java思想外文翻譯---java編程思想 (java和因特網(wǎng))

1、<p>　　文獻(xiàn)來(lái)源：Bruce Eckel. Thinking in Java [J]. Pearson Higher Isia Education,2006-2-20.</p><p>　　Java編程思想 (Java和因特網(wǎng))</p><p>　　既然Java不過(guò)另一種類型的程序設(shè)計(jì)語(yǔ)言，大家可能會(huì)奇怪它為什么值得如此重視，為什么還有這么多的人認(rèn)為它是計(jì)算機(jī)程序設(shè)計(jì)的一個(gè)里

2、程碑呢？如果您來(lái)自一個(gè)傳統(tǒng)的程序設(shè)計(jì)背景，那么答案在剛開(kāi)始的時(shí)候并不是很明顯。Java除了可解決傳統(tǒng)的程序設(shè)計(jì)問(wèn)題以外，還能解決World Wide Web(萬(wàn)維網(wǎng))上的編程問(wèn)題。</p><p><b>　　1、客戶端編程</b></p><p>　　Web最初采用的“服務(wù)器－瀏覽器”方案可提供交互式內(nèi)容，但這種交互能力完全由服務(wù)器提供，為服務(wù)器和因特網(wǎng)帶來(lái)了不小的

3、負(fù)擔(dān)。服務(wù)器一般為客戶瀏覽器產(chǎn)生靜態(tài)網(wǎng)頁(yè)，由后者簡(jiǎn)單地解釋并顯示出來(lái)?；綡TML語(yǔ)言提供了簡(jiǎn)單的數(shù)據(jù)收集機(jī)制：文字輸入框、復(fù)選框、單選鈕、列表以及下拉列表等，另外還有一個(gè)按鈕，只能由程序規(guī)定重新設(shè)置表單中的數(shù)據(jù)，以便回傳給服務(wù)器。用戶提交的信息通過(guò)所有Web服務(wù)器均能支持的“通用網(wǎng)關(guān)接口”（CGI）回傳到服務(wù)器。包含在提交數(shù)據(jù)中的文字指示CGI該如何操作。最常見(jiàn)的行動(dòng)是運(yùn)行位于服務(wù)器的一個(gè)程序。那個(gè)程序一般保存在一個(gè)名為“cgi-bi

4、n”的目錄中（按下Web頁(yè)內(nèi)的一個(gè)按鈕時(shí)，請(qǐng)注意一下瀏覽器頂部的地址窗，經(jīng)常都能發(fā)現(xiàn)“cgi-bin”的字樣）。大多數(shù)語(yǔ)言都可用來(lái)編制這些程序，但其中最常見(jiàn)的是Perl。這是由于Perl是專為文字的處理及解釋而設(shè)計(jì)的，所以能在任何服務(wù)器上安裝和使用，無(wú)論采用的處理器或操作系統(tǒng)是什么。</p><p><b>　　2、腳本編制語(yǔ)言</b></p><p>　　插件造成了

5、腳本編制語(yǔ)言的爆炸性增長(zhǎng)。通過(guò)這種腳本語(yǔ)言，可將用于自己客戶端程序的源碼直接插入HTML頁(yè)，而對(duì)那種語(yǔ)言進(jìn)行解釋的插件會(huì)在HTML頁(yè)顯示的時(shí)候自動(dòng)激活。腳本語(yǔ)言一般都傾向于盡量簡(jiǎn)化，易于理解。而且由于它們是從屬于HTML頁(yè)的一些簡(jiǎn)單正文，所以只需向服務(wù)器發(fā)出對(duì)那個(gè)頁(yè)的一次請(qǐng)求，即可非?？斓剌d入。缺點(diǎn)是我們的代碼全部暴露在人們面前。另一方面，由于通常不用腳本編制語(yǔ)言做過(guò)分復(fù)雜的事情，所以這個(gè)問(wèn)題暫且可以放在一邊。</p>&

