Overview of JSP Technology
Benefits of JSP
JSP pages are translated into servlets. So, fundamentally, any task JSP pages can perform could also be accomplished by servlets. However, this underlying equivalence does not mean that servlets and JSP pages are equally appropriate in all scenarios. The issue is not the power of the technology, it is the convenience, productivity, and maintainability of one or the other. After all, anything you can do on a particular computer platform in the Java programming language you could also do in assembly language. But it still matters which you choose.
JSP provides the following benefits over servlets alone:
• It is easier to write and maintain the HTML. Your static code is ordinary HTML: no extra backslashes, no double quotes, and no lurking Java syntax.
• You can use standard Web-site development tools. Even HTML tools that know nothing about JSP can be used because they simply ignore the JSP tags.
• You can divide up your development team. The Java programmers can work on the dynamic code. The Web developers can concentrate on the presentation layer. On large projects, this division is very important. Depending on the size of your team and the complexity of your project, you can enforce a weaker or stronger separation between the static HTML and the dynamic content.
Now, this discussion is not to say that you should stop using servlets and use only JSP instead. By no means. Almost all projects will use both. For some requests in your project, you will use servlets. For others, you will use JSP. For still others, you will combine them with the MVC architecture . You want the appropriate tool for the job, and servlets, by themselves, do not complete your toolkit.
Advantages of JSP Over Competing Technologies
A number of years ago, Marty was invited to attend a small 20-person industry roundtable discussion on software technology. Sitting in the seat next to Marty was James Gosling, inventor of the Java programming language. Sitting several seats away was a high-level manager from a very large software company in Redmond, Washington. During the discussion, the moderator brought up the subject of Jini, which at that time was a new Java technology. The moderator asked the manager what he thought of it, and the manager responded that it was too early to tell, but that it seemed to be an excellent idea. He went on to say that they would keep an eye on it, and if it seemed to be catching on, they would follow his company's usual "embrace and extend" strategy. At this point, Gosling lightheartedly interjected "You mean disgrace and distend."
Now, the grievance that Gosling was airing was that he felt that this company would take technology from other companies and suborn it for their own purposes. But guess what? The shoe is on the other foot here. The Java community did not invent the idea of designing pages as a mixture of static HTML and dynamic code marked with special tags. For example, ColdFusion did it years earlier. Even ASP (a product from the very software company of the aforementioned manager) popularized this approach before JSP came along and decided to jump on the bandwagon. In fact, JSP not only adopted the general idea, it even used many of the same special tags as ASP did.
So, the question becomes: why use JSP instead of one of these other technologies? Our first response is that we are not arguing that everyone should. Several of those other technologies are quite good and are reasonable options in some situations. In other situations, however, JSP is clearly better. Here are a few of the reasons.
Versus .NET and Active Server Pages (ASP)
.NET is well-designed technology from Microsoft. ASP.NET is the part that directly competes with servlets and JSP. The advantages of JSP are twofold.
First, JSP is portable to multiple operating systems and Web servers; you aren't locked into deploying on Windows and IIS. Although the core .NET platform runs on a few non-Windows platforms, the ASP part does not. You cannot expect to deploy serious ASP.NET applications on multiple servers and operating systems. For some applications, this difference does not matter. For others, it matters greatly.
Second, for some applications the choice of the underlying language matters greatly. For example, although .NET's C# language is very well designed and is similar to Java, fewer programmers are familiar with either the core C# syntax or the many auxiliary libraries. In addition, many developers still use the original version of ASP. With this version, JSP has a clear advantage for the dynamic code. With JSP, the dynamic part is written in Java, not VBScript or another ASP-specific language, so JSP is more powerful and better suited to complex applications that require reusable components.
You could make the same argument when comparing JSP to the previous version of ColdFusion; with JSP you can use Java for the "real code" and are not tied to a particular server product. However, the current release of ColdFusion is within the context of a J2EE server, allowing developers to easily mix ColdFusion and servlet/JSP code.
