JAVA外文文献+翻译

JAVA外文文献+翻译 燕山大学本科生毕业 设计 领导形象设计圆作业设计ao工艺污水处理厂设计附属工程施工组织设计清扫机器人结构设计 (翻译) Java and the Internet If Java is, in fact, yet another computer programming language, you may question why it is so important and why it is being promoted as a revolutionary step in computer programming. The answer isn’t immediately obvious if you’re coming from a traditional programming perspective. Although Java is very useful for solving traditional stand-alone programming problems, it is also important because it will solve programming problems on the World Wide Web. 1. Client-side programming The Web’s initial server-browser design provided for interactive content, but the interactivity was completely provided by the server. The server produced static pages for the client browser, which would simply interpret and display them. Basic HTML contains simple mechanisms for data gathering: text-entry boxes, check boxes, radio boxes, lists and drop-down lists, as well as a button that can only be programmed to reset the data on the form or “submit” the data on the form back to the server. This submission passes through the Common Gateway Interface (CGI) provided on all Web servers. The text within the submission tells CGI what to do with it. The most common action is to run a program located on the server in a directory that’s typically called “cgi-bin.” (If you watch the address window at the top of your browser when you push a button on a Web page, you can sometimes see “cgi-bin” within all the gobbledygook there.) These programs can be written in most languages. Perl is a common choice because it is designed for text manipulation and is interpreted, so it can be installed on any server regardless of processor or operating system. Many powerful Web sites today are built strictly on CGI, and you can in fact do nearly anything with it. However, Web sites built on CGI programs can rapidly become overly complicated to maintain, and there is also the problem of response time. The response of a CGI program depends on how much data must 1 燕山大学本科生毕业设计(翻译) be sent, as well as the load on both the server and the Internet. (On top of this, starting a CGI program tends to be slow.) The initial designers of the Web did not foresee how rapidly this bandwidth would be exhausted for the kinds of applications people developed. For example, any sort of dynamic graphing is nearly impossible to perform with consistency because a GIF file must be created and moved from the server to the client for each version of the graph. And you’ve no doubt had direct experience with something as simple as validating the data on an input form. You press the submit button on a page; the data is shipped back to the server; the server starts a CGI program that discovers an error, formats an HTML page informing you of the error, and then sends the page back to you; you must then back up a page and try again. Not only is this slow, it’s inelegant. The solution is client-side programming. Most machines that run Web browsers are powerful engines capable of doing vast work, and with the original static HTML approach they are sitting there, just idly waiting for the server to dish up the next page. Client-side programming means that the Web browser is harnessed to do whatever work it can, and the result for the user is a much speedier and more interactive experience at your Web site. The problem with discussions of client-side programming is that they aren’t very different from discussions of programming in general. The parameters are almost the same, but the platform is different: a Web browser is like a limited operating system. In the end, you must still program, and this accounts for the dizzying array of problems and solutions produced by client-side programming. The rest of this section provides an overview of the issues and approaches in client-side programming. 