关于java技术毕业论文设计外文翻译.docx
关于java技术 毕业论文设计外文翻译 附录一外文资料原文： ABOUT JA V A TECHNOLOGY Servlets and JSP: An Overview 1.What are Java Servlets? Servlets are Java technology's answer to CGI programming. They are programs that run on a Web server and build Web pages. Building Web pages on the fly is useful (and commonly done) for a number of reasons: ?The Web page is based on data submitted by the user. For example the results pages from search engines are generated this way, and programs that process orders for e-commerce sites do this as well. ?The data changes frequently. For example, a weather-report or news headlines page might build the page dynamically, perhaps returning a previously built page if it is still up to date. ?The Web page uses information from corporate databases or other such sources. For example, you would use this for making a Web page at an on-line store that lists current prices and number of items in stock. 2.What are the Advantage of Servlets Over "Traditional" CGI? Java servlets are more efficient, easier to use, more powerful, more portable, and cheaper than traditional CGI and than many alternative CGI-like technologies. (More importantly, servlet developers get paid more than Perl programmers :-). ?Efficient. With traditional CGI, a new process is started for each HTTP request. If the CGI program does a relatively fast operation, the overhead of starting the process can dominate the execution time. With servlets, the Java Virtual Machine stays up, and each request is handled by a lightweight Java thread, not a heavyweight operating system process. Similarly, in traditional CGI, if there are N simultaneous request to the same CGI program, then the code for the CGI program is loaded into memory N times. With servlets, however, there are N threads but only a single copy of the servlet class. Servlets also have more alternatives than do regular CGI programs for optimizations such as caching previous computations, keeping database connections open, and the like. ?Convenient. Hey, you already know Java. Why learn Perl too? Besides the convenience of being able to use a familiar language, servlets have an extensive infrastructure for automatically parsing and decoding HTML form data, reading and setting HTTP headers, handling cookies, tracking sessions, and many other such utilities. ?Powerful. Java servlets let you easily do several things that are difficult or impossible with regular CGI. For one thing, servlets can talk directly to the Web server (regular CGI programs can't). This simplifies operations that need to look up images and other data stored in standard places. Servlets can also share data among each other, making useful things like database connection pools easy to implement. They can also maintain information from request to request, simplifying things like session tracking and caching of previous computations. ?Portable. Servlets are written in Java and follow a well-standardized API. Consequently, servlets written for, say I-Planet Enterprise Server can run virtually unchanged on Apache, Microsoft IIS, or WebStar. Servlets are supported directly or via a plugin on almost every major Web server. ?Inexpensive. There are a number of free or very inexpensive Web servers available that are good for "personal" use or low-volume Web sites. However, with the major exception of Apache, which is free, most commercial-quality Web servers are relatively expensive. Nevertheless, once you have a Web server, no matter the cost of that server, adding servlet support to it (if it doesn't come preconfigured to support servlets) is generally free or cheap. 3. What is JSP? Java Server Pages (JSP) is a technology that lets you mix regular, static HTML with dynamically-generated HTML. Many Web pages that are built by CGI programs are mostly static, with the dynamic part limited to a few small locations. But most CGI variations, including servlets, make you generate the entire page via your program, even though most of it is always the same. JSP lets you create the two parts separately. Here's an example: Welcome to Our Store Welcome to Our Store Welcome, To access your account settings, click here. Regular HTML for all the rest of the on-line store's Web page. 4. What are the Advantages of JSP? ?vs. Active Server Pages (ASP). ASP is a similar technology from Microsoft. The advantages of JSP are twofold. First, the dynamic part is written in Java, not Visual Basic or other MS-specific language, so it is more powerful and easier to use. Second, it is portable to other operating systems and non-Microsoft Web servers. ?vs. Pure Servlets. JSP doesn't give you anything that you couldn't in principle do with a servlet. But it is more convenient to write (and to modify!) regular HTML than to have a zillion println statements that generate the HTML. Plus, by separating the look from the content you can put different people on different tasks: your Web page design experts can build the HTML, leaving places for your servlet programmers to insert the dynamic content. ?vs. Server-Side Includes (SSI). SSI is a widely-supported technology for including externally-defined pieces into a static Web page. JSP is better because it lets you use servlets instead of a separate program to generate that dynamic part. Besides, SSI is really only intended for simple inclusions, not for "real" programs that use form data, make database connections, and the like. ?vs. JavaScript. JavaScript can generate HTML dynamically on the client. This is a useful capability, but only handles situations where the dynamic information