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1、An Overview of Servlet and JSP TechnologyGildas Avoine and Philippe OechslinEPFL, Lausanne, Switzerland1.1 A Servlets JobServlets 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 applic
2、ations on the HTTP server. Their job is to perform the following tasks, as illustrated in Figure 1-1.Figure 1-11Read 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 a custom HTTP client pro
3、gram.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 and the behind-the-scenes
4、 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 call, invoking a Web service,
5、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 probably would not want it to. The same
6、 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.4Send the explicit data (i.e., the document) to the client.This document can be sent in a variety of formats
7、, 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.5Send the implicit HTTP response data.
8、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 respons
9、e 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 r
10、equests 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 f
11、rom 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 of data: explicit (i.e., HTML form data) and implicit (i.e., HTTP requ
12、est 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.2The Web page is derived from data that changes frequently.If the page changes for every request, then you certainly need to build the resp
13、onse 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, bu
14、t 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.3The Web page uses information from corporate databases or
15、 other server-side sources.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 nee
16、d 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 t
17、he 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 ca
18、n 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
19、use of servlets has not caught on, and well only be discussing HTTP servlets.1.3 The Advantages of Servlets Over Traditional CGIJava servlets are more efficient, easier to use, more powerful, more portable, safer, and cheaper than traditional CGI and many alternative CGI-like technologies.1Efficient
20、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, no
21、t 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 ap
22、proach 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
23、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.2ConvenientServlets have an extensive infrastructure for automatically parsing and decoding HTML form data, reading and sett
24、ing 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 reus
25、able code than does Visual Basic, VBScript, or C+. Why go back to those languages for server-side programming?3PowerfulServlets support several capabilities that are difficult or impossible to accomplish with regular CGI. Servlets can talk directly to the Web server, whereas regular CGI programs can
26、not, 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 can also share data, making it easy to implement database connection pooling and similar resource-sharing opti
27、mizations. Servlets can also maintain information from request to request, simplifying techniques like session tracking and caching of previous computations.4PortableServlets are written in the Java programming language and follow a standard API. Servlets are supported directly or by a plugin on vir
28、tually 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 plugin), IBM WebSphere, iPlanet Enterprise Server, Oracle9i AS, or StarNine WebStar. They are part of the Java
29、2 Platform, Enterprise Edition (J2EE; see so industry support for servlets is becoming even more pervasive.5InexpensiveA number of free or very inexpensive Web servers are good for development use or deployment of low- or medium-volume Web sites. Thus, with servlets and JSP you can start with a free
30、 or inexpensive server and migrate to more expensive servers with high-performance capabilities or advanced administration utilities only after your project meets initial success. This is in contrast to many of the other CGI alternatives, which require a significant initial investment for the purcha
31、se 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. India was near the top of the list, probably #2 behind the U.S. Marty also taught one of his JSP and servlet training courses
32、 (see in Manila, and there was great interest in servlet and JSP technology there.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 tha
33、t time) highly unfavorable currency exchange rates against the 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 regu
34、lar Apache Web server, or embedded in Microsoft IIS). Once the 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
35、 becomes even larger, they might want to move to a distributed (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 larg
36、e and complex, they might want to use Enterprise JavaBeans (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
37、Linux box and onto an IBM mainframe running IBM WebSphere. But once again, none of their servlets or JSP pages have to be rewritten.6SecureOne 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
38、. So, the CGI programmer must be careful to filter out characters 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 librar
39、ies.A second source of problems is the fact that some CGI 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 999th element, which is really some random part
40、 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 Runtime.exec or JNI) to invoke a program on the local ope
41、rating 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.7MainstreamThere are a lot of good technologies out there. But if vendors dont support them and developers dont know how to
42、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 many others. Several low-cost plugins add support to Micr
43、osoft 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 medium to large Web applications. They are used by the
44、 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 hundreds of other sites that you visit every day.Of course, po
45、pularity 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.Servlet和JSP技术简述Gildas Avoine and Philippe OechslinEPFL, Lausanne, Switzerland1.1 Servlet的功能Servlets是
46、运行在Web或应用服务器上的Java程序，它是一个中间层，负责连接来自Web浏览器或其他HTTP客户程序的请求和HTTP服务器上的数据库或应用程序。Servlet的工作是执行西门的任务，如图1.1所示 。图1.1Web中间件的作用（1） 读取客户发送的显式数据。最终用户一般在页面的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
47、息包括cookie、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。该数据库可能不理解HTTP，或者不能返回HTML形式的结果，所有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，我们也不希望让它这么做。对应大多数其他应用程序，也存在类似的问题。因此，我们需要Web中间层从HTTP流中提取输入数据，与应用程序会话，并将结果嵌入到文档中。（4） 向客户发送显式数据（即文档）。这个文档可以用各种格式发送，包括文本（HTML或XML
48、），二进制（GIF图），甚至可以式建立在其他底层格式之上的压缩格式，如gzip。但是，到目前为止，HTML式最常用的格式，故而servelt和JSP的重要任务之一就式将结果包装到HTML中。（5） 发送隐式的HTTP响应数据。图1.1中显示了一条从Web中间层到客户端的单箭头。但是，实际发送的数据有两种：文档本身，以及后台的HTTP信息。同样，两种数据对开发来说都式至关重要的。HTTP响应数据的发送过程涉及告知浏览器或其他客户程序所返回文档的类型（如HTML），设置cookie和缓存参数，以及其他类似的任务。1.2 动态构建网页的原因预先建立的文档可以满足客户的许多请求，服务器无需调用serv
49、let就可以处理这些请求。然而，许多情况下静态的结果不能满足要求，我们需要针对每个请求生成一个页面。实时构建页面的理由有很多种：1、网页基于客户发送的数据。例如，搜索引擎生成的页面，以及在线商店的订单确认页面，都要针对特定的用户请求而产生。在没有读取到用户提交的数据之前，我们不知道应该显示什么。要记住，用户提交两种类型的数据：显示（即HTML表单的数据）和隐式（即HTTP请求的报头）。两种输入都可用来构建输出页面。基于cookie值针对具体用户构建页面的情况尤其普遍。2、页面由频繁改变的数据导出。如果页面需要根据每个具体的请求做出相应的改变，当然需要在请求发生时构建响应。但是，如果页面周期性地改变，我们可以用两种方式来处理它：周期性地在服务器上构建新的页面（和客户请求无关），或