Versus PHP
PHP (a recursive acronym for "PHP: Hypertext Preprocessor") is a free, open-source, HTML-embedded scripting language that is somewhat similar to both ASP and JSP. One advantage of JSP is that the dynamic part is written in Java, which already has an extensive API for networking, database access, distributed objects, and the like, whereas PHP requires learning an entirely new, less widely used language. A second advantage is that JSP is much more widely supported by tool and server vendors than is PHP.
Versus Pure Servlets
JSP doesn't provide any capabilities that couldn't, in principle, be accomplished with servlets. In fact, JSP documents are automatically translated into servlets behind the scenes. But it is more convenient to write (and to modify!) regular HTML than to use a zillion println statements to generate the HTML. Plus, by separating the presentation from the content, you can put different people on different tasks: your Web page design experts can build the HTML by using familiar tools and either leave places for your servlet programmers to insert the dynamic content or invoke the dynamic content indirectly by means of XML tags.
Does this mean that you can just learn JSP and forget about servlets? Absolutely not! JSP developers need to know servlets for four reasons:
1. JSP pages get translated into servlets. You can't understand how JSP works without understanding servlets.
2. JSP consists of static HTML, special-purpose JSP tags, and Java code. What kind of Java code? Servlet code! You can't write that code if you don't understand servlet programming.
3. Some tasks are better accomplished by servlets than by JSP. JSP is good at generating pages that consist of large sections of fairly well structured HTML or other character data. Servlets are better for generating binary data, building pages with highly variable structure, and performing tasks (such as redirection) that involve little or no output.
4. Some tasks are better accomplished by a combination of servlets and JSP than by either servlets or JSP alone.
Versus JavaScript
JavaScript, which is completely distinct from the Java programming language, is normally used to dynamically generate HTML on the client, building parts of the Web page as the browser loads the document. This is a useful capability and does not normally overlap with the capabilities of JSP (which runs only on the server). JSP pages still include SCRIPT tags for JavaScript, just as normal HTML pages do. In fact, JSP can even be used to dynamically generate the JavaScript that will be sent to the client. So, JavaScript is not a competing technology; it is a complementary one.
It is also possible to use JavaScript on the server, most notably on Sun ONE (formerly iPlanet), IIS, and BroadVision servers. However, Java is more powerful, flexible, reliable, and portable.
Versus WebMacro or Velocity
JSP is by no means perfect. Many people have pointed out features that could be improved. This is a good thing, and one of the advantages of JSP is that the specification is controlled by a community that draws from many different companies. So, the technology can incorporate improvements in successive releases.
However, some groups have developed alternative Java-based technologies to try to address these deficiencies. This, in our judgment, is a mistake. Using a third-party tool like Apache Struts that augments JSP and servlet technology is a good idea when that tool adds sufficient benefit to compensate for the additional complexity. But using a nonstandard tool that tries to replace JSP is a bad idea. When choosing a technology, you need to weigh many factors: standardization, portability, integration, industry support, and technical features. The arguments for JSP alternatives have focused almost exclusively on the technical features part. But portability, standardization, and integration are also very important. For example, the servlet and JSP specifications define a standard directory structure for Web applications and provide standard files (.war files) for deploying Web applications. All JSP-compatible servers must support these standards. Filters can be set up to apply to any number of servlets or JSP pages, but not to nonstandard resources. The same goes for Web application security settings.
Besides, the tremendous industry support for JSP and servlet technology results in improvements that mitigate many of the criticisms of JSP. For example, the JSP Standard Tag Library and the JSP 2.0 expression language address two of the most well-founded criticisms: the lack of good iteration constructs and the difficulty of accessing dynamic results without using either explicit Java code or verbose jsp:useBean elements.
10.4 Misconceptions About JSP
Forgetting JSP Is Server-Side Technology
Here are some typical questions Marty has received (most of them repeatedly).
• Our server is running JDK 1.4. So, how do I put a Swing component in a JSP page?
• How do I put an image into a JSP page? I do not know the proper Java I/O commands to read image files.
• Since Tomcat does not support JavaScript, how do I make images that are highlighted when the user moves the mouse over them?
• Our clients use older browsers that do not understand JSP. What should we do?
• When our clients use "View Source" in a browser, how can I prevent them from seeing the JSP tags?
