eve o r~entwit
`
`~~.~~~.~
`
`~~~~,'~.
`
`i' i ! t 1 1
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`# 1
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`! ~
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`'i X11 ~ 1
`
`Duane K.~ields
`Mark A. Kolb
`Shawn Bayern
`
`ll~ MANNING
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`Web Develo~~e~t with
`~~~ ev~~e~ ~ es
`
`SECOND EDITION
`
`DUANE K. FIELDS
`MARK A. KOLB
`~.
`
`MANNING
`Greenwich
`(74° w. long.)
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`Extev~nal content ~
`
`483
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`technique, it allows static HTML pages to include soiree form of dynamic content,
`even if the local server does not support a technology such as JSP.
`
`WARNING One downside is that older browsers don't support external style sheets or
`JavaScript source, however the 4.0 and 5.0 browsers do, as do some of the
`3.0 browsers. If you plan to rely on these features in your site, make certain
`that your audience can take advantage of them.
`
`16.4.E JSP style sheets
`The most common use for style sheets in web publishing is to provide a standazd-
`ized look and feel for all of the pages on the site, without having to stylize the col-
`ors, fonts, and layout of each page separately. To this end browsers can make use of
`the <link> tag to embed references to external style sheet documents into ~heiz
`HTML. Typically, the link tag looks like this:
`
`clink rel="stylesheet" type="text/css" href="style.css">
`It is important to understand that the href attribute that defines the external style
`sheet is pointing to the UKL of a document somewhere out on the network. The
`document doesn't have to be local to the current server, and it doesn't even have to
`have a .css extension. After loading the initial page, the browser will spawn an addi-
`tional request to download the style sheet. Regardless of the location or file exten-
`sion, the browser will expect what comes back from the server to be only a list of
`CSS style directives. No HTML is allowed. A typical style sheet then might contain
`something like the following:
`
`P { color: red; font-size=l4pt; margin-left: gem; }
`P.hilite { background-color: yellow; }
`Since this style information is just text, we are free to create our own style sheet
`through JSP, allowing us to customize the style on aper-request, orper-session basis.
`We're not going to go into the details of how style sheets work. If you aren't familiar
`with -them all you need to know is that they tell the browser how different content
`elements (such as tables, paragraphs, etc.) should look and be laid out. In this simple
`example (listing 16.7) we'll create a style sheet that creates a random color scheme
`by selecting a new combination of background, text, and anchor colors for each
`page request.
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`CHAPTER 16
`Genev~atin8 non-HTML content
`
`ao@ page contentType="text/css" o>
`<o!
`String[] colors = { ~~red", "green", "blue", "yellow", "black", "white" };
`
`private String getRandomColor() {
`return colors[(int)(Math.random() * colors.length)];
`
`} %
`
`>
`
`BODY { background-color: <%= getRandomColor() o> }
`P { color: <o= getRandomColor() %> }
`A:link { color: <o= getRandomColor() %> }
`A:active { color: <a= getRandomColor() o> }
`A:visited { color: <o= getRandomColor() o> }
`
`You can load this page into your browser directly to see the results. Notice how it
`changes with each reload. (If it doesn't, you'll probably need to modify your brows-
`ers caching behavior to assure it reloads the style sheet from the network each time,
`or see the caching section in this chapter for information on how to have the page
`direct the browser's caching behavior itself . Here's atypical result:
`
`BODY { background-color: green }
`P { color: black }
`A:link { color: yellow }
`A:active { color: red }
`A:visited { color: blue }
`
`To reference this style sheet in our documents (table 16.8), we make use of the link
`tag, substituting the URL of the dynamic style sheet we just created:
`
`Listing 16.8 random.html
`
`<html>
`<head>
`<lirik rel="stylesheet" type="text/css" href="random-css.jsp">
`</head>
`
`<body>
`<p>
`Welcome to a page with a random style.
`</p>
`c/body>
`</html>
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`F_,.xtev~nczl contevct (
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`4R5
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`This style sheet can be referenced by any HTML document, including static HTML
`and JSP files. It is now accessible from other servers as well. Dynamic style sheets are
`a useful way to modify the look and feel of your existing pages without having to
`convert the entire site's web pages to jSP. There are layout effects that can't be done
`without style sheets, like carefully controlling the font size, margins, and borders ~~f
`the content. We could take advantage of this technique to allow the user to cc~ntroi
`the style of the page recording their choices in their session, and by retrieving this
`information to construct our dynamic style sheet. Another useful technic~iie is
`adapting the style sheet based on the platform or the vendor of the client's browser.
`For example, we might need to bump up the default font size for UNIX brotivsers,
`or modify a table style to achieve parity between Netscape and Internet Explorer.
`
`16.4.2 JavaScript
`Like style sheets, JavaScript can be either included directly in the HTML page, or
`imported by the browser at regl~est time. The src attribute of the <script> tag can
`be used to specify an external file that contains javaScript functions, variables, or
`commands that should be made available to the page. Typically, this capability is
`used to create libraries of JavaScript functions that are available to multiple pages on
`the site. A typical reference looks like this:
`
`<script src="loans.js">
`</script>
`And the file loans.js (which could be anywhere on the network) would contain func-
`tions to calculate loan payments, interest rates, and so forth. As with style sheets this
`file could be a JSP that modified the commands or functions based on the current
`users, their browsers, and so on. For example, the functions for calculating loans
`might take the user's local interest or tax rates into account. This provides a way of
`centralizing application logic in asingle-client side module, by introducing server-
`side dependencies.
`A more interesting technique is using dynamic imported JavaScript to pass infor-
`mation between the client and the server. In this example we'll pretend that we run
`an online bookstore and we are loolung for a way to provide a "book of the month"
`that our associates and partners can promote through their own web sites. It's sim-
`ple enough to select a book each month, but we want a way that the partners can
`promote it without having to manually change the book's information each month
`and would like to display the current number available for purchase. We also can't
`assume that every affiliate site is running a JSP capable server or has access to serv-
`lets. What we can do is provide a dynamic javaScript library on our server that can
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