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Definition
Servlets are modules that extend request/response-oriented servers, such
as Java-enabled web servers. For example, a servlet might be responsible for taking
data in an HTML order-entry form and applying the business logic used to update
a company's order database. Servlets are to servers what applets are to browsers.
Unlike applets, however, servlets have no graphical user interface. Servlets can
be embedded in many different servers because the servlet API, which you use to
write servlets, assumes nothing about the server's environment or protocol. Servlets
have become most widely used within HTTP servers; many web servers support Java
Servlet technology.
Use
Servlets instead of CGI Scripts.
Servlets are an effective replacement
for CGI scripts. They provide a way to generate dynamic documents that is both
easier to write and faster to run. Servlets also address the problem of doing
server-side programming with platform-specific APIs: they are developed with the
Java Servlet API, a standard Java extension.
So use servlets to handle HTTP
client requests. For example, have servlets process data POSTed over HTTPS using
an HTML form, including purchase order or credit card data. A servlet like this
could be part of an order-entry and processing system, working with product and
inventory databases, and perhaps an on-line payment system.
Other
Uses for Servlets
Here are a few more of the many applications for servlets:
" Allowing
collaboration between people. A servlet can handle multiple requests concurrently,
and can synchronize requests. This allows servlets to support systems such as
on-line conferencing.
" Forwarding requests. Servlets can forward
requests to other servers and servlets. Thus servlets can be used to balance load
among several servers that mirror the same content, and to partition a single
logical service over several servers, according to task type or organizational
boundaries.
Architecture of the Servlet Package
The javax.servlet
package provides interfaces and classes for writing servlets. The architecture
of the package is described below.
The
Servlet Interface
The central abstraction in the Servlet API is the Servlet
interface. All servlets implement this interface, either directly or, more commonly,
by extending a class that implements it such as HttpServlet The
Servlet interface declares, but does not implement, methods that manage the servlet
and its communications with clients. Servlet writers provide some or all of these
methods when developing a servlet.
Client
Interaction
When a servlet accepts a call from a client, it receives two
objects:
" A ServletRequest , which encapsulates the communication from
the client to the server.
" A ServletResponse , which encapsulates
the communication from the servlet back to the client.
ServletRequest and
ServletResponse are interfaces defined by the javax.servlet package.
The
Servlet Request Interface
The ServletRequest interface allows the servlet
access to:
" Information such as the names of the parameters passed
in by the client, the protocol (scheme) being used by the client, and the names
of the remote host that made the request and the server that received it.
" The input stream, ServletInputStream . Servlets use the input stream to
get data from clients that use application protocols such as the HTTP POST and
PUT methods.
Interfaces that extend ServletRequest interface allow the servlet
to retrieve more protocol-specific data. For example, the HttpServletRequest interface
contains methods for accessing HTTP-specific header information.
The
Servlet Response Interface
The ServletResponse interface gives the servlet
methods for replying to the client. It:
" Allows the servlet to set
the content length and MIME type of the reply.
" Provides an output
stream, ServletOutputStream , and a Writer through which the servlet can send
the reply data.
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