2.Plug-ins One of the most significant steps forward in client-side programming is the development of the plug-in. This is a way for a programmer to add new functionality to the browser by downloading a piece of code that plugs itself into the appropriate spot in the browser. It tells the browser “from now on you can 2 燕山大学本科生毕业设计(翻译) perform this new activity.” (You need to download the plug-in only once.) Some fast and powerful behavior is added to browsers via plug-ins, but writing a plug-in is not a trivial task, and isn’t something you’d want to do as part of the process of building a particular site. The value of the plug-in for client-side programming is that it allows an expert programmer to develop a new language and add that language to a browser without the permission of the browser manufacturer. Thus, plug-ins provide a “back door” that allows the creation of new client-side programming languages (although not all languages are implemented as plug-ins). 3.Scripting languages Plug-ins resulted in an explosion of scripting languages. With a scripting language you embed the source code for your client-side program directly into the HTML page, and the plug-in that interprets that language is automatically activated while the HTML page is being displayed. Scripting languages tend to be reasonably easy to understand and, because they are simply text that is part of an HTML page, they load very quickly as part of the single server hit required to procure that page. The trade-off is that your code is exposed for everyone to see (and steal). Generally, however, you aren’t doing amazingly sophisticated things with scripting languages so this is not too much of a hardship. This points out that the scripting languages used inside Web browsers are really intended to solve specific types of problems, primarily the creation of richer and more interactive graphical user interfaces (GUIs). However, a scripting language might solve 80 percent of the problems encountered in client-side programming. Your problems might very well fit completely within that 80 percent, and since scripting languages can allow easier and faster development, you should probably consider a scripting language before looking at a more involved solution such as Java or ActiveX programming. The most commonly discussed browser scripting languages are JavaScript (which has nothing to do with Java; it’s named that way just to grab some of Java’s marketing momentum), VBScript (which looks like Visual Basic), and 3 燕山大学本科生毕业设计(翻译) Tcl/Tk, which comes from the popular cross-platform GUI-building language. There are others out there, and no doubt more in development. JavaScript is probably the most commonly supported. It comes built into both Netscape Navigator and the Microsoft Internet Explorer (IE). In addition, there are probably more JavaScript books available than there are for the other browser languages, and some tools automatically create pages using JavaScript. However, if you’re already fluent in Visual Basic or Tcl/Tk, you’ll be more productive using those scripting languages rather than learning a new one. (You’ll have your hands full dealing with the Web issues already.) 