- Does the page specifically request a decorator using a meta decorator tag?
- Is the page a frame set (if so, don't apply a decorator)?
- Does the page have a
printable=true parameter> (If so, use the printable decorator.)
- Does the page specifically request a decorator by the decorator file name?
- Does the page match a pattern in the decorators.xml file?
Conceptually, the first rule that evaluates to true determines the decorator that is used. In the example above, when the printable=true parameter is present, the printable.jsp decorator (rule #3) is used instead of the main.jsp (matched in rule #5). In SiteMesh, these rules are described as mappers.
decorators/*.jsp
The three files in the decorators directory are the various decorator JSP files, as described by decorators.xml. We saw an example of a simple decorator above, and we'll look at a more sophisticated example later in this article.
sitemesh-2.0.1.jar
This is the main SiteMesh binary, typically installed in the WEB-INF/lib directory. You can find the Javadoc for this library at www.opensymphony.com/sitemesh/api.
*.tld
SiteMesh uses two tag libraries, but most users will only need the decorator tags (sitemesh-decorator.tld). You can find documentation on these at www.opensymphony.com/sitemesh/tags.html. We've already touched on the main tags, used to retrieve the head, title, and body. We'll look at the remaining tag, getProperty, in the next section.
Advanced SiteMesh
One of the more powerful abilities of SiteMesh is to use the ordinary HTML meta tag (for example, <meta name="foo" content="bar">) to pass information from the base page to the decorator. For example, let's say that we would like to define the author of an HTML page using a meta tag, as shown below.
<html>
<meta name="author" content="test@example.com">
<head>
<title>Simple Document</title>
</head>
<body>
Hello World! <br />
<%= 1+1 %>
</body>
</html>
We can make a decorator "smart" enough to know to look for this meta tag, and if present, generate the appropriate HTML:
<%@ taglib uri="sitemesh-decorator" prefix="decorator" %>
<decorator:usePage id="myPage" />
<html>
<head>
<title>My Site -
<decorator:title default="Welcome!" />
</title>
<decorator:head />
</head>
<body>
<h1><decorator:title default="Welcome!" /></h1>
<h3>
<a href="mailto:<decorator:getProperty property="meta.author"
default="staff@example.com" />">
<decorator:getProperty property="meta.author"
default="staff@example.com" />
</a></h3><hr />
<decorator:body />
<p><small>
(<a href="?printable=true">printable version</a>)
</small>
</p>
</body>
</html>
You'll notice that we use a default attribute in the getProperty tag -- if no author is specified, we'll just assume that it was written by the staff. If you decide to use this model for storing page metadata, you'll want to work with your content developers and other team members to determine what tags you want to use and how you'll be using them. At the simple end, you may want to use meta tags to describe things like the author and page timestamp. At the complex end, you may do things like standardize on an XML file to manage your site navigation and use a meta tag to pass the page's node to the decorator.
The page that results from applying this decorator to the JSP page above is shown in Figure 6.
Figure 6. meta Tag Displayed
These page attributes are powerful, and you can retrieve many different properties, not just the meta tags (here's a list of generated page properties). After using SiteMesh for a while, you'll start thinking about HTML and JSP as a mechanism for generating simple markup -- closer to the original intent of HTML -- without having to make a full switch to an XML/XSL or other template engine.
Summary
As we've seen, SiteMesh provides for a powerful, easy-to-use, non-intrusive mechanism for applying page templates. It's easy to envision a wide range of possible uses. For example, you might define a decorator that emits extra debugging information about the page, as determined by the browser (this is especially powerful when combined with a web browser that lets you set an arbitrary user-agent). You might define a decorator with a stripped-down XML output, allowing for easier automated testing. You can even use the decorator to grab content from other pages, for example, to simple portal-like capability.
Once you've gotten comfortable with sitemesh-blank.war, I'd suggest looking at sitemesh-example.war for more features and ideas.
Regardless of how you use SiteMesh, I've found that it lets me centralize a tremendous amount of code, moving it out of my presentation layer and into my decorators, without having to learn a new programming language or templating system.
Oh, and as a final note for those of you still interested in building web pages in assembly, check out home.worldonline.dk/viksoe/asmil.htm.
Good luck and happy coding!
Will Iverson served as Developer Relations Manager for the VisualCafé
group at Symantec, as the Java & Runtimes Product Manager at Apple
Computer and has led Cascade Technology Group, a private consulting
firm, since 1999. |
