12-10-2012, 04:28 PM
INTRODUCTION TO BASIC SERVLET PROGRAMMING
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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 2–1 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 the end user enters in a form
and the behind-the-scenes HTTP information. Both varieties are critical
to effective development. The HTTP information includes cookies,
media types and compression schemes the browser understands, and
so forth.
3. Generate the results.
This process may require talking to a database, executing an RMI or
CORBA call, invoking a legacy application, 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. 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 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), binary (GIF images), or even a compressed format like gzip
that is layered on top of some other underlying format.
5. Send the implicit HTTP response data.
Figure 2–1 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.
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.
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 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.
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. Besides, 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?
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.
Portable
Servlets are written in the Java programming language and follow a standard API.
Servlets are supported directly or by a plug-in on virtually every major Web server.
Consequently, servlets written for, say, iPlanet Enterprise Server can run virtually
unchanged on Apache, Microsoft Internet Information Server (IIS), IBM Web-
Sphere, or StarNine WebStar. They are part of the Java 2 Platform, Enterprise Edition
(J2EE; see http://java.sunj2ee/), so industry support for servlets is
becoming even more pervasive.
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.
A second source of problems is the fact that some CGI programs are processed by
languages that do not automatically check array or string bounds. For example, in C
and C++ it is perfectly legal to allocate a 100-element array and then write into the
999th “element,” which is really some random part of program memory. So, programmers
who forget to perform this check open up their system to deliberate or
accidental buffer overflow attacks.