Transcript Java Servlet

Presented by
Hsiuling Hsieh
Christine Liu
Copyright, 1996 © Dale Carnegie & Associates, Inc.
Introduction
Java technology isn't just for programming applets
which run on the client side in web browsers, or for
writing Internet applications. The Servlet API brings
the power of Java to your servers,too.
JavaTM Servlet technology provides web
developers with a simple, consistent mechanism for
extending the functionality of a web server and for
accessing existing business systems. A servlet can
almost be thought of as an applet that runs on the
server side -- without a face.
What is a Servlet?
• Servlets are protocol- and platformindependent server side components, which
dynamically extend Java enabled servers.
They provide a general framework for
services built using the request-response
paradigm.
• Servlets run inside servers. They are Java
application components which are
downloaded, on demand.
–
– Clients talking to Java Servlets in servers. Clients may
range in complexity from simple HTML forms to
sophisticated Java applets. Servlets will frequently use
some kind of persistent storage, such as files or a
database.
Servlet OOP Architecture
• Servlet is an instance of a Java class which
implements the javax.servlet.Servlet
interface (provided by the JSDK).
• Most servlets do this indirectly, by extending
javax.servlet.http.HttpServlet or
javax.servlet.http.GenericServlet, two
of javax.servlet.Servlet's standard
implementations.
Hierarchy :
Servlet
HttpServlet|GenericServlet
MyServlet
Ways to use Servlet
A simple way:
Servlet can process data which was POSTed
over HTTPS using an HTML FORM, say, an
order-entry, and applying the business logic
used to update a company's order database.
A simple servlet
Some other uses:
• Since servlets handle multiple requests
concurrently, the requests can be synchronized
with each other to support collaborative
applications such as on-line conferencing.
• One could define a community of active agents,
which share work among each other. The code for
each agent would be loaded as a servlet, and the
agents would pass data to each other.
• One servlet could forward requests other servers.
This technique can balance load among several
servers which mirror the same content.
What does Servlet look like?
•
Servlets implement the Servlet interface, usually by extending either
the generic or an HTTP-specific implementation. The simplest
possible servlet defines a single method, service:
• import java.servlet.*;
• public class MyServlet extends GenericServlet {
•
public void service (ServletRequest request,
•
ServletResponse response
•
) throws ServletException, IOException
•
{ ...
•
}
•
}
(cont’d)
• The service method is provided with Request and Response
parameters. These encapsulate the data sent by the client,
and allowing servlets to report status including errors.
• Servlets normally retrieve most of their parameters through
an input stream, and send their responses using an output
stream:
ServletInputStream
in = request.getInputStream ();
ServletOutputStream out = response.getOutputStream ();
• These input and output streams may be used with data in
whatever format is appropriate. For example, an applet and
servlet might exchange data using object serialization; HTML,
and numerous image formats, may also be appropriate data
formats.
Primary Servlet methods
After being loaded, three methods are involved in the lifecycle
of a servlet:
• Servlets are activated by the server through an init call. To
provide implementation of this call is to perform potentially
costly (usually, I/O intensive) setup only once, rather than
once per request.
• After initialization, servlets handle many requests. Each client
request generates one service upcall. These requests may be
concurrent; this allows servlets to coordinate activities among
many clients.
• Requests are processed until the servlet is explicitly shut down
by the web server, by calling the destroy method. The
servlet's class may then become eligible for garbage
collection.
HTML-Aware Servlets
• Many servlets will directly generate HTML
formatted text, use Java formatted
output classes such as java.io.PrintWriter.
There is no need to use scripting
languages to dynamically modify or
generate HTML pages.
HTTP-Specific Servlets
• Servlets which are being used with the HTTP
protocol may support any HTTP method,
including GET, POST, HEAD, and more.
• They may redirect requests to other locations,
and send HTTP-specific error messages.
• They can get access to parameters which were
passed through standard HTML forms.
Examples:
• String method = request.getMethod (); // e.g. POST
• String name = request.getParameter ("name")
• String phone = request.getParameter ("phone");
• String card = request.getParameter ("creditcard");
Performance Feature
• Servlets do not require creation of a new
process for each request. In most
environments, many servlets run in parallel
within the same process as the server.
When used in such environments with
HTTP, servlets provide compelling
performance advantages over both the CGI
approach and the Fast-CGI approach.
Performance (cont’d)
Security Features
• Servlets have the Java advantage: memory
access violations and strong typing
violations are not possible
• Strong security policy support. All Java
environments provide a Security Manager
which can be used to control whether
actions such as network or file access are to
be permitted.
End