I started working on this because I think that the need for it will
continue to grow. I'm aware of the Sun FAQ that gets posted weekly, but
I was thinking that something a little more detailed might do some
good. Please take a look at this, and let me know what you think...
It's far from complete, but I wanted to get it out because these always
work much better when there's a community effort... :-)
--
Within C++ is a smaller, cleaner language
struggling to get out.
It's called Java.
Thomas Moore
[EMAIL PROTECTED] Home Account
Software.Engineer [EMAIL PROTECTED]
phone://732.462.1880:268 NJ Patterns Group Home Page
employer://Celwave, RF http://members.home.net/twmoore/cjpg
The Java Servlet FAQ
0. About This FAQ
1. Introduction To Servlets
2. Just The FAQs - Answers to common questions
3. Servlet Engine Specific Questions
4. Other Servlet Resources
---
Section 0 - About This FAQ
0.1 Why did you write this FAQ?
Servlets are a very popular topic right now. They enable Java
programmers to write powerful applications for that thin and
ubiquitous client, the web browser, without the overhead or
implementation issues that are still inherent in today's
browsers. The backgrounds of people seem to be a mix of
experienced Java appication/applet programmers and experienced
perl/C/C++/etc. CGI programmers (as well as people fairly new
to the field in general). The parties each bring their own skill
sets to the table. This FAQ is meant to be a starting point
for both groups, as well as serving as a general introduction
to the technology.
Besides, servlets (not unlike Java itself) can be somewhat of a
moving target. There are enough changes in the works, and enough
new information being introduced, that a single resource pointing
to that information would be very useful.
0.2 What does this FAQ cover?
This FAQ consists of answers to some of the topics that come up
most frequently on Sun's Servlet-Interest mailing list, as well
as contributions from servlet programmers. It is meant to be a
quick source for general information as well as a pointer to
more in-depth resources.
0.3 What isn't in this FAQ?
First, this FAQ is not a tutorial on Java, servlets, or anything else.
There are countless FAQs on the Java programming language, and people
who have questions regarding Java programming in areas other than
servlets are encouraged to look to them for answers. This is also not
meant to take the place of in depth resources and tutorials. There
are several books on the market today, as well as several websites, that
step new programmers through learning the servlet interface. See the
section on Other Resources for more information.
0.4 I disagree with something the FAQ says. How can I correct it?
Send corrections/opinions/flames to [EMAIL PROTECTED]
0.5 You don't say anything about <foo>. I think the FAQ should have this
information. What should I do?
See Question 0.4
---
Section 1 - Introduction to Servlets
1.1 What is a servlet?
A servlet is an application or a script that is written in Java and
executed on a server, as opposed to on a client. It is analagous to
CGI, although servlets are more than simply CGI scripts written in Java.
See question 1.2 for more detail.
1.2 How is a servlet different from CGI?
CGI (the Common Gateway Interface) is a protocol that allows clients
using web browsers to send information to and retrieve information
from servers via programs set up on the server. CGI provides a set of
methods that can parse information coming from a web browser. The
information can then be handled by the program, often written in C, C++,
or (perhaps most commonly) perl. It is frequently used to generate
"dynamic" web content, that is, it generates HTML on the fly, which
is displayed on the client's browser as if it were a standard .html
file located on the server.
Servlets are similar in that, in their most common form, they allow
for dynamic interaction between a web browser and programs on the
server, accepting data via http and sending back HTML code. However,
servlets are not simply CGI programs written in Java. It is possible to
cross the two technologies, but it generally involves using something
to intermediate between the OS and the Java VM (often a shell script).
Servlet technology makes this intermediary scripting unnecessary.
Sevlets run on a server engine, often running within, or in cooperation
with, the server's Web software.
1.3 Are servlets better than CGI?
This is something of a religious question (See "Holy Wars" in the
Jargon File). It is somewhat akin to asking whether C++ or Java
Servlets offer several advantages over CGI. They fall into two
main categories: design and performance.
In terms of design, Java is an object oriented language. While it
is not impossible to construct badly designed Java programs, the
language itself encourages adherence to object oriented
principles. While this is not the place to carry on Java/C++/
Smalltalk/etc. debates, I will just say in general that Java
(and servlets in particular) enable and encourage handling
requests with a cascade of specialized objects. It makes it less
likely that a programmer will do everything in one big script. This
segmented design can lead to easier revision and maintainence, and
encourages software reuse.
For performance, servlets are only instantiated when the client
first accesses the program. That is, the first time any client
hits the URL that is used to access the servlet, the servlet
engine instantiates that object. All subsiquent accesses are done
to that instance. This keeps the response time of servlets lower
than that of CGI programs, which must be run once per hit. Also,
becasue a servlet is instantiated only once, all accesses are put
through that one object. While this does cause issues with regard
to current threads of execution, the programmer can address them
by having certain objects be unique to a user's session and others
shared (with synchronized concurrency locks where necessary) among
all users. Java's simple multithreading syntax makes this fairly
easy.
1.4 What servlet engines are available? What servlet engines will
work with my web server?
There are several servlet engines available. The one you select
will be determined by you web server platform, your budget, and/or
your personal preferences.
<insert table here with hardware/web server/engine/non-commercial
cost/commercial cost>
1.5 What discussion forums are there for servlets?
The servlet mailing list hosted by Sun is currently the largest
single forum for discussing servlet related issues. It has its
own FAQ, but it does stand to mention that the list has
historically been a very valuable resource for programmers, with
both a high signal-to-noise ratio and a fast response time to
member's questions. Please consult the list's FAQ before posting,
and remember that your emails go out to the entire list membership
- people who subscribed specifically to get servlet related
information.
--
Section 2 - Just The FAQS
2.1 How do I upload a file?
Form based file upload requires a couple of steps. The server
must supply (and the client must support) encoding type
multipart/form-data. Most current browsers do, but it's not a
guarantee. Secondly (and this is usually the trickiest part),
your servlet has to parse the binary data and do something with it
(e.g., write it to a file on the server). The intrepid programmer
is referred to RFC 1867 for cluefulness on how to parse this
data. Less brave souls can use either Jason Hunter's
implementation of a MultipartRequest (available from
www.servlets.com), or CParseRFC1867 (available
from www.servletcentral.com). Note that the source code is
available for both of these examples, but both assume that you
will be writing the file to a file on the server. Other uses
(e.g. storing the file as a binary object in a database) will
require adaptation.
2.2 How can I get a servlet to send EMail?
[This section will mention some of the more popular Java email
packages, and will make cautious mention of Jason's use of the
internal sun package, since that seems to be a topic of
discussion/flame]
2.3 What is the Session object and how do I use it?
[This section will mention what a session is, how it works, and
rudiments on how to use it to do different things, e.g., hold
objects like database connections or specific data generated
during the session]
2.4 What is servlet chaining? How do I chain servlets?
[This section will explain the difference between chaining
servlets and forwarding requests]
--
Section 3 - Servlet Engine Specific Questions
3.1 Sun's Java Web Server
3.2 Live Software's JRun
3.3 Apache JServ
--
Section 4 - Other Servlet Resources
4.1 What web based sources talk about servlets?
4.2 What are some good books about servlets?
4.3 Where can I get my servlets hosted?