4.Java If a scripting language can solve 80 percent of the client-side programming problems, what about the other 20 percent—the “really hard stuff?” The most popular solution today is Java. Not only is it a powerful programming language built to be secure, cross-platform, and international, but Java is being continually extended to provide language features and libraries that elegantly handle problems that are difficult in traditional programming languages, such as multithreading, database access, network programming, and distributed computing. Java allows client-side programming via the applet. An applet is a mini-program that will run only under a Web browser. The applet is downloaded automatically as part of a Web page (just as, for example, a graphic is automatically downloaded). When the applet is activated it executes a program. This is part of its beauty—it provides you with a way to automatically distribute the client software from the server at the time the user needs the client software, and no sooner. The user gets the latest version of the client software without fail and without difficult reinstallation. Because of the way Java is designed, the programmer needs to create only a single program, and that program automatically works with all computers that have browsers with built-in Java interpreters. (This safely includes the vast majority of machines.) Since Java is a full-fledged programming language, you can do as much work as possible on the client before and after making requests of the 4 燕山大学本科生毕业设计(翻译) server. For example, you won’t need to send a request form across the Internet to discover that you’ve gotten a date or some other parameter wrong, and your client computer can quickly do the work of plotting data instead of waiting for the server to make a plot and ship a graphic image back to you. Not only do you get the immediate win of speed and responsiveness, but the general network traffic and load on servers can be reduced, preventing the entire Internet from slowing down. One advantage a Java applet has over a scripted program is that it’s in compiled form, so the source code isn’t available to the client. On the other hand, a Java applet can be decompiled without too much trouble, but hiding your code is often not an important issue. Two other factors can be important. As you will see later in this book, a compiled Java applet can comprise many modules and take multiple server “hits” (accesses) to download. (In Java 1.1 and higher this is minimized by Java archives, called JAR files, that allow all the required modules to be packaged together and compressed for a single download.) A scripted program will just be integrated into the Web page as part of its text (and will generally be smaller and reduce server hits). This could be important to the responsiveness of your Web site. Another factor is the all-important learning curve. Regardless of what you’ve heard, Java is not a trivial language to learn. If you’re a Visual Basic programmer, moving to VBScript will be your fastest solution, and since it will probably solve most typical client/server problems you might be hard pressed to