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 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.
