Perl too? Besides the convenience of being able to use a
familiar language, servlets have an extensive
infrastructure for automatically parsing and decoding
HTML form data, reading and setting HTTP headers,
handling cookies, tracking sessions, and many other
such utilities.
8. • Powerful. Java servlets let you easily do several things
that are difficult or impossible with regular CGI. For one
thing, servlets can talk directly to the Web server (regular
CGI programs can't). This simplifies operations that need
to look up images and other data stored in standard
places. Servlets can also share data among each other,
making useful things like database connection pools easy
to implement.
9. • Portable. Servlets are written in Java and follow a well-
standardized API. Consequently, servlets written for, say I-
Planet Enterprise Server can run virtually unchanged on
Apache, Microsoft IIS, or WebStar. Servlets are supported
directly or via a plugin on almost every major Web server.
• Inexpensive. There are number of inexpensive Web
servers available that are good for "personal" use or low-
volume Web sites.
10.
11.
12. • A Servlet component can delegate the requests to its back-end
tier such as a database management system, RMI, EAI, or other
Enterprise Information System (EIS).
• A Servlet is deployed as a middle tier just like other web
components such as JSP components.
• The Servlet components are building block components, which
always work together with other components such as JSP
components, JavaBean components, Enterprise Java Bean (EJB)
components, and web service components.
• A Servlet component is also a distributed component,
which can provide services to remote clients and also
access remote resources.
13. • A Java Servlet application is supported by its Servlet
container.
• The Apache Tomcat web server is the official reference
implementation of Servlet containers, supporting Servlets
and JSP.
14. • The Java Servlet is a server-side web
component that takes a HTTP request from a
client, handles it, talks to a database, talks to a
JavaBean component, and responds with a
HTTP response or dispatches the request to
other Servlets or JSP components.
• Servlets can dynamically produce text-based
HTML markup contents and binary contents as
well contents based on the client’s request.
15. • A Java Servlet is a typical Java class that extends the
abstract class HttpServlet.
• The HttpServlet class extends another abstract class
called GenericServlet.
• The GenericServlet class implements three interfaces:
javax.servlet.Servlet, javax.servlet.ServletConfig, and
java.io.Serializable.
16.
17. • A Servlet has a lifecycle just like a Java applet. The
lifecycle is managed by the Servlet container.
• Three methods are central to the life cycle of a servlet.
These are init(), service() and destroy().
• They are implemented by every servlet and are invoked
at specific times by te server.
18. • Firstly, let us assume that a user enters a URL to a web
browser. The browser will then generate an HTTP request
for this URL. This request is sent to the appropriate server.
• Second, this HTTP request is received by the Web Server.
The server maps this request to a particular servlet. The
servlet is dynamically retrieved and loaded into the address
space of the server.
19. • Third, the server now invokes the init() method of the
servlet. This method is invoked only when the servlet is
first loaded into the memory. It is possible to pass initial
parameters to the servlet so it may configure itself.
20. • Fourth, the server invokes the service() method of the
servlet. This method is called to process the HTTP
request. The servlet to read the data that has been
provided in the HTTP request. It may also formulate an
HTTP response for the client. The servlet remains in the
server’s address space and is available to process any
other HTTP request received from the client. It is called
for each HTTP request.
21. • Finally, the server may decise to unload the servlet from
the memory. The server calls the destroy() method to
relinquish any resources such as file handles that are
allocated for the servlet. The memory allocated for the
servlet and its objects can then be garbage collected.
22.
23. To become familiar with the key servlet components. We
will begin by building and testing a simple servlet.
The basic steps are as follows:
1. Create and compile the servlet source code.
2. Start Tomcat.
3. Start the web browser and request the servlet.
24. import java.io.*;
import javax.servlet.*;
public class HelloServlet extends GenericServlet {
public void service(ServletRequest request,
ServletResponse response)
throws ServletException, IOException{
response.setContentType(“text/html”);
PrintWriter pw= response.getWriter();
26. • You can download Tomcat through the Sun
Microsystems Web site at java.sun.com. The
current version is 7.0. . The default location for
Tomcat 7.0 is
C:Program FilesApache Tomcat 7.0
• To start Tomcat, select Start Tomcat in the Start |
Programs menu, or run startup.bat from the
C:Program FilesApache Tomcat 7.0bin
directory. When you are done testing servlets, you
can stop Tomcat by selecting Stop Tomcat in the
Start | Programs menu, or run shutdown.bat.
27. • Your first step is to download software that implements
the Java Servlet 2.1 or 2.2 and Java Server Pages 1.0 or
1.1 specifications. You can get a free version from Sun,
known as the JavaServer Web Development Kit
(JSWDK), at http://java.sun.com/products/servlet/.
• Next, you need to tell javac where to find the servlet and
JSP classes when you compile a servlet file. The JSWDK
installation instructions explain this, but it basically
amounts to putting servlet.jar and jsp.jar(which come with
the JSWDK) on your CLASSPATH. If you've never dealt
with the CLASSPATH before, it is the variable that
specifies where Java looks for classes
28. • If it is unspecified, Java looks in the current directory and
the standard system libraries. If you set it yourself, you
need to be sure to include ".", signifying the current
directory.
29. • Start a web browser and enter the URL shown here:
• http://localhost:8080/examples/servlets/HelloServlets
OR
• http://127.0.0.1:8080/examples/servlets/HelloServlets
30. • This can be done because 127.0.0.1 is defined as the IP
address of the local machine.
• You will observe the output of the servlet in the browser
display area. And it will contain the string Hello! In bold
type.
31. • Two packages contain the classes and interfaces that are
required to build servlets. These are javax.servlet and
javax.servlet.http.
32. • The javax.servlet package contains a number of classes
and interfaces that describe and define the contracts
between a servlet class and the runtime environment
provided for an instance of such a class by a conforming
servlet container.
33. • The javax.servlet.http package contains a number of
classes and interfaces that describe and define the
contracts between a servlet class running under the
HTTP protocol and the runtime environment provided for
an instance of such a class by a conforming servlet
container
