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毕业设计

外文文献原文及译文

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

using dynamic Web content such as an applet. Imagine using an applet by itself for a search

engine site:

"Downloading 50 terabyte applet, please wait!" Obviously, that is silly; you need to talk to

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 http://jcp.org/en/jsr/detail?

id=116). In practice, however, this use of servlets has not caught on, and we'll only be

discussing HTTP servlets.

Java servlets are more efficient, easier to use, more powerful, more portable, safer, and

cheaper than traditional CGI and many alternative CGI-like technologies.

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.

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