All of these questions are based upon the assumption that browsers know something about the server-side process. But they do not. Thus:
• For putting applets with Swing components into Web pages, what matters is the browser's Java version—the server's version is irrelevant. If the browser supports the Java 2 platform, you use the normal APPLET (or Java plug-in) tag and would do so even if you were using non-Java technology on the server.
• You do not need Java I/O to read image files; you just put the image in the directory for Web resources (i.e., two levels up from WEB-INF/classes) and output a normal IMG tag.
• You create images that change under the mouse by using client-side JavaScript, referenced with the SCRIPT tag; this does not change just because the server is using JSP.
• Browsers do not "support" JSP at all—they merely see the output of the JSP page. So, make sure your JSP outputs HTML compatible with the browser, just as you would do with static HTML pages.
• And, of course you need not do anything to prevent clients from seeing JSP tags; those tags are processed on the server and are not part of the output that is sent to the client.
Confusing Translation Time with Request Time
A JSP page is converted into a servlet. The servlet is compiled, loaded into the server's memory, initialized, and executed. But which step happens when? To answer that question, remember two points:
• The JSP page is translated into a servlet and compiled only the first time it is accessed after having been modified.
• Loading into memory, initialization, and execution follow the normal rules for servlets.
Table 1 gives some common scenarios and tells whether or not each step occurs in that scenario. The most frequently misunderstood entries are highlighted. When referring to the table, note that servlets resulting from JSP pages use the _jspService method (called for both GET and POST requests), not doGet or doPost. Also, for initialization, they use the jspInit method, not the init method.
Table 1. JSP Operations in Various Scenarios
  JSP page translated into servlet Servlet compiled Servlet loaded into server's memory jspInit called _jspService called
Page first written
Request 1 Yes Yes Yes Yes Yes
Request 2 No No No No Yes
Server restarted
Request 3 No No Yes Yes Yes
Request 4 No No No No Yes
Page modified
Request 5 Yes Yes Yes Yes Yes
Request 6 No No No No Yes












 JSP中HTML的编写与维护更为简单。JSP中可以使用常规的HTML:没有额外的反斜杠,没有额外的双引号,也没有暗含的Java语法。
 能够使用标准的网站开发工具。即使对那些对JSP一无所知的HTML工具,我们也可以使用,因为它们会忽略JSP标签(JSP tags)。