justify learning Java. If you’re experienced with a scripting language you will certainly benefit from looking at JavaScript or VBScript before committing to Java, since they might fit your needs handily and you’ll be more productive sooner.to run its applets withi 5.ActiveX To some degree, the competitor to Java is Microsoft’s ActiveX, although it takes a completely different approach. ActiveX was originally a Windows-only solution, although it is now being developed via an independent consortium to become cross-platform. Effectively, ActiveX says “if your program connects to 5 燕山大学本科生毕业设计(翻译) its environment just so, it can be dropped into a Web page and run under a browser that supports ActiveX.” (IE directly supports ActiveX and Netscape does so using a plug-in.) Thus, ActiveX does not constrain you to a particular language. If, for example, you’re already an experienced Windows programmer using a language such as C++, Visual Basic, or Borland’s Delphi, you can create ActiveX components with almost no changes to your programming knowledge. ActiveX also provides a path for the use of legacy code in your Web pages. 6.Security Automatically downloading and running programs across the Internet can sound like a virus-builder’s dream. ActiveX especially brings up the thorny issue of security in client-side programming. If you click on a Web site, you might automatically download any number of things along with the HTML page: GIF files, script code, compiled Java code, and ActiveX components. Some of these are benign; GIF files can’t do any harm, and scripting languages are generally limited in what they can do. Java was also designed to run its applets within a “sandbox” of safety, which prevents it from writing to disk or accessing memory outside the sandbox. ActiveX is at the opposite end of the spectrum. Programming with ActiveX is like programming Windows—you can do anything you want. So if you click on a page that downloads an ActiveX component, that component might cause damage to the files on your disk. Of course, programs that you load onto your computer that are not restricted to running inside a Web browser can do the same thing. Viruses downloaded from Bulletin-Board Systems (BBSs) have long been a problem, but the speed of the Internet amplifies the difficulty. The solution seems to be “digital signatures,” whereby code is verified to show who the author is. This is based on the idea that a virus works because its creator can be anonymous, so if you remove the anonymity individuals will be forced to be responsible for their actions. This seems like a good plan because it allows programs to be much more functional, and I suspect it will eliminate malicious mischief. If, however, a program has an unintentional destructive bug 6 燕山大学本科生毕业设计(翻译) it will still cause problems. The Java approach is to prevent these problems from occurring, via the sandbox. The Java interpreter that lives on your local Web browser examines the applet for any untoward instructions as the applet is being loaded. In particular, the applet cannot write files to disk or erase files (one of the mainstays of viruses). Applets are generally considered to be safe, and since this is essential for reliable client/server systems, any bugs in the Java language that allow viruses are rapidly repaired. (It’s worth noting that the browser software actually enforces these security restrictions, and some browsers allow you to select different security levels to provide varying degrees of access to your system.) You might be skeptical of this rather draconian restriction against writing files to your local disk. For example, you may want to build a local database or save data for later use offline. The initial vision seemed to be that eventually everyone would get online to do anything important, but that was soon seen to be impractical (although low-cost “Internet appliances” might someday satisfy the needs of a significant segment of users). The solution is the “signed applet” that uses public-key encryption to verify that an applet does indeed come from where it claims it does. A signed applet can still trash your disk, but the theory is that since you can now hold the applet creator accountable they won’t do vicious things. Java provides a framework for digital signatures so that you will eventually be able to allow an applet to step outside the sandbox if necessary. Digital signatures have missed an important issue, which is the speed that people move around on the Internet. If you download a buggy program and it does something untoward, how long will it be before you discover the damage? It could be days or even weeks. By then, how will you track down the program that’s done it? And what good will it do you at that point? 7.Internet vs. intranet The Web is the most general solution to the client/server problem, so it makes sense that you can use the same technology to solve a subset of the problem, in particular the classic client/server problem within a company. With traditional 7 燕山大学本科生毕业设计(翻译) client/server approaches you have the problem of multiple types of client computers, as well as the difficulty of installing new client software, both of which are handily solved with Web browsers and client-side programming. When Web technology is used for an information network that is restricted to a particular company, it is referred to as an intranet. Intranets provide much greater security than the Internet, since you can physically control access to the servers within your company. In terms of training, it seems that once people understand the general concept of a browser it’s much easier for them to deal with differences in the way pages and applets look, so the learning curve for new kinds of systems seems to be reduced. The security problem brings us to one of the divisions that seems to be automatically forming in the world of client-side programming. If your program is running on the Internet, you don’t know what platform it will be working under, and you want to be extra careful that you don’t disseminate buggy code. You need something cross-platform and secure, like a scripting language or Java. If you’re running on an intranet, you might have a different set of constraints. It’s not uncommon that your machines could all be Intel/Windows platforms. On an intranet, you’re responsible for the quality of your own code and can repair bugs when they’re discovered. In addition, you might already have a body of legacy code that you’ve been using in a more traditional client/server approach, whereby you must physically install client programs every time you do an upgrade. The time wasted in installing upgrades is the most compelling reason to move to browsers, because upgrades are invisible and automatic. If you are involved in such