is based on the client's environment. With the exception of cookies, HTTP and form submission data is not available to JavaScript. And, since it runs on the client, JavaScript can't access server-side resources like databases, catalogs, pricing information, and the like. ?vs. Static HTML. Regular HTML, of course, cannot contain dynamic information. JSP is so easy and convenient that it is quite feasible to augment HTML pages that only benefit marginally by the insertion of small amounts of dynamic data. Previously, the cost of using dynamic data would preclude its use in all but the most valuable instances. Overview JavaServer Pages (JSP) lets you separate the dynamic part of your pages from the static HTML. You simply write the regular HTML in the normal manner, using whatever Web-page-building tools you normally use. You then enclose the code for the dynamic parts in special tags, most of which start with "". For example, here is a section of a JSP page that results in something like "Thanks for ordering Core Web Programming"for URL of http:/host/OrderConfirmation.jsp?title=Core+Web+Programming: Thanks for ordering a You normally give your file a .jsp extension, and typically install it in any place you could place a normal Web page. Although what you write often looks more like a regular HTML file than a servlet, behind the scenes, the JSP page just gets converted to a normal servlet, with the static HTML simply being printed to the output stream associated with the servlet's service method. This is normally done the first time the page is requested, and developers can simply request the page themselves when first installing it if they want to be sure that the first real user doesn't get a momentary delay when the JSP page is translated to a servlet and the servlet is compiled and loaded. Note also that many Web servers let you define aliases that so that a URL that appears to reference an HTML file really points to a servlet or JSP page. Aside from the regular HTML, there are three main types of JSP constructs that you embed in a page: scripting elements, directives, and actions. Scripting elements let you specify Java code that will become part of the resultant servlet, directives let you control the overall structure of the servlet, and actions let you specify existing components that should be used, and otherwise control the behavior of the JSP engine. To simplify the scripting elements, you have access to a number of predefined variables such as request in the snippet above. Note that this tutorial covers version 1.0 of the JSP specification. JSP has changed dramatically since version 0.92, and although these changes were almost entirely for the better, you should note that version 1.0 JSP pages are almost totally incompatible with the earlier JSP engines. 2. Syntax Summary 3. Template Text: Static HTML In many cases, a large percent of your JSP page just consists of static HTML, known as template text. In all respects except one, this HTML looks just like normal HTML, follows all the same syntax rules, and is simply "passed through" to the client by the servlet created to handle the page. Not only does the HTML look normal, it can be created by whatever tools you already are using for building Web pages. For example, I used Allaire's HomeSite for most of the JSP pages in this tutorial. The one minor exception to the "template text is passed straight through" rule is that, if you want to have " Note that code inside a scriptlet gets inserted exactly as written, and any static HTML (template text) before or after a scriptlet gets converted to print statements. This means that scriptlets need not contain complete Java statements, and blocks left open can affect the static HTML outside of the scriptlets. For example, the following JSP fragment, containing mixed template text and scriptlets " inside a scriptlet, enter "%>" instead. Finally, note that the XML equivalent of is Code 4.3 JSP Declarations A JSP declaration lets you define methods or fields that get inserted into the main body of the servlet class (outside of the service method processing the request). It has the following form: Since declarations do not generate any output, they are normally used in conjunction with JSP expressions or scriptlets. For example, here is a JSP fragment that prints out the number of times the current page has been requested since the server booted (or the servlet class was changed and reloaded): Accesses to page since server reboot: As with scriptlets, if you want to use the characters "%>", enter "%>" instead. Finally, note that the XML equivalent of is Code 5. JSP Directives A JSP directive affects the overall structure of the servlet class. It usually has the following form: However, you can also combine multiple attribute settings for a single directive, as follows: There are two main types of directive: page, which lets you do things like import classes, customize the servlet superclass, and the like; and include, which lets you insert a file into the servlet class at the time the JSP file is translated into a servlet. The specification also mentions the taglib directive, which is not supported in JSP version 1.0, but is intended to let JSP authors define their own tags. It is expected that this will be the main new contribution of JSP 1.1. 5.1 The JSP page Directive The page directive lets you define one or more of the following case-sensitive attributes: import="package.class" or import="package.class1,.,package.classN". This lets you specify what packages should be imported. For example:
