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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 an
2、d databases or applications on the HTTP server. Their job is to perform the following tasks, as illustrated in Figure 1-1. Figure 1-1 1Read 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 or
3、a custom HTTP client program. 2Read 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 form
4、 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. 3Generate the results. This process may require talking to a database, executing an RMI or EJB ca
5、ll, 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 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 1 页，共 13 页 - - - - - - - - - results in HTML, s
6、o the Web browser cant 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. 4Send the explicit data (i.e., the document)
7、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 t
8、he results inside of HTML. 5Send 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 var
9、ieties 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 s
10、atisfied 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
11、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 dont know what to display until you read the data that the user submits. Just remember that the user submits two kinds o
12、f 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. 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - -
13、 - - - - 第 2 页，共 13 页 - - - - - - - - - 2The 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 changes only periodically, however, you could do it two ways: you could periodically build a
14、 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 hea
15、dlines site might build the pages dynamically, perhaps returning a previously built page if that page is still up to date. 3The Web page uses information from corporate databases or other server-side sources. If the information is in a database, you need server-side processing even if the client is
16、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 f
17、rom 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 o
18、ften 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 exte
19、nd 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. 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - -
20、 - - - - - - - - - 名师精心整理 - - - - - - - 第 3 页，共 13 页 - - - - - - - - - 1.3 The Advantages of Servlets Over Traditional CGI Java servlets are more efficient, easier to use, more powerful, more portable, safer, and cheaper than traditional CGI and many alternative CGI-like technologies. 1Efficient Wit
21、h 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
22、 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 appro
23、ach 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
24、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. 2Convenient Servlets have an extensive infrastructure for automatically parsing and decoding HTML form data, reading and setti
25、ng HTTP headers, handling cookies, tracking sessions, and many other such high-level utilities. In CGI, you have to do much of this yourself. Besides, if you already know the Java programming language, why learn Perl too? Youre already convinced that Java technology makes for more reliable and reusa
26、ble code than does Visual Basic, VBScript, or C+. Why go back to those languages for server-side programming? 3Powerful Servlets support several capabilities that are difficult or impossible to accomplish with regular CGI. Servlets can talk directly to the Web server, whereas regular CGI 名师资料总结 - -
27、-精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 4 页，共 13 页 - - - - - - - - - programs cannot, at least not without using a server-specific API. Communicating with the Web server makes it easier to translate relative URLs into concrete path names, for instance. Multiple servlets
28、can also share data, making it easy to implement database connection pooling and similar resource-sharing optimizations. Servlets can also maintain information from request to request, simplifying techniques like session tracking and caching of previous computations. 4Portable Servlets are written i
29、n the Java programming language and follow a standard API. Servlets are supported directly or by a plugin on virtually every major Web server. Consequently, servlets written for, say, Macromedia JRun can run virtually unchanged on Apache Tomcat, Microsoft Internet Information Server (with a separate
30、 plugin), IBM WebSphere, iPlanet Enterprise Server, Oracle9i AS, or StarNine WebStar. They are part of the Java 2 Platform, Enterprise Edition (J2EE; see http:/ so industry support for servlets is becoming even more pervasive. 5Inexpensive A number of free or very inexpensive Web servers are good fo
31、r development use or deployment of low- or medium-volume Web sites. Thus, with servlets and JSP you can start with a free or inexpensive server and migrate to more expensive servers with high-performance capabilities or advanced administration utilities only after your project meets initial success.