6、lt;p>　　腳本語(yǔ)言真正面向的是特定類型問(wèn)題的解決，其中主要涉及如何創(chuàng)建更豐富、更具有互動(dòng)能力的圖形用戶界面（GUI）。然而，腳本語(yǔ)言也許能解決客戶端編程中80％的問(wèn)題。你碰到的問(wèn)題可能完全就在那80％里面。而且由于腳本編制語(yǔ)言的宗旨是盡可能地簡(jiǎn)化與快速，所以在考慮其他更復(fù)雜的方案之前（如Java及ActiveX），首先應(yīng)想一下腳本語(yǔ)言是否可行。</p><p>　　目前討論得最多的腳本編制語(yǔ)言包括Jav

7、aScript（它與Java沒(méi)有任何關(guān)系；之所以叫那個(gè)名字，完全是一種市場(chǎng)策略）、VBScript（同Visual Basic很相似）以及Tcl/Tk（來(lái)源于流行的跨平臺(tái)GUI構(gòu)造語(yǔ)言）。當(dāng)然還有其他許多語(yǔ)言，也有許多正在開(kāi)發(fā)中。</p><p>　　JavaScript也許是日常用的，它得到的支持也最全面。無(wú)論NetscapeNavigator，Microsoft Internet Explorer，還是Ope

8、ra，目前都提供了對(duì)JavaScript的支持。除此以外，市面上講述JavaScript的書(shū)籍也要比講述其他語(yǔ)言的書(shū)多得多。有些工具還能利用JavaScript自動(dòng)產(chǎn)生網(wǎng)頁(yè)。當(dāng)然，如果你已經(jīng)有Visual Basic或者Tcl/Tk的深厚功底，當(dāng)然用它們要簡(jiǎn)單得多，起碼可以避免學(xué)習(xí)新語(yǔ)言的煩惱（解決Web方面的問(wèn)題就已經(jīng)夠讓人頭痛了）。</p><p><b>　　3、Java</b><

9、;/p><p>　　如果說(shuō)一種腳本編制語(yǔ)言能解決80％的客戶端程序設(shè)計(jì)問(wèn)題，那么剩下的20％又該怎么辦呢？它們屬于一些高難度的問(wèn)題嗎？目前最流行的方案就是Java。它不僅是一種功能強(qiáng)大、高度安全、可以跨平臺(tái)使用以及國(guó)際通用的程序設(shè)計(jì)語(yǔ)言，也是一種具有旺盛生命力的語(yǔ)言。對(duì)Java的擴(kuò)展是不斷進(jìn)行的，提供的語(yǔ)言特性和庫(kù)能夠很好地解決傳統(tǒng)語(yǔ)言不能解決的問(wèn)題，比如多線程操作、數(shù)據(jù)庫(kù)訪問(wèn)、連網(wǎng)程序設(shè)計(jì)以及分布式計(jì)算等等。Jav

10、a通過(guò)“程序片”（Applet）巧妙地解決了客戶端編程的問(wèn)題。</p><p>　　程序片（或“小應(yīng)用程序”）是一種非常小的程序，只能在Web瀏覽器中運(yùn)行。作為Web頁(yè)的一部分，程序片代碼會(huì)自動(dòng)下載回來(lái)（這和網(wǎng)頁(yè)中的圖片差不多）。激活程序片后，它會(huì)執(zhí)行一個(gè)程序。程序片的一個(gè)優(yōu)點(diǎn)體現(xiàn)在：通過(guò)程序片，一旦用戶需要客戶軟件，軟件就可從服務(wù)器自動(dòng)下載回來(lái)。它們能自動(dòng)取得客戶軟件的最新版本，不會(huì)出錯(cuò)，也沒(méi)有重新安裝的麻煩。