an intranet, the most sensible approach to take is the shortest path that allows you to use your existing code base, rather than trying to recode your programs in a new language. When faced with this bewildering array of solutions to the client-side programming problem, the best plan of attack is a cost-benefit analysis. Consider the constraints of your problem and what would be the shortest path to your solution. Since client-side programming is still programming, it’s always a good idea to take the fastest development approach for your particular situation. 8 燕山大学本科生毕业设计(翻译) This is an aggressive stance to prepare for inevitable encounters with the problems of program development. 8.Server-side programming This whole discussion has ignored the issue of server-side programming. What happens when you make a request of a server? Most of the time the request is simply “send me this file.” Your browser then interprets the file in some appropriate fashion: as an HTML page, a graphic image, a Java applet, a script program, etc. A more complicated request to a server generally involves a database transaction. A common scenario involves a request for a complex database search, which the server then formats into an HTML page and sends to you as the result. (Of course, if the client has more intelligence via Java or a scripting language, the raw data can be sent and formatted at the client end, which will be faster and less load on the server.) Or you might want to register your name in a database when you join a group or place an order, which will involve changes to that database. These database requests must be processed via some code on the server side, which is generally referred to as server-side programming. Traditionally, server-side programming has been performed using Perl and CGI scripts, but more sophisticated systems have been appearing. These include Java-based Web servers that allow you to perform all your server-side programming in Java by writing what are called servlets. Servlets and their offspring, JSPs, are two of the most compelling reasons that companies who develop Web sites are moving to Java, especially because they eliminate the problems of dealing with differently abled browsers. 9. separate arena: applications Much of the brouhaha over Java has been over applets. Java is actually a general-purpose programming language that can solve any type of problem—at least in theory. And as pointed out previously, there might be more effective 9 燕山大学本科生毕业设计(翻译) ways to solve most client/server problems. When you move out of the applet arena (and simultaneously release the restrictions, such as the one against writing to disk) you enter the world of general-purpose applications that run standalone, without a Web browser, just like any ordinary program does. Here, Java’s strength is not only in its portability, but also its programmability. As you’ll see throughout this book, Java has many features that allow you to create robust programs in a shorter period than with previous programming languages. Be aware that this is a mixed blessing. You pay for the improvements through slower execution speed (although there is significant work going on in this area—JDK 1.3, in particular, introduces the so-called “hotspot” performance improvements). Like any language, Java has built-in limitations that might make it inappropriate to solve certain types of programming problems. Java is a rapidly evolving language, however, and as each new release comes out it becomes more and more attractive for solving larger sets of problems. 