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An Overview of Servlet and JSP Technology

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.

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.

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.

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 results inside a document.

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.

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.

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:

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.

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 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.

If the information is in a database, you need server-side processing even if the client is

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