11、由于Java的設(shè)計(jì)原理，程序員只需要?jiǎng)?chuàng)建程序的一個(gè)版本，那個(gè)程序能在幾乎所有計(jì)算機(jī)以及安裝了Java解釋器的瀏覽器中運(yùn)行。由于Java是一種全功能的編程語(yǔ)言，所以在向服務(wù)器發(fā)出一個(gè)請(qǐng)求之前，我們能先在客戶端做完盡可能多的工作。例如，再也不必通過(guò)因特網(wǎng)傳送一個(gè)請(qǐng)求表單，再由服務(wù)器確定其中是否存在一個(gè)拼寫或者其他參數(shù)錯(cuò)誤。大多數(shù)數(shù)據(jù)校驗(yàn)工作均可在客戶端完成，沒(méi)有必要坐在計(jì)算機(jī)前面焦急地等待服務(wù)器的響應(yīng)。這樣一來(lái)，不僅速度和響應(yīng)的靈敏度得到了

12、極大的提高，對(duì)網(wǎng)絡(luò)和服務(wù)器造成的負(fù)擔(dān)也可以明顯減輕，這對(duì)保障因特網(wǎng)的暢通是至關(guān)重要的。 與腳本程序相比，Java程序片的另一個(gè)優(yōu)點(diǎn)是它采用編譯好的形式，所以客戶端看不到源碼。當(dāng)然在另一方面，反編譯Java程序片也并不是件</p><p><b>　　4、因特網(wǎng)和內(nèi)聯(lián)網(wǎng)</b></p><p>　　Web是解決客戶機(jī)／服務(wù)器問(wèn)題的一種常用方案，所以最好能用相同的

13、技術(shù)解決此類問(wèn)題的一些“子集”，特別是公司內(nèi)部的傳統(tǒng)客戶機(jī)／服務(wù)器問(wèn)題。對(duì)于傳統(tǒng)的客戶機(jī)／服務(wù)器模式，我們面臨的問(wèn)題是擁有多種不同類型的客戶計(jì)算機(jī)，而且很難安裝新的客戶軟件。但通過(guò)Web瀏覽器和客戶端編程，這兩類問(wèn)題都可得到很好的解決。若一個(gè)信息網(wǎng)絡(luò)局限于一家特定的公司，那么在將Web技術(shù)應(yīng)用于它之后，即可稱其為“內(nèi)聯(lián)網(wǎng)”（Intranet），以示與國(guó)際性的“因特網(wǎng)”（Internet）有別。內(nèi)聯(lián)網(wǎng)提供了比因特網(wǎng)更高的安全級(jí)別，因?yàn)榭梢?/p>

14、物理性地控制對(duì)公司內(nèi)部服務(wù)器的使用。說(shuō)到培訓(xùn)，一般只要人們理解了瀏覽器的常規(guī)概念，就可以非常輕松地掌握網(wǎng)頁(yè)和程序片之間的差異，所以學(xué)習(xí)新型系統(tǒng)的開(kāi)銷會(huì)大幅度減少。</p><p>　　安全問(wèn)題將我們引入客戶端編程領(lǐng)域一個(gè)似乎是自動(dòng)形成的分支。若程序是在因特網(wǎng)上運(yùn)行，由于無(wú)從知曉它會(huì)在什么平臺(tái)上運(yùn)行，所以編程時(shí)要特別留意，防范可能出現(xiàn)的編程錯(cuò)誤。需作一些跨平臺(tái)處理，以及適當(dāng)?shù)陌踩婪?，比如采用某種腳本語(yǔ)言或者Jav

15、a。</p><p>　　但假如在內(nèi)聯(lián)網(wǎng)中運(yùn)行，面臨的一些制約因素就會(huì)發(fā)生變化。全部機(jī)器均為Intel/Windows平臺(tái)是件很平常的事情。在內(nèi)聯(lián)網(wǎng)中，需要對(duì)自己代碼的質(zhì)量負(fù)責(zé)。而且一旦發(fā)現(xiàn)錯(cuò)誤，就可以馬上改正。除此以外，可能已經(jīng)有了一些“歷史遺留”的代碼，并用較傳統(tǒng)的客戶機(jī)／服務(wù)器方式使用那些代碼。但在進(jìn)行升級(jí)時(shí)，每次都要物理性地安裝一道客戶程序。浪費(fèi)在升級(jí)安裝上的時(shí)間是轉(zhuǎn)移到瀏覽器的一項(xiàng)重要原因。使用了瀏覽器

