616 Chapter 18 |
Applets and Multimedia |
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<applet |
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applet class |
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code = "DisplayLabel.class" |
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width = 250 |
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height = 50 |
> |
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</applet> |
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</body> |
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</html> |
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A tag is an instruction to the Web browser. The browser interprets the tag and decides how to |
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display or otherwise treat the subsequent contents of the HTML document. Tags are enclosed |
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inside brackets. The first word in a tag, called the tag name, describes tag functions. A tag can |
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have additional attributes, sometimes with values after an equals sign, which further define |
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the tag’s action. For example, in the preceding HTML file, <applet> is the tag name, and |
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code, width, and height are attributes. The width and height attributes specify the rec- |
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tangular viewing area of the applet. |
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Most tags have a start tag and a corresponding end tag. The tag has a specific effect on the |
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region between the start tag and the end tag. For example, <applet...>...</applet> |
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tells the browser to display an applet. An end tag is always the start tag’s name preceded by a |
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slash. |
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HTML tag |
An HTML document begins with the <html> tag, which declares that the document is |
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written in HTML. Each document has two parts, a head and a body, defined by <head> and |
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<body> tags, respectively. The head part contains the document title, including the <title> |
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tag and other information the browser can use when rendering the document, and the body |
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part holds the actual contents of the document. The header is optional. For more information, |
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refer to Supplement V.A, “HTML and XHTML Tutorial.” |
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<applet> tag |
The complete syntax of the <applet> tag is as follows: |
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<applet |
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[codebase = applet_url] |
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code = classfilename.class |
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width = applet_viewing_width_in_pixels |
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height = applet_viewing_height_in_pixels |
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[archive = archivefile] |
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[vspace = vertical_margin] |
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[hspace = horizontal_margin] |
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[align = applet_alignment] |
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[alt = alternative_text] |
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> |
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<param> tag |
<param name = param_name1 value = param_value1> |
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<param name = param_name2 value = param_value2> |
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... |
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<param name = param_name3 value = param_value3> |
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</applet> |
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The code, width, and height attributes are required; all the others are optional. The |
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<param> tag will be introduced in §18.7, “Passing Strings to Applets.” The other attributes |
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are explained below. |
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codebase attribute |
■ codebase specifies a base where your classes are loaded. If this attribute is not used, |
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the Web browser loads the applet from the directory in which the HTML page is |
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located. If your applet is located in a different directory from the HTML page, you |
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must specify the applet_url for the browser to load the applet. This attribute |
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enables you to load the class from anywhere on the Internet. The classes used by the |
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applet are dynamically loaded when needed. |
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archive attribute |
■ archive instructs the browser to load an archive file that contains all the class files |
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needed to run the applet. Archiving allows the Web browser to load all the classes |
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18.3 The HTML File and the <applet> Tag 617
from a single compressed file at one time, thus reducing loading time and improving performance. To create archives, see Supplement III.Q, “Packaging and Deploying Java Projects.”
■ vspace and hspace specify the size, in pixels, of the blank margin to pad around the applet vertically and horizontally.
■align specifies how the applet will be aligned in the browser. One of nine values is used: left, right, top, texttop, middle, absmiddle, baseline, bottom, or absbottom.
■alt specifies the text to be displayed in case the browser cannot run Java.
18.3.1Viewing Applets from a Web Browser
To display an applet from a Web browser, open the applet’s HTML file (e.g., DisplayLabel.html). Its output is shown in Figure 18.3(a).
(a) |
(b) |
FIGURE 18.3 The DisplayLabel program is loaded from a local host in (a) and from a Web server in (b).
To make your applet accessible on the Web, you need to store the DisplayLabel.class and DisplayLabel.html on a Web server, as shown in Figure 18.4. You can view the applet from an appropriate URL. For example, I have uploaded these two files on Web server www.cs.armstrong.edu/. As shown in Figure 18.3(b), you can access the applet from www.cs.armstrong.edu/liang/intro8e/book/DisplayLabel.html.
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http://www.webserver.com/appropriatepath/DisplayLabel.html |
Web Server |
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Web Browser |
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The .html file and applet’s .class files |
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HTML Page |
are stored in the Web server. |
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FIGURE 18.4 A Web browser requests an HTML page from a Web server.
18.3.2Viewing Applets Using the Applet Viewer Utility
You can test the applet using the applet viewer utility, which can be invoked from the DOS
prompt using the appletviewer command from c:\book, as shown in Figure 18.5(a). Its out- appletviewer put is shown in Figure 18.5(b).
The applet viewer functions as a browser. It is convenient for testing applets during development without launching a Web browser.