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本科毕业设计（论文）
英文翻译
题
目
创业平台设计与实现
学生姓名
专业班级
网
学
号
院
（系）
指导教师（职称）
完成时间
2008
年
6
月
6
日
Ser
vlet
和
JSP
技术简述
专业班级：
姓名：
学号
英文原文
An Overview
of Servlet and JSP Technology
Gildas
Avoine and Philippe Oechslin
EPFL,
Lausanne, Switzerland
1.1 A Servlet's
Job
Servlets are Java programs that run
on Web or application servers, acting as a middle
layer between requests coming from Web
browsers or other HTTP clients and databases or
applications on the HTTP server. Their
job is to perform the following tasks, as
illustrated
in Figure 1-1.
Figure 1-1
1
．
Read the
explicit data sent by the client.
The
end user normally enters this data in an HTML form
on a Web page. However, the
data could
also come from an applet or a custom HTTP client
program.
2
．
Read
the implicit HTTP request data sent by the
browser.
Figure
1-1
shows
a
single
arrow
going
from
the
client
to
the
Web
server
(the
layer
where
servlets and JSP execute), but there are really
two varieties of data: the explicit data
that
the
end
user
enters
in
a
form
and
the
behind-the-scenes
HTTP
information.
Both
varieties
are
critical.
The
HTTP
information
includes
cookies,
information
about
media
types and compression
schemes the browser understands, and so on.
3
．
Generate the
results.
This process may require
talking to a database, executing an RMI or EJB
call, invoking
a Web service, or
computing the response directly. Your real data
may be in a relational
database. Fine.
But your database probably doesn't speak HTTP or
return results in HTML,
so the Web
browser can't talk directly to the database. Even
if it could, for security reasons,
you
probably would not want it to. The same argument
applies to most other applications.
You
need the Web middle layer to extract the incoming
data from the HTTP stream, talk to
1
Servlet
和
JSP
< br>技术简述
专业班级：
姓名：
学号
the
application, and embed the results inside a
document.
4
．
Send
the explicit data (i.e., the document) to the
client.
This
document
can
be
sent
in
a
variety
of
formats,
including
text
(HTML
or
XML),
binary (GIF images), or even a
compressed format like gzip that is layered on top
of some
other underlying format. But,
HTML is by far the most common format, so an
important
servlet/JSP task is to wrap
the results inside of HTML.
5
．
Send the
implicit HTTP response data.
Figure 1-1
shows a single arrow going from the Web middle
layer (the servlet or JSP
page) to the
client. But, there are really two varieties of
data sent: the document itself and
the
behind-the-scenes
HTTP
information.
Again,
both
varieties
are
critical
to
effective
development.
Sending
HTTP
response
data
involves
telling
the
browser
or
other
client
what
type
of
document
is
being
returned
(e.g.,
HTML),
setting
cookies
and
caching
parameters, and other such tasks.
1.2 Why Build Web Pages
Dynamically?
many
client
requests
can
be
satisfied
by
prebuilt
documents,
and
the
server
would
handle these requests
without invoking servlets. In many cases, however,
a static result is
not sufficient, and
a page needs to be generated for each request.
There are a number of
reasons why Web
pages need to be built on-the-fly:
1
．
The
Web page is based on data sent by the client.
For
instance,
the
results
page
from
search
engines
and
order-confirmation
pages
at
online
stores are specific to particular user requests.
You don't know what to display until
you read the data that the user
submits. Just remember that the user submits two
kinds of
data: explicit (i.e., HTML
form data) and implicit (i.e., HTTP request
headers). Either kind
of input can be
used to build the output page. In particular, it
is quite common to build a
user-
specific page based on a cookie value.
2
．
The Web page is
derived from data that changes frequently.
If the page changes for every request,
then you certainly need to build the response at
request time. If it changes only
periodically, however, you could do it two ways:
you could
periodically build a new Web
page on the server (independently of client
requests), or you
could wait and only
build the page when the user requests it. The
right approach depends
on
the
situation,
but
sometimes
it
is
more
convenient
to
do
the
latter:
wait
for
the
user
2
Servlet
和
JSP
技术简述
专业班级：
姓名：
学号
request.
For
example,
a
weather
report
or
news
headlines
site
might
build
the
pages
dynamically, perhaps returning a
previously built page if that page is still up to
date.
3
．
The Web
page uses information from corporate databases or
other server-side sources.
If the
information is in a database, you need server-side
processing even if the client is
using
dynamic
Web
content
such
as
an
applet.
Imagine
using
an
applet
by
itself
for
a
search
engine site:
the database.
Going from the client to the Web tier to the
database (a three-tier approach)
instead of from
an applet
directly to
a database (a two-tier
approach) provides increased
flexibility and security with little or
no performance penalty. After all, the database
call is
usually the rate-limiting step,
so going through the Web server does not slow
things down.
In fact, a three-tier
approach is often faster because the middle tier
can perform caching
and connection
pooling.
In principle, servlets are not
restricted to Web or application servers that
handle HTTP
requests but can be used
for other types of servers as well. For example,
servlets could be
embedded in FTP or
mail servers to extend their functionality. And, a
servlet API for SIP
(Session
Initiation
Protocol)
servers
was
recently
standardized
(see
/en/jsr/detail?id=116).
In practice, however, this use of servlets has not
caught
on, and we'll only be discussing
HTTP servlets.
1.3 The Advantages of
Servlets Over
Java servlets are more
efficient, easier to use, more powerful, more
portable, safer, and
cheaper than
traditional CGI and many alternative CGI-like
technologies.
1
．
Efficient
With
traditional
CGI,
a
new
process
is
started
for
each
HTTP
request.