10 燕山大学本科生毕业设计(翻译) Java和因特网 既然Java不过另一种类型的程序设计语言，大家可能会奇怪它为什么值得如此重视，为什么还有这么多的人认为它是计算机程序设计的一个里程碑呢,如果您来自一个传统的程序设计背景，那么答案在刚开始的时候并不是很明显。Java除了可解决传统的程序设计问题以外，还能解决World Wide Web(万维网)上的编程问题。 1.客户端编程 Web最初采用的“服务器,浏览器” 方案 气瓶 现场处置方案 .pdf气瓶 现场处置方案 .doc见习基地管理方案.doc关于群访事件的化解方案建筑工地扬尘治理专项方案下载 可提供交互式 内容 财务内部控制制度的内容财务内部控制制度的内容人员招聘与配置的内容项目成本控制的内容消防安全演练内容 ，但这种交互能力完全由服务器提供，为服务器和因特网带来了不小的负担。服务器一般为客户浏览器产生静态网页，由后者简单地解释并显示出来。基本HTML语言提供了简单的数据收集机制:文字输入框、复选框、单选钮、列表以及下拉列表等，另外还有一个按钮，只能由程序规定重新设置表单中的数据，以便回传给服务器。用户提交的信息通过所有Web服务器均能支持的“通用网关接口”(CGI)回传到服务器。包含在提交数据中的文字指示CGI该如何操作。最常见的行动是运行位于服务器的一个程序。那个程序一般保存在一个名为“cgi-bin”的目录中(按下Web页内的一个按钮时，请注意一下浏览器顶部的地址窗，经常都能发现“cgi-bin”的字样)。大多数语言都可用来编制这些程序，但其中最常见的是Perl。这是由于Perl是专为文字的处理及解释而设计的，所以能在任何服务器上安装和使用，无论采用的处理器或操作系统是什么。 2. 插件 朝客户端编程迈进的时候，最重要的一个问题就是插件的设计。利用插件，程序员可以方便地为浏览器添加新功能，用户只需下载一些代码，把它们“插入”浏览器的适当位置即可。这些代码的作用是告诉浏览器“从现在开始，你可以进行这些新活动了”(仅需下载这些插入一次)。有些快速和功能强大的行为是通过插件添加到浏览器的。但插件的编写并不是一件 11 燕山大学本科生毕业设计(翻译) 简单的任务。在我们构建一个特定的站点时，可能并不希望涉及这方面的工作。对客户端程序设计来说，插件的价值在于它允许专业程序员设计出一种新的语言，并将那种语言添加到浏览器，同时不必经过浏览器原创者的许可。由此可以看出，插件实际是浏览器的一个“后门”，允许创建新的客户端程序设计语言(尽管并非所有语言都是作为插件实现的)。 3. 脚本编制语言 插件造成了脚本编制语言的爆炸性增长。通过这种脚本语言，可将用于自己客户端程序的源码直接插入HTML页，而对那种语言进行解释的插件会在HTML页显示的时候自动激活。脚本语言一般都倾向于尽量简化，易于理解。而且由于它们是从属于HTML页的一些简单正文，所以只需向服务器发出对那个页的一次请求，即可非常快地载入。缺点是我们的代码全部暴露在人们面前。另一方面，由于通常不用脚本编制语言做过分复杂的事情，所以这个问题暂且可以放在一边。 脚本语言真正面向的是特定类型问题的解决，其中主要涉及如何创建更丰富、更具有互动能力的图形用户界面(GUI)。然而，脚本语言也许能解决客户端编程中80,的问题。你碰到的问题可能完全就在那80,里面。而且由于脚本编制语言的宗旨是尽可能地简化与快速，所以在考虑其他更复杂的方案之前(如Java及ActiveX)，首先应想一下脚本语言是否可行。 目前讨论得最多的脚本编制语言包括JavaScript(它与Java没有任何关系;之所以叫那个名字，完全是一种市场策略)、VBScript(同Visual Basic 很相似)以及Tcl/Tk(来源于流行的跨平台GUI构造语言)。当然还有其他许多语言，也有许多正在开发中。 JavaScript也许是日常用的，它得到的支持也最全面。无论NetscapeNavigator，Microsoft Internet Explorer，还是Opera，目前都提供了对JavaScript的支持。除此以外，市面上讲述JavaScript的书籍也要比讲述其他语言的书多得多。有些工具还能利用JavaScript自动产生网页。当然，如果你已经有Visual Basic或者Tcl/Tk的深厚功底，当然用它们要简单得多，起码可以避免学习新语言的烦恼(解决Web方面的问题就已经够让人头痛了)。 12 燕山大学本科生毕业设计(翻译) 4. Java 如果说一种脚本编制语言能解决80,的客户端程序设计问题，那么剩下的20,又该怎么办呢,它们属于一些高难度的问题吗,目前最流行的方案就是Java。它不仅是一种功能强大、高度安全、可以跨平台使用以及国际通用的程序设计语言，也是一种具有旺盛生命力的语言。对Java的扩展是不断进行的，提供的语言特性和库能够很好地解决传统语言不能解决的问题，比如多线程操作、数据库访问、连网程序设计以及分布式计算等等。Java通过“程序片”(Applet)巧妙地解决了客户端编程的问题。 程序片(或“小应用程序”)是一种非常小的程序，只能在Web浏览器中运行。作为Web页的一部分，程序片代码会自动下载回来(这和网页中的图片差不多)。激活程序片后，它会执行一个程序。程序片的一个优点体现在:通过程序片，一旦用户需要客户软件，软件就可从服务器自动下载回来。它们能自动取得客户软件的最新版本，不会出错，也没有重新安装的麻烦。由于Java的设计原理，程序员只需要创建程序的一个版本，那个程序能在几乎所有计算机以及安装了Java解释器的浏览器中运行。由于Java是一种全功能的编程语言，所以在向服务器发出一个请求之前，我们能先在客户端做完尽可能多的工作。例如，再也不必通过因特网传送一个请求表单，再由服务器确定其中是否存在一个拼写或者其他参数错误。大多数数据校验工作均可在客户端完成，没有必要坐在计算机前面焦急地等待服务器的响应。这样一来，不仅速度和响应的灵敏度得到了极大的提高，对网络和服务器造成的负担也可以明显减轻，这对保障因特网的畅通是至关重要的。 与脚本程序相比，Java程序片的另一个优点是它采用编译好的形式，所以客户端看不到源码。当然在另一方面，反编译Java程序片也并不是件难事，而且代码的隐藏一般并不是个重要的问题。大家要注意另外两个重要的问题。正如本书以前会讲到的那样，编译好的Java程序片可能包含了许多模块，所以要多次“命中”(访问)服务器以便下载(在Java 1.1中，这个问题得到了有效的改善——利用Java压缩档，即JAR文件——它允许设计者将所有必要的模块都封装到一起，供用户统一下载)。在另一方面，脚本程序是作为Web页正文的一部分集成到Web页内的。这种程序一般都非 13 燕山大学本科生毕业设计(翻译) 常小，可有效减少对服务器的点击数。另一个因素是学习方面的问题。不管你平时听别人怎么说，Java都不是一种十分容易便可学会的语言。如果你以前是一名Visual Basic程序员，那么转向VBScript会是一种最快捷的方案。由于VBScript可以解决大多数典型的客户机,服务器问题，所以一旦上手，就很难下定决心再去学习Java。如果对脚本编制语言比较熟，那么在转向Java之前，建议先熟悉一下JavaScript或者VBScript，因为它们可能已经能够满足你的需要，不必经历学习Java的艰苦过程。 5. ActiveX 在某种程度上，Java的一个有力竞争对手应该是微软的ActiveX，尽管它采用的是完全不同的一套实现机制。ActiveX最早是一种纯Windows的方案。经过一家独立的专业协会的努力，ActiveX现在已具备了跨平台使用的能力。