16、后，升級(jí)就變得易如反掌，而且整個(gè)過(guò)程是透明和自動(dòng)進(jìn)行的。如果真的是牽涉到這樣的一個(gè)內(nèi)聯(lián)網(wǎng)中，最明智的方法是采用ActiveX，而非試圖采用一種新的語(yǔ)言來(lái)改寫程序代碼。</p><p><b>　　5、服務(wù)器端編程</b></p><p>　　我們的整個(gè)討論都忽略了服務(wù)器端編程的問(wèn)題。如果向服務(wù)器發(fā)出一個(gè)請(qǐng)求，會(huì)發(fā)生什么事情？大多數(shù)時(shí)候的請(qǐng)求都是很簡(jiǎn)單的一個(gè)“把這個(gè)文件

17、發(fā)給我”。瀏覽器隨后會(huì)按適當(dāng)?shù)男问浇忉屵@個(gè)文件：作為HTML頁(yè)、一幅圖、一個(gè)Java程序片、一個(gè)腳本程序等等。向服務(wù)器發(fā)出的較復(fù)雜的請(qǐng)求通常涉及到對(duì)一個(gè)數(shù)據(jù)庫(kù)進(jìn)行操作(事務(wù)處理)。其中最常見(jiàn)的就是發(fā)出一個(gè)數(shù)據(jù)庫(kù)檢索命令，得到結(jié)果后，服務(wù)器會(huì)把它格式化成HTML頁(yè)，并作為結(jié)果傳回來(lái)(當(dāng)然，假如客戶通過(guò)Java或者某種腳本語(yǔ)言具有了更高的智能，那么原始數(shù)據(jù)就能在客戶端發(fā)送和格式化；這樣做速度可以更快，也能減輕服務(wù)器的負(fù)擔(dān))。另外，有時(shí)需要在

18、數(shù)據(jù)庫(kù)中注冊(cè)自己的名字（比如加入一個(gè)組時(shí)），或者向服務(wù)器發(fā)出一份訂單，這就涉及到對(duì)那個(gè)數(shù)據(jù)庫(kù)的修改。這類服務(wù)器請(qǐng)求必須通過(guò)服務(wù)器端的一些代碼進(jìn)行，我們稱其為“服務(wù)器端的編程”。在傳統(tǒng)意義上，服務(wù)器端編程是用Perl和CGI腳本進(jìn)行的，但更復(fù)雜的系統(tǒng)已經(jīng)出現(xiàn)。其中包括基于Java的Web服務(wù)器，它允許我們用Java進(jìn)行所有服務(wù)器端編程，寫出的程序就叫做“小服務(wù)程序”(Servlet)。</p><p>　　6、一個(gè)

19、獨(dú)立的領(lǐng)域：應(yīng)用程序</p><p>　　與Java有關(guān)的大多數(shù)爭(zhēng)論都是與程序片有關(guān)的。Java實(shí)際是一種常規(guī)用途的程序設(shè)計(jì)語(yǔ)言，可解決任何類型的問(wèn)題，至少理論上如此。而且正如前面指出的，可以用更有效的方式來(lái)解決大多數(shù)客戶機(jī)／服務(wù)器問(wèn)題。如果將視線從程序片身上轉(zhuǎn)開(kāi)（同時(shí)放寬一些限制，比如禁止寫盤等），就進(jìn)入了常規(guī)用途的應(yīng)用程序的廣闊領(lǐng)域。這種應(yīng)用程序可獨(dú)立運(yùn)行，毋需瀏覽器，就象普通的執(zhí)行程序那樣。在這兒，Java

