An Overview of Servlet and JSP Technology
Nagle ,Wiegley
Abstract: Servlet program running in the rver-side, dynamically generated Web page with the traditional CGI and many other similar compared to CGI technology, Java Servlet with a more efficient, easier to u, more powerful and has better portability, more savings to invest. 空调功率.
Keywords: JSP Technology;Servlet定安粽子;HTTP rver
1. 微信多开电脑A Servlet's Job
Servlets are Java programs that run on Web or application rvers, acting as a middle layer between requests coming from Web browrs or other HTTP clients and databas or applications on the HTTP rver. Their job is to perform the following tasks, as illustrated in Figure 1-1.
Figure 1-1 Web middleware role
1.1 Read the explicit data nt by the client.
The end ur 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.
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1八字阴阳.2 Read the implicit HTTP request data nt by the browr.
Figure 1-1 shows a single arrow going from the client to the Web rver (the layer where rvlets and JSP execute), but there are really two varieties of data: the explicit data that the end ur 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 browr understands, and so on.
1.3 Generate the results.
This process may require talking to a databa, executing an RMI or EJB call, invoking a Web rvice, or computing the respon directly. Your real data may be in a relational databa. Fine. But your databa probably doesn't speak HTTP or return results in HTML, so the Web browr can't talk directly to the databa. Even if it could, for curity 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 实践论读后感inside a document.
1.4 Send the explicit data (i.e., the document) to the client.
This document can be nt in a variety of formats, including text (HTML or XML), binary (GIF images), or even a compresd 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 rvlet/JSP task is to wrap the results inside of HTML.
1.5 Send the implicit HTTP respon data.
imply
Figure 1-1 shows a single arrow going from the Web middle layer (the rvlet or JSP page) to the client. But, there are really two varieties of data nt: the document itlf and the behind-the-scenes HTTP information. Again, both varieties are critical to effective development. Sending HTTP respon data involves telling the browr or other client what type of document is being returned (e.g., HTML), tting cookies and caching parameters, and other such tasks.
2 Why Build Web Pages Dynamically?
many client requests can be satisfied by prebuilt documents, and the rver would handle the requests without invoking rvlets. In many cas, 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:
2.1 The Web page is bad on data nt by the client.
For instance, the results page from arch engines and order-confirmation pages at onlin
e stores are specific to particular ur requests. You don't know what to display until you read the data that the ur submits. Just remember that the ur submits two kinds of data: explicit (i.e., HTML form data) and implicit (i.e., HTTP request headers). Either kind of input can be ud to build the output page. In particular, it is quite common to build a ur-specific page bad on a cookie value.
2.2 The Web page is derived from data that changes frequently.
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If the page changes for every request, then you certainly need to build the respon 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 rver (independently of client requests), or you could wait and only build the page when the ur requests it. The right approach depends on the situation, but sometimes it is more convenient to do the latter: wait for the ur 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.
2.3 The Web page us information from corporate databas or other rver-side sourc
es.
If the information is in a databa, you need rver-side processing even if the client is using dynamic Web content such as an applet. Imagine using an applet by itlf for a arch engine site:
