【正文】 然而，瀏覽器仍然顯得很原始，在用戶迅速增多的要求面前顯得有些力不從心。在 Web 的發(fā)展過程中，我們可以很清晰地看出這些令人心喜的變化。他們希望獲得完全的客戶機(jī)／服務(wù)器能力，使客戶（程序）也能反饋一些信息到服務(wù)器。我們向一個服務(wù)器發(fā)出請求，它向我們回傳一個文件，由于本機(jī)的瀏覽器軟件（亦即 “ 客戶 ” 或 “ 客戶程序 ” ）負(fù)責(zé)解釋和格式化，并在我們面前的屏幕上正確地顯示出來。但我們沒必要了解更進(jìn)一步的細(xì)節(jié)，因?yàn)槲ㄒ灰P(guān)心的就是一次建立同一個服務(wù)器的連接，并同它打交道（即使可能要在全世界的范圍內(nèi)搜索正確的服務(wù)器）?？蛻魴C(jī)／服務(wù)器問題需要從根本上加以變 革！ Web 實(shí)際就是一套規(guī)模巨大的客戶機(jī)／服務(wù)器系統(tǒng)。在過去，我們一般為單獨(dú)的問題采取單獨(dú)的解決方案；每次都要設(shè)計一套新方案。事實(shí)上，大約有一半的程序設(shè)計活動都可以采用客戶機(jī)／服務(wù)器的結(jié)構(gòu)。有時甚至?xí)谷水a(chǎn)生完全無從著手的感覺。一般可以考慮讓客戶機(jī)負(fù)擔(dān)部分處理任務(wù)，但有時亦可分派給服務(wù)器所在地的其他機(jī)器，那些機(jī)器亦叫作 “ 中間件 ” （中間件也用于改進(jìn)對系統(tǒng)的 維護(hù)）。所以任何微小的延誤都是不能忽視的。另外，對多種類型的計算機(jī)和操作系統(tǒng)的支持也是一個大問題。客戶軟件發(fā)生了改變之后，它們必須在客戶機(jī)器上構(gòu)建、調(diào)試以及安裝。除此以外，系統(tǒng)經(jīng)常允許客戶將新信息插入一個服務(wù)器。這里要注意的一個主要問題是單個服務(wù)器需要同時向多個客戶提供服務(wù)。而對那些駐留在遠(yuǎn)程機(jī)器上的軟件，它們需要與服務(wù)器通信，取回信息，進(jìn)行適當(dāng)?shù)奶幚?，然后在遠(yuǎn)程機(jī)器上顯示出來，這些就叫作 “ 客戶 ” （ Client）。所以我們能方便地更改信息，然后將修改過的信息發(fā)放給信息的消費(fèi)者。一般將數(shù)據(jù)集中保存在某個數(shù)據(jù)庫中，根據(jù)其他人或者機(jī)器的請求將信息投遞給對方。我們在這里有必 要作一些深入的探討，但在這之前，必須理解客戶機(jī)／服務(wù)器系統(tǒng)的概念，這是充斥著許多令人迷惑的問題的又一個計算領(lǐng)域。 什么是 Web？ Web 這個詞剛開始顯得有些泛泛，似乎 “ 沖浪 ” 、 “ 網(wǎng)上存在 ” 以及 “ 主頁 ” 等等都和它拉上了一些關(guān)系。 petitive pricing and greater access to information. For anizations it increases their customer value and the building of sustainable capabilities, next to the increased profits. 中文翻譯 Java 和因特網(wǎng) 既然 Java 不過另一種類型的程序設(shè)計語言，大家可能會奇怪它為什么值得如此重視，為什么還有這么多的人認(rèn)為它是計算機(jī)程序設(shè)計的一個里程碑呢？如果您來自一個傳統(tǒng)的程序設(shè)計背景，那么答案在剛開始的時候并不是很明顯。 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. Microsoft’s 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 ponent, that ponent might cause damage to the files on your disk. Of course, programs that you load onto your puter that are not restricted to running inside a Web browser can do the same thing. Viruses downloaded from BulletinBoard Systems (BBSs) have long been . 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, 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 lowcost “Inter appliances” might someday satisfy the needs of a significant segment of users). The solution is the “signed applet” that uses publickey encryption to verify that an applet does indeed e 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 creators 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. Chapter 14 contains an example of how to sign an applet. In addition, Java Web Start is a relatively new way to easily distribute standalone programs that don’t need a web browser in which to run. This technology has the potential of solving many client side problems associated with running programs inside a browser. Web Start programs can either be signed, or they can ask the client for permission every time they are doing something potentially dangerous on the local system. Chapter 1 4 has a simple example and explanation of Java Web Start. Digital signatures have missed an important issue, which is the speed that people move around on the Inter. 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? vs. intra The Web is the most general solution to the client/server problem, so it makes sense to use the same technology to solve a subset of the problem, in particular the classic client/server problem within a pany. With traditional client/server approaches you have the problem of multiple types of client puters, as well as the difficulty of installing new client software, both of which are handily solved with Web browsers and clientside programming. When Web technology is used for an information work that is restricted to a particular pany, it is referred to as an intra. Intras provide much greater security than the Inter, since you can physically control access to the servers within your pany. In terms of training, it seems that once people understand the general concept of a browser it’s much easier for them to deal