【導(dǎo)讀】類(lèi)似CGI的技術(shù)相比，JavaServlet具有更高的效率，更容易使用，功能更強(qiáng)大，具有更好的可移植性，更節(jié)省投資。Servlet的工作是執(zhí)行西門(mén)的任務(wù)，如圖所示。最終用戶(hù)一般在頁(yè)面的HTML表單中輸入這些數(shù)據(jù)。applet或定制的HTTP客戶(hù)程序。兩種數(shù)據(jù)都很重要。HTTP信息包括cookie、瀏覽器所能識(shí)別的媒體類(lèi)型。計(jì)算得出對(duì)應(yīng)的響應(yīng)。實(shí)際的數(shù)據(jù)可能存儲(chǔ)在關(guān)系型數(shù)據(jù)庫(kù)中。HTTP，或者不能返回HTML形式的結(jié)果，所有Web瀏覽器不能直接與數(shù)據(jù)庫(kù)進(jìn)行會(huì)話(huà)。因此，我們需要Web中間層從HTTP流中提取輸。入數(shù)據(jù)，與應(yīng)用程序會(huì)話(huà)，并將結(jié)果嵌入到文檔中。式最常用的格式，故而servelt和JSP的重要任務(wù)之一就式將結(jié)果包裝到HTML中。發(fā)送隱式的HTTP響應(yīng)數(shù)據(jù)。然而，許多情況下靜態(tài)的結(jié)果不能滿(mǎn)足要求，我們需要針對(duì)每個(gè)請(qǐng)求生成。損失很少甚至沒(méi)有。而且，用于會(huì)話(huà)啟動(dòng)協(xié)議服務(wù)器的servletAPI最近已經(jīng)被標(biāo)準(zhǔn)化。單僅僅載入servlet類(lèi)的單一副本。最后，當(dāng)CGI程序結(jié)束對(duì)請(qǐng)求的處理之后，程序結(jié)束。

　　

【正文】 untries have large pools of welleducated software developers. Second, both countries have (or had, at that time) highly unfavorable currency exchange rates against the . dollar. So, buying a specialpurpose Web server from a . pany consumed a large part of early project funds. 12 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 the project starts to bee 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 bees 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 bees quite large and plex, 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 bees even bigger, they might move it off of their Linux box and onto an IBM mainframe running IBM WebSphere. But once again, none of their servlets or JSP pages have to be rewritten. 6． Secure One of the main sources of vulnerabilities in traditional CGI stems from the fact that the programs are often executed by generalpurpose operating system shells. 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 libraries. 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 100element array and then write into the 999th element, which is really some random part of program memory. So, programmers who fet 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 (., with 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. 7． Mainstream There are a lot of good technologies out there. But if vendors don39。t support them and developers don39。t 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/iPla, New Atlanta, ATG, Fujitsu, Lutris, Silverstream, the World Wide Web Consortium (W3C), and many others. Several lowcost plugins add support to Microsoft IIS and Zeus as well. They run on Windows, Unix/Linux, MacOS, VMS, and IBM mainframe operating systems. 13 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 Airlines Web sites), emerce (), 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 counterexamples abound. But our point is that you are not experimenting with a new and unproven technology when you work with serverside Java.