20、的特色并不僅僅反應(yīng)在它的移植能力，也反映在編程本身上。就像貫穿全書(shū)都會(huì)講到的那樣，Java提供了許多有用的特性，使我們能在較短的時(shí)間里創(chuàng)建出比用從前的程序設(shè)計(jì)語(yǔ)言更健壯的程序。</p><p>　　但要注意任何東西都不是十全十美的，我們?yōu)榇艘惨冻鲆恍┐鷥r(jià)。其中最明顯的是執(zhí)行速度放慢了（盡管可對(duì)此進(jìn)行多方面的調(diào)整）。和任何語(yǔ)言一樣，Java本身也存在一些限制，使得它不十分適合解決某些特殊的編程問(wèn)題。但不管怎樣，J

21、ava都是一種正在快速發(fā)展的語(yǔ)言。隨著每個(gè)新版本的發(fā)布，它變得越來(lái)越可愛(ài)，能充分解決的問(wèn)題也變得越來(lái)越多。</p><p>　　Thingking in Java（Java and the Internet）</p><p>　　If Java is, in fact, yet another computer programming language, you may question

22、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

23、 useful for solving traditional stand-alone programming problems, it is also important because it will solve programming problems on the World Wide Web.</p><p>　　1.Client-side programming</p><p>

24、;　　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 interp

25、ret 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

26、the form or “submit” the data on the form back to the server. T</p><p>　　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 C

27、GI 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 be sent, as well as the load on both the server a

28、nd 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 band</p><p>　　The solution is client-side programming. Most machi

29、nes 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 program

30、ming 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.</p><p>　　The problem with discussions

31、 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 syst

32、em. 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 i

33、n client-side programming.</p><p>　　2.Scripting languages</p><p>　　Plug-ins resulted in an explosion of scripting languages. With a scripting language you embed the source code for your client-s

34、ide 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,

35、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 tra</p><p>　　This points out that the scripting languages

36、 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 perc

37、ent 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

38、consider a scrip</p><p>　　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), VBScr

39、ipt (which looks like Visual Basic), and Tcl/Tk, which comes from the popular cross-platform GUI-building language. There are others out there, and no doubt more in development.</p><p>　　JavaScript is probab

40、ly 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 l

41、anguages, 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.

42、 (You’ll have your hands full dealing wi</p><p><b>　　3.Java</b></p><p>　　If a scripting language can solve 80 percent of the client-side programming problems, what about the other 20

43、 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 p

44、rovide language features and libraries that elegantly handle problems that are difficult in traditional programming languages, such as multithreading, database access, net</p><p>　　An applet is a mini-progra

45、m 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. Th

46、is 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 c

47、lient software without fail and wi</p><p>　　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

48、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 mo

49、dules and take multiple server “hits” (accesses) to download. (In Java 1.1 and higher this is minimized by Java archi</p><p>　　4.Internet vs. intranet</p><p>　　The Web is the most general soluti

50、on 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 client/server a

51、pproaches 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

52、technology is used for an </p><p>　　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 In

53、ternet, 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. <

54、/p><p>　　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 qu

55、ality 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 physical

56、ly install client programs every time you do an upgrade. The time wasted in ins</p><p>　　5.Server-side programming</p><p>　　This whole discussion has ignored the issue of server-side programming

57、. 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

58、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</p>

59、<p>　　6.separate arena: applications</p><p>　　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

60、in theory. And as pointed out previously, there might be more effective ways to solve most client/server problems. When you move out of the applet arena (and simultaneously release the restrictions, such as the one again

61、st writing to disk) you enter the world of general-purpose applications that run standalone, without a Web browser, just like </p><p>　　Be aware that this is a mixed blessing. You pay for the improvements th

62、rough 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 limitation

63、s 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 solvin</p>