 可以对开发团队进行划分。Java程序员可以致力于动态代码。Web开发人员可以将经理集中在表示层(presentation layer)上。对于大型的项目,这种划分极为重要。依据开发团队的大小,及项目的复杂程度,可以对静态HTML和动态内容进行弱分离(weaker separation)和强分离(stronger separation)。
许多年前,Marty受到邀请,参加一个有关软件技术的小型(20个人)研讨会.做在Marty旁边的人是James Gosling--- Java编程语言的发明者。隔几个位置,是来自华盛顿一家大型软件公司的高级经理。在讨论过程中,研讨会的主席提出了Jini的议题,这在当时是一项新的Java技术.主席向该经理询问他的想法.他继续说,他们会持续关注这项技术,如果这项技术变得流行起来,他们会遵循公司的“接受并扩充(embrace and extend)”的策略.此时, Gosling随意地插话说“你的意思其实就是不接受且不扩充(disgrace and distend)。”
在此, Gosling的抱怨显示出,他感到这个公司会从其他公司那里拿走技术,用于他们自己的目的.但你猜这次怎么样?这次鞋子穿在了另一只脚上。Java社团没有发明这一思想----将页面设计成由静态HTML和用特殊标签标记的动态代码混合组成.。ColdFusion多年前就已经这样做了。甚至ASP(来自于前述经理所在公司的一项产品)都在JSP出现之前推广了这种方式。实际上,JSP不只采用了这种通用概念,它甚至使用许多和ASP相同的特殊标签。
与.NET和Active Server Pages (ASP)相比
首先,JSP可以移植到多种操作系统和Web服务器,您不必仅仅局限于部署在Windows 和IIS上尽管核心.NET平台可以在好几种非Windows平台上运行,但ASP这一部分不可以。您不能期望可以将重要的ASP.NET应用部署到多种服务器和操作系统。对于某些应用,这种差异没有什么影响。但有些应用,这种差异却非常重要。
PHP(“PHP:Hypertext Preprocessor”的递归字母缩写词)是免费的、开放源代码的、HTML嵌入其中的脚本语言,与ASP和JSP都有某种程度的类似。JSP的一项优势是动态部分用Java编写,而Java已经在联网、数据库访问、分布式对象等方面拥有广泛的API,而PHP需要学习全新的、应用相对广泛的语言。JSP的第二项优势是,和PHP相比,JSP拥有极为广泛的工具和服务器提供商的支持。
JavaScript也可以用在服务器端,最因人注意的是SUN ONE(以前的iPlanet)、IIS和BroadVision服务器。然而,Java更为强大灵活、可靠且可移植。
但是,一些组织已经开发出了基于Java的替代技术,试图弥补这些不足。据我们的判断,这样做是错误的。使用扩充JSP和Servlet技术的第三方工具,如Apache Structs,是一种很好的思路,只要该工具带来的好处能够补偿工具带来的额外复杂性。但是,试图使用非标准的工具代替JSP则不理想。在选择一项技术时,需要权衡许多方面的因素:标准化、可移植性、集成性、行业支持和技术特性。对于JSP替代技术的争论几乎只是集中在技术特性上,而可移植性、标准化和集成性也十分重要。例如,Servlet和JSP规范为Web应用定义了一个标准的目录结构,并提供用于部署Web应用的标准文件(.war文件)。所有JSP兼容的服务器必须支持这些标准。我们可以建立过滤器作用到任意树木的Servlet和JSP页面上,但不能用于非标准资源。Web应用安全设置也同样如此。
此外,业界对JSP和Servlet技术的巨大支持使得这两项技术都有了巨大的进步,从而减轻了对JSP的许多批评。例如,JSP标准标签库和JSP 2.0表达式语言解决了两种最广泛的批评:缺乏良好的迭代结构;不使用显式的Java代码或冗长的jsp:useBean元素难以访问动态结果。
 我们的服务器正在运行JDK1.4。我如何将Swing组件用到JSP页面中呢?
 我如何将图像放到JSP页面中?我不知道读取图像文件应该使用哪些Java I/O命令。
 Tomcat不支持JavaScript,当用户在图像上移动鼠标时,我如何使图像突出显示呢?
 我们的客户使用不理解JSP的旧浏览器。我应该怎么做?
 当我们的客户在浏览器中使用“View Source”(查看源代码)时,如何阻止他们看到JSP标签?
 如果要将使用Swing组件的applet放到网页中,重要的是浏览器的Java版本,和服务器的Java版本无关。如果浏览器支持Java 2平台,您可以使用正常的APPLET(或Java插件)标签,即使在服务器上使用了非Java技术也须如此。
 您不需要Java I/O来读取图像文件,您只需将图像放在存储Web资源的目录中(即WEB-INF/classes向上两级的目录),并输出一个正常的IMG标签。
 您应该用SCRIPT标签,使用客户端JavaScript创建在鼠标下会更改的图像,这不会由于服务器使用JSP而改变。
 浏览器根本不“支持”JSP----它们看到的只是JSP页面的输出。因此,如同对待静态HTML页面一样,只需确保JSP输出的HTML与浏览器兼容。
 当然,您不需要采取什么措施来阻止客户看到JSP标签,这些标签在服务器上进行处理,发送给客户的输出中并不出现。
 JSP页面仅在修改后第一次被访问时,才会被转换成servlet并进行编译;
 载入到内存中、初始化和执行遵循servlet的一般规则。

表1 各种情况下的JSP操作
  将JSP 页面转换成servlet 编译Servlet  将Servlet 载入到服务器内存中 调用jspInit  调用_jspService
请求 1 有 有 有 有 有
请求 2 无 无 无 无 有
请求3 无 无 有 有 有
请求 4 无 无 无 无 有
请求 5 有 有 有 有 有
请求 6 无 无 无 无 有