实际上，ActiveX的意思是“假如你的程序同它的工作环境正常连接，它就能进入Web页，并在支持ActiveX的浏览器中运行”(IE固化了对ActiveX的支持，而Netscape需要一个插件)。所以，ActiveX并没有限制我们使用一种特定的语言。比如，假设我们已经是一名有经验的Windows程序员，能熟练地使用像C++、Visual Basic或者Borland Delphi那样的语言，就能几乎不加任何学习地创建出ActiveX组件。事实上，ActiveX是在我们的Web页中使用“历史遗留”代码的最佳途径。 6. 安全 自动下载和通过因特网运行程序听起来就像是一个病毒制造者的梦想。在客户端的编程中，ActiveX带来了最让人头痛的安全问题。点击一个Web站点的时候，可能会随同HTML网页传回任何数量的东西:GIF文件、脚本代码、编译好的Java代码以及ActiveX组件。有些是无害的;GIF文件不会对我们造成任何危害，而脚本编制语言通常在自己可做的事情上有着很大的限制。Java也设计成在一个安全“沙箱”里在它的程序片中运行，这样可防止操作位于沙箱以外的磁盘或者内存区域。 ActiveX是所有这些里面最让人担心的。用ActiveX编写程序就像编制 14 燕山大学本科生毕业设计(翻译) Windows应用程序——可以做自己想做的任何事情。下载回一个ActiveX组件后，它完全可能对我们磁盘上的文件造成破坏。当然，对那些下载回来并不限于在Web浏览器内部运行的程序，它们同样也可能破坏我们的系统。从BBS下载回来的病毒一直是个大问题，但因特网的速度使得这个问题变得更加复杂。 目前解决的办法是“数字签名”，代码会得到权威机构的验证，显示出它的作者是谁。这一机制的基础是认为病毒之所以会传播，是由于它的编制者匿名的缘故。所以假如去掉了匿名的因素，所有设计者都不得不为它们的行为负责。这似乎是一个很好的主意，因为它使程序显得更加正规。但我对它能消除恶意因素持怀疑态度，因为假如一个程序便含有Bug，那么同样会造成问题。 Java通过“沙箱”来防止这些问题的发生。Java解释器内嵌于我们本地的Web浏览器中，在程序片装载时会检查所有有嫌疑的指令。特别地，程序片根本没有权力将文件写进磁盘，或者删除文件(这是病毒最喜欢做的事情之一)。我们通常认为程序片是安全的。而且由于安全对于营建一套可靠的客户机,服务器系统至关重要，所以会给病毒留下漏洞的所有错误都能很快得到修复(浏览器软件实际需要强行遵守这些安全规则;而有些浏览器则允许我们选择不同的安全级别，防止对系统不同程度的访问)。 大家或许会怀疑这种限制是否会妨碍我们将文件写到本地磁盘。比如，我们有时需要构建一个本地数据库，或将数据保存下来，以便日后离线使用。最早的版本似乎每个人都能在线做任何敏感的事情，但这很快就变得非常不现实(尽管低价“互联网工具”有一天可能会满足大多数用户的需要)。解决的方案是“签了名的程序片”，它用公共密钥加密算法验证程序片确实来自它所声称的地方。当然在通过验证后，签了名的一个程序片仍然可以开始清除你的磁盘。但从理论上说，既然现在能够找到创建人“算帐”，他们一般不会干这种蠢事。Java 1.1为数字签名提供了一个框架，在必要时，可让一个程序片“走”到沙箱的外面来。 数字签名遗漏了一个重要的问题，那就是人们在因特网上移动的速度。如下载回一个错误百出的程序，而它很不幸地真的干了某些蠢事，需要多久的时间才能发觉这一点呢,这也许是几天，也可能几周之后。发现了之后，又如何追踪当初肇事的程序呢(以及它当时的责任有多大), 15 燕山大学本科生毕业设计(翻译) 7. 因特网和内联网 Web是解决客户机,服务器问题的一种常用方案，所以最好能用相同的技术解决此类问题的一些“子集”，特别是公司内部的传统客户机,服务器问题。对于传统的客户机,服务器模式，我们面临的问题是拥有多种不同类型的客户计算机，而且很难安装新的客户软件。但通过Web浏览器和客户端编程，这两类问题都可得到很好的解决。若一个信息网络局限于一家特定的公司，那么在将Web技术应用于它之后，即可称其为“内联网”(Intranet)，以示与国际性的“因特网”(Internet)有别。内联网提供了比因特网更高的安全级别，因为可以物理性地控制对公司内部服务器的使用。说到培训，一般只要人们理解了浏览器的常规概念，就可以非常轻松地掌握网页和程序片之间的差异，所以学习新型系统的开销会大幅度减少。 安全问题将我们引入客户端编程领域一个似乎是自动形成的分支。若程序是在因特网上运行，由于无从知晓它会在什么平台上运行，所以编程时要特别留意，防范可能出现的编程错误。需作一些跨平台处理，以及适当的安全防范，比如采用某种脚本语言或者Java。 但假如在内联网中运行，面临的一些制约因素就会发生变化。全部机器均为Intel/Windows平台是件很平常的事情。在内联网中，需要对自己代码的质量负责。而且一旦发现错误，就可以马上改正。除此以外，可能已经有了一些“历史遗留”的代码，并用较传统的客户机,服务器方式使用那些代码。但在进行升级时，每次都要物理性地安装一道客户程序。浪费在升级安装上的时间是转移到浏览器的一项重要原因。使用了浏览器后，升级就变得易如反掌，而且整个过程是透明和自动进行的。如果真的是牵涉到这样的一个内联网中，最明智的方法是采用ActiveX，而非试图采用一种新的语言来改写程序代码。 面临客户端编程问题令人困惑的一系列解决方案时，最好的方案是先做一次投资,回报分析。请 总结 初级经济法重点总结下载党员个人总结TXt高中句型全总结.doc高中句型全总结.doc理论力学知识点总结pdf 出问题的全部制约因素，以及什么才是最快的方案。由于客户端程序设计仍然要编程，所以无论如何都该针对自己的特定情况采取最好的开发途径。这是准备面对程序开发中一些不可避免的问题时，我们可以做出的最佳姿态。 16 燕山大学本科生毕业设计(翻译) 8服务器端编程 我们的整个讨论都忽略了服务器端编程的问题。如果向服务器发出一个请求，会发生什么事情,大多数时候的请求都是很简单的一个“把这个文件发给我”。浏览器随后会按适当的形式解释这个文件:作为HTML页、一幅图、一个Java程序片、一个脚本程序等等。向服务器发出的较复杂的请求通常涉及到对一个数据库进行操作(事务处理)。其中最常见的就是发出一个数据库检索命令，得到结果后，服务器会把它格式化成HTML页，并作为结果传回来(当然，假如客户通过Java或者某种脚本语言具有了更高的智能，那么原始数据就能在客户端发送和格式化;这样做速度可以更快，也能减轻服务器的负担)。另外，有时需要在数据库中注册自己的名字(比如加入一个组时)，或者向服务器发出一份订单，这就涉及到对那个数据库的修改。这类服务器请求必须通过服务器端的一些代码进行，我们称其为“服务器端的编程”。在传统意义上，服务器端编程是用Perl和CGI脚本进行的，但更复杂的系统已经出现。其中包括基于Java的Web服务器，它允许我们用Java进行所有服务器端编程，写出的程序就叫做“小服务程序”(Servlet)。 9.一个独立的领域:应用程序 与Java有关的大多数争论都是与程序片有关的。Java实际是一种常规用途的程序设计语言，可解决任何类型的问题，至少理论上如此。而且正如前面指出的，可以用更有效的方式来解决大多数客户机,服务器问题。如果将视线从程序片身上转开(同时放宽一些限制，比如禁止写盘等)，就进入了常规用途的应用程序的广阔领域。这种应用程序可独立运行，毋需浏览器，就象普通的执行程序那样。在这儿，Java的特色并不仅仅反应在它的移植能力，也反映在编程本身上。就像贯穿全书都会讲到的那样，Java提供了许多有用的特性，使我们能在较短的时间里创建出比用从前的程序设计语言更健壮的程序。 但要注意任何东西都不是十全十美的，我们为此也要付出一些代价。其中最明显的是执行速度放慢了(尽管可对此进行多方面的调整)。和任何语言一样，Java本身也存在一些限制，使得它不十分适合解决某些特殊的 17 燕山大学本科生毕业设计(翻译) 编程问题。但不管怎样，Java都是一种正在快速发展的语言。随着每个新版本的发布，它变得越来越可爱，能充分解决的问题也变得越来越多。 18