1、An Overview of Servlet and JSP Technology Marty Hall and Larry Brown， 2000-07, Core Servlets and JavaServer Pages chapter 1 1.1 A Servlets Job 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 a
2、nd databases or applications on the HTTP server. Their job is to perform the following tasks, as illustrated in Figure 1-1. Figure 1-1 1 Read the explicit data sent by the client. The end user normally enters this data in an HTML form on a Web page. However, the data could also come from an applet o
3、r a custom HTTP client program. 2 Read the implicit HTTP request data sent by the browser. Figure 1-1 shows 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 f
4、orm 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. 3 Generate the results. This process may require talking to a database, executing an RMI or EJ
5、B call, invoking a Web service, or computing the response directly. Your real data may be in a relational database. Fine. But your database probably doesnt speak HTTP or return results in HTML, so the Web browser cant talk directly to the database. Even if it could, for security reasons, you probabl
6、y 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. 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 (
7、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
8、 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 o
9、r 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. I
10、n 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-confi
11、rmation pages at online stores are specific to particular user requests. You dont 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
12、 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. 2 The Web page is derived from data that changes frequently. If the page changes for every request, then you certainly need to build the response at request time. If it cha
13、nges 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 convenie
14、nt 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. 3 The Web page uses information from corporate databases or other server-side sources. I
15、f 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.
16、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 g
17、oing 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
18、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 well only be discussing HTTP servlets.