If
the
CGI
program
itself
is
relatively
short,
the
overhead
of
starting
the
process
can
dominate
the
execution
time.
With
servlets,
the
Java
virtual
machine
stays
running
and
handles
each
request
with
a
lightweight
Java
thread,
not
a
heavyweight
operating
system
process.
Similarly, in traditional CGI, if there
are N requests to the same CGI program, the code
for
the CGI program is loaded into
memory N times. With servlets, however, there
would be N
threads, but only a single
copy of the servlet class would be loaded. This
approach reduces
server memory
requirements and saves time by instantiating fewer
objects. Finally, when a
CGI program
finishes handling a request, the program
terminates. This approach makes it
3
Servlet
和
JSP
< br>技术简述
专业班级：
姓名：
学号
difficult
to
cache
computations,
keep
database
connections
open,
and
perform
other
optimizations that rely on persistent
data. Servlets, however, remain in memory even
after
they complete a response, so it
is straightforward to store arbitrarily complex
data between
client requests.
2
．
Convenient
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
high-level
utilities.
In
CGI,
you
have
to
do
much
of
this
yourself.
Besides, if you
already know the Java programming language, why
learn Perl too? You're
already
convinced
that
Java
technology
makes
for
more
reliable
and
reusable
code
than
does
Visual
Basic,
VBScript,
or
C++.
Why
go
back
to
those
languages
for
server-side
programming?
3
．
Powerful
Servlets support several capabilities
that are difficult or impossible to accomplish
with
regular CGI. Servlets can talk
directly to the Web server, whereas regular CGI
programs
cannot,
at
least
not
without
using
a
server-
specific
API.
Communicating
with
the
Web
server
makes
it
easier
to
translate
relative
URLs
into
concrete
path
names,
for
instance.
Multiple
servlets
can
also
share
data,
making
it
easy
to
implement
database
connection
pooling and similar resource-sharing
optimizations. Servlets can also maintain
information
from
request
to
request,
simplifying
techniques
like
session
tracking
and
caching
of
previous computations.
4
．
Portable
Servlets
are
written
in
the
Java
programming
language
and
follow
a
standard
API.
Servlets
are
supported
directly
or
by
a
plugin
on
virtually
every
major
Web
server.
Consequently, servlets written for,
say, Macromedia JRun can run virtually unchanged
on
Apache
Tomcat,
Microsoft
Internet
Information
Server
(with
a
separate
plugin),
IBM
WebSphere, iPlanet Enterprise Server,
Oracle9i AS, or StarNine WebStar. They are part of
the Java 2 Platform,
Enterprise Edition (J2EE; see /j2ee/),
so
industry
support for
servlets is becoming even more pervasive.
5
．
Inexpensive
A
number
of
free
or
very
inexpensive
Web
servers
are
good
for
development
use
or
deployment
of
low-
or
medium-volume
Web
sites.
Thus,
with
servlets
and
JSP
you
can
start
with
a
free
or
inexpensive
server
and
migrate
to
more
expensive
servers
with
4
Servlet
和
JSP
技术简述
专业班级：
姓名：
学号
high-performance capabilities or
advanced administration utilities only after
your project
meets
initial
success.
This
is
in
contrast
to
many
of
the
other
CGI
alternatives,
which
require a significant
initial investment for the purchase of a
proprietary package.
Price and
portability are somewhat connected. For example,
Marty tries to keep track
of the
countries of readers that send him questions by
email. India was near the top of the
list,
probably
#2
behind
the
U.S.
Marty
also
taught
one
of
his
JSP
and
servlet
training
courses
(see
/)
in
Manila,
and
there
was
great
interest
in
servlet and JSP technology there.
Now, why are India and the Philippines
both so interested? We surmise that the answer
is
twofold.
First,
both
countries
have
large
pools
of
well-educated
software
developers.
Second, both
countries have (or had, at that time) highly
unfavorable currency exchange
rates
against
the
U.S.
dollar.
So,
buying
a
special-
purpose
Web
server
from
a
U.S.
company consumed a large part of early
project funds.
But, with servlets and
JSP, they could start with a free server: Apache
Tomcat (either
standalone, embedded in
the regular Apache Web server, or embedded in
Microsoft IIS).
Once
the
project
starts
to
become
successful,
they
could
move
to
a
server
like
Caucho
Resin
that had higher performance and easier
administration but that is not free. But none
of
their
servlets
or
JSP pages
have
to
be
rewritten.
If
their
project
becomes
even
larger,
they might want to
move to a distributed (clustered) environment. No
problem: they could
move
to
Macromedia
JRun
Professional,
which
supports
distributed
applications
(Web
farms).
Again,
none
of
their
servlets
or
JSP
pages
have
to
be
rewritten.
If
the
project
becomes quite large and complex, they
might want to use Enterprise JavaBeans (EJB) to
encapsulate their business logic. So,
they might switch to BEA WebLogic or Oracle9i AS.
Again,
none
of
their
servlets
or
JSP
pages
have
to
be
rewritten.
Finally,
if
their
project
becomes
even
bigger,
they
might
move
it
off
of
their
Linux
box
and
onto
an
IBM
mainframe running IBM WebSphere. But
once again, none of their servlets or JSP pages
have to be rewritten.
6
．
Secure
One of the main sources of
vulnerabilities in traditional CGI stems from the
fact that
the programs are often
executed by general-purpose operating system
shells. So, the CGI
programmer must be
careful to filter out characters such as
backquotes and semicolons that
are
treated specially by
the shell.
Implementing
this precaution
is
harder than one might
think,
and
weaknesses
stemming
from
this
problem
are
constantly
being
uncovered
in
widely used CGI
libraries.
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