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1、An Overview of Servlet and JSP TechnologyMarty Hall and Larry Brown，2000-07, Core Servlets and JavaServer Pages chapter 11.1 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 d
2、atabases or applications 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 cust
3、om 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 and the
4、 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 call, invoki
5、ng 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 probably would not wa
6、nt 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) to the client.This document can be sent in a variety of formats, including text (HTML or XML), binary (GIF images), or e
7、ven 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.Figure 1-1 shows a single arrow going from the Web middle
8、 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
9、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. In many cases, however
10、, 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-confirmation pages at online
11、 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 can be used to build t
12、he 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 response at request time. If it changes only periodically, ho
13、wever, 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
14、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 other server-side sources.If the information is in a dat
15、abase, 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 W
16、eb 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 do
17、es 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
18、, 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 discu
19、ssing 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.1EfficientWith traditional CGI, a new process is started for each H
20、TTP 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 t
21、raditional 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
22、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 e
23、ven 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 setting HTTP headers, handling cookies, tracking sessions, an
24、d 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 reusable code than does Visual Basic, VBScript, or C+. Why go
25、 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 cannot, at least not without using a server-specific API. Co
26、mmunicating 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 optimizations. Servlets can also maintain information from re
27、quest 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 virtually every major Web server. Consequently, servlets wri
28、tten 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 2 Platform, Enterprise Edition (J2EE; see so industry sup
29、port 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 or inexpensive server and migrate to more expensive serv
30、ers 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 purchase of a proprietary package.Price and portability are som
31、ewhat 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 (see in Manila, and there was great interest in servlet
32、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 that time) highly unfavorable currency exchange rates agains
33、t 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 regular Apache Web server, or embedded in Microsoft IIS). Onc
34、e 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 becomes even larger, they might want to move to a distri
35、buted (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 JavaBean
36、s (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
37、. 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. So, the CGI programmer must be careful to filter out ch
38、aracters 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
39、 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 of program memory. So, programmers who forget to perform
40、 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 operating system, it does not use a shell to do so. And, of
41、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 use them, what good are they? Servlet and JSP technology
42、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 Microsoft IIS and Zeus as well. They run on Windows, Unix/Lin
43、ux, 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 airline industry (most United Airlines and Delta Airline
44、s 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, popularity alone is no proof of good technology. Numerous c
45、ounter-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技术简介Marty Hall and Larry Brown，2000-07,chapter 11.1 Servlet的功能Servlets是运行在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客户端和HTTP服务器上的数据库或应用程序。Servl
46、et的工作是执行下面的任务，如图1-1所示 。图1-11. 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
47、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），二进制（GIF图像），甚至可以是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Servlet/JSP的重要任务之一就是将结果包
48、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是至关重要的。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servlet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
49、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），或者仅仅在用户请求该页面时再构建。具体应该采用哪种方式要根据具体情况而定，但后一种方式常常更为方便，因为它只需简单地等待用户的请求。例如，天气预报或新闻网站可能会动态地构建