Illustrating Tapestry

-Illustration 2-

Illustration 2

If you thought the previous illustration was a lot of work to show a simple welcome page, don't fret; this illustration is for you. Here, we'll use the same Insert JWC as in Listing 1, but we'll use it to show the user's visitng time instead. Before we continue, let me boost your spirits by showing you the code that is needed for this illustration.

Page Template

<html>
    <head>
        <title>Welcome to Tapestry!</title>
    </head>
    <body>
        Hello Jack!  Welcome to Tapestry!
        Your visiting time is <span jwcid="now@Insert" value="ognl:new java.util.Date()">Current time</span>
    </body>
</html>

Listing 6. Home.html

That's all we need for this illustration, of course other than the web.xml which we can reuse from the previous illustration. You don't need the page specification or the JavaBeans component class; neither do you need the application specification. The framework has default values defined for all these parts; we'll talk about these a little later, but for now let's focus our attention on the template.

The template, if you notice, looks pretty much the same as in Listing 1 except for the placeholder tag. The placeholder tag now has the component type (Insert) embedded into the jwcid attribute value using the @ symbol. This is just another notation for declaring a component. A component thus declared is known as an Implicit component. now is the unique ID for the component and may be omitted if you don't have the need to reference this component in your code. If the ID is omitted, the component is called an "Anonymous Implicit" component, and the framework would provide an arbitrary ID to such components for internal use. An Implicit component has to be declared and configured in the placeholder tag in its entirety thus avoiding the need for a separate specification. Parameters of Implicit JWCs are bound to their values right in the placeholder tag in the form of attribute-value pairs. So the value parameter of the Insert JWC, in our example, is bound to an instance of java.util.Date, as shown in Listing 6, using OGNL. Yes, OGNL expressions are perfectly acceptable for attribute values in templates. Any attribute value prefixed with ognl: is considered an OGNL expression. This type of binding is identical, in meaning, to the binding specified in the Page specification from our previous illustration.

The component declaration was only one part of our Page specification, the other parts being the Page class declaration (demo.Home) and the template declaration ($template asset). The framework by default searches for the template by the name of the component with a .html extension, so we can skip the $template declaration. We also do without the Page class, and thus the Page specification itself, with the help of the framework default Page class, org.apache.tapestry.html.BasePage. The missing application specification is also owed to the framework's default behavior. Knowing that the framework will forward all unknown requests to a Page component named Home, we skip the application specification too. Another point worth mentioning here is that the framework has the ability to search and locate pages and templates not explicitly specified in the specification. The framework performs this search in a methodical fashion, the details of which can be found in the JavaDocs of org.apache.tapestry.resolver.PageSpecificationResolver and org.apache.tapestry.engine.DefaultTemplateSource classes.

Now, if we place our template (Home.html) under the application context Welcome, and place the web.xml file under the WEB-INF folder, and ran our Welcome example application within Tomcat, running at port 8080, the request URL http://localhost:8080/Welcome/app will take us to the Home Page of our Welcome application that we just built.

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