32、 This is in contrast to many of the other CGI alternatives, which require a significant initial investment for the purchase of a proprietary package. Price and portability are somewhat connected. For example, Marty tries to keep track of the countries of readers that send him questions by email. Ind
33、ia was near the top of the list, probably #2 behind the U.S. Marty also taught one of his JSP and servlet training courses (see http:/ in Manila, and there was great interest in servlet and JSP technology there. 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 5 页，共 13
34、 页 - - - - - - - - - Now, why are India and the Philippines both so interested? We surmise that the answer is twofold. First, both countries have large pools of well-educated software developers. Second, both countries have (or had, at that time) highly unfavorable currency exchange rates against th
35、e U.S. dollar. So, buying a special-purpose Web server from a U.S. company consumed a large part of early project funds. But, with servlets and JSP, they could start with a free server: Apache Tomcat (either standalone, embedded in the regular Apache Web server, or embedded in Microsoft IIS). Once t
36、he project starts to become successful, they could move to a server like Caucho Resin that had higher performance and easier administration but that is not free. But none of their servlets or JSP pages have to be rewritten. If their project becomes even larger, they might want to move to a distribut
37、ed (clustered) environment. No problem: they could move to Macromedia JRun Professional, which supports distributed applications (Web farms). Again, none of their servlets or JSP pages have to be rewritten. If the project becomes quite large and complex, they might want to use Enterprise JavaBeans (
38、EJB) to encapsulate their business logic. So, they might switch to BEA WebLogic or Oracle9i AS. Again, none of their servlets or JSP pages have to be rewritten. Finally, if their project becomes even bigger, they might move it off of their Linux box and onto an IBM mainframe running IBM WebSphere. B
39、ut once again, none of their servlets or JSP pages have to be rewritten. 6Secure One of the main sources of vulnerabilities in traditional CGI stems from the fact that the programs are often executed by general-purpose operating system shells. So, the CGI programmer must be careful to filter out cha
40、racters such as backquotes and semicolons that are treated specially by the shell. Implementing this precaution is harder than one might think, and weaknesses stemming from this problem are constantly being uncovered in widely used CGI libraries. A second source of problems is the fact that some CGI
41、 programs are processed by languages that do not automatically check array or string bounds. For example, in C and C+ it is perfectly legal to allocate a 100-element array and then write into the 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 6 页，共 13 页 - - - - - - -
42、 - - 999th element, which is really some random part of program memory. So, programmers who forget to perform this check open up their system to deliberate or accidental buffer overflow attacks. Servlets suffer from neither of these problems. Even if a servlet executes a system call (e.g., with Runt
43、ime.exec or JNI) to invoke a program on the local operating system, it does not use a shell to do so. And, of course, array bounds checking and other memory protection features are a central part of the Java programming language. 7Mainstream There are a lot of good technologies out there. But if ven
44、dors dont support them and developers dont know how to use them, what good are they? Servlet and JSP technology is supported by servers from Apache, Oracle, IBM, Sybase, BEA, Macromedia, Caucho, Sun/iPlanet, New Atlanta, ATG, Fujitsu, Lutris, Silverstream, the World Wide Web Consortium (W3C), and ma
45、ny others. Several low-cost plugins add support to Microsoft IIS and Zeus as well. They run on Windows, Unix/Linux, MacOS, VMS, and IBM mainframe operating systems. They are the single most popular application of the Java programming language. They are arguably the most popular choice for developing
46、 medium to large Web applications. They are used by the airline industry (most United Airlines and Delta Airlines Web sites), e-commerce (), online banking (First USA Bank, Banco Popular de Puerto Rico), Web search engines/portals (), large financial sites (American Century Investments), and hundred
47、s of other sites that you visit every day. Of course, popularity alone is no proof of good technology. Numerous counter-examples abound. But our point is that you are not experimenting with a new and unproven technology when you work with server-side Java. 摘要： Servlet 程序运行在服务器端，在动态的生成Web页面方面与传统的CGI名
48、师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 7 页，共 13 页 - - - - - - - - - 和许多其他的类似CGI 技术相比， Java Servlet 更高效，更容易使用，功能更强大，并且拥有更好的可移植性，更节省投资。关键词： JSP 技术， Servlet，HTTP 服务Servlet 和 JSP 技术简介Marty Hall and Larry Brown ，2000-07,chapter 1 1.1Servlet的功能Servlets是运行在 Web或应用服务器
49、上的Java程序，它是一个中间层，负责连接来自 Web 浏览器或其他 HTTP 客户端和 HTTP 服务器上的数据库或应用程序。Servlet的工作是执行下面的任务，如图1-1 所示 。图 1-1 1.读取客户端发送的显式数据。客户端用户一般在页面的HTML 表单中输入这些数据。然而，数据还有可能来自 applet或定制的 HTTP 客户程序。2.读取由浏览器发送的隐式请求数据。图 1-1 中显示了一条从客户端到Web 服务器的单箭头，但实际上从客户端传送到 Web 服务器的数据有两种，它们分别为用户在表单中输入的显式数据，以及后台的 HTTP 信息。两种数据都很重要。 HTTP 信息包括 c
50、ookie、浏览器所能识别的媒体类型和压缩模式等。3.生成结果。这个过程可能需要访问数据库、执行RMI 或 EJB调用、调用 Web服务，或者直接计算得出对应的响应。 实际的数据可能存储在关系型数据库中。但是该数据库可能不理解HTTP，或者不能返回HTML 形式的结果，所以Web浏览器不能直接与数据库进行会话。 即使它能够做到这一点， 为了安全上的考虑， 我们也名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 8 页，共 13 页 - - - - - - - - - 不希望让它这么做。