Creating Component Libraries
Nearly every Tapestry application includes a least a couple of custom components, specific to the application. What's exciting about Tapestry is how easy it is to package components for reuse across many applications ... and the fact that applications using a component library need no special configuration.
We're going to create a somewhat insipid component that displays a large happy face icon.
Tapestry doesn't mandate that you use any build system, but we'll assume for the moment that you are using Maven 2. In that case, you'll have a pom.xml file something like the following:
You will need to modify the Tapestry release version number ("5.2.0" in the listing above) to reflect the current version of Tapestry when you create your component library.
We'll go into more detail about the relevant portions of this POM in the later sections.
Step 1: Choose a base package name
Just as with Tapestry applications, Tapestry component libraries should have a unique base package name. In this example, we'll use
As with an application, we'll follow the conventions: we'll place the module for this library inside the services package, and place pages and components under their respective packages.
Step 2: Create your pages and/or components
Our component is very simple:
HappyIcon appears inside the components sub-package. The happyIcon field is injected with the the Asset for the file
happy.jpg. The path specified with the @Path annotation is relative to the
HappyIcon.class file; it should be stored in the project under
Tapestry ensures that the
happy.jpg asset can be accessed from the client web browser; the src attribute of the <img> tag will be a URL that directly accesses the image file ... there's no need to unpackage the
happy.jpg file. This works for any asset file stored under the library's root package.
This component renders out an
<img> tag for the icon.
Often, a component library will have many different components, or even pages.
Step 3: Choose a virtual folder name
In Tapestry, components that have been packaged in a library are referenced using a virtual folder name. It's effectively as if the application had a new root-level folder containing the components.
In our example, we'll use "happy" as the folder name. That means the application can include the HappyIcon component in the template using any of the following, which are all equivalent:
- <img t:type="happy.happyicon"/>
- <img t:type="happy/icon"/>
Why "icon" vs. "happyicon"? Tapestry notices that the folder name, "happy" is a prefix or suffix of the class name ("HappyIcon") and creates an alias that strips off the prefix (or suffix). To Tapestry, they are completely identical: two different aliases for the same component class name.
The above naming is somewhat clumsy, and can be improved by introducing an additional namespace into the template:
The special namespace mapping for sets up namespace prefix "h:" to mean the same as "happy/". It then becomes possible to reference components within the happy virtual folder directly.
Step 4: Configure the virtual folder
Tapestry needs to know where to search for your component class. This is accomplished in your library's IoC module class, by making a contribution to the ComponentClassResolver service configuration.
At application startup, Tapestry will read the library module along with all other modules and configure the ComponentClassResolver service using information in the module:
The ComponentClassResolver service is responsible for mapping libraries to packages; it takes as a contribution a collection of these LibraryMapping objects. Every module may make its own contribution to the ComponentClassResolver service, mapping its own package ("org.example.happylib") to its own folder ("happy").
This module class is also where you would define new services that can be accessed by your components (or other parts of the application).
Step 5: Configure the module to autoload
For Tapestry to load your module at application startup, it is necessary to put an entry in the JAR manifest. This is taken care of in the pom.xml above:
Step 6: Extending Client Access
As of Tapestry 5.2, a new step is needed: extending access for the assets. This is accomplished in your library's module class, HappyModule:
This contribution uses a regular expression to identify that any resource on the classpath under the org/example/happylib folder with a jpg extension is allowed. If you had a mix of different image types, you could replace jpg with (jpg|gif|png).
Step 7: Versioning Assets
Classpath assets, those packaged in JAR files (such as the happy.jpg asset) are retrieved by the client web browser using a URL that reflects the package name. Tapestry users a special virtual folder, /assets, under the context folder for this purpose.
The image file here is exposed to the web browser via the URL /happyapp/assets/org/example/happylib/components/happy.jpg (this assumes that the application was deployed as happyapp.war).
Tapestry uses a far-future expiration date for classpath assets; this allows browsers to aggressively cache the file, but in Tapestry 5.1 and earlier this causes a problem when a later version of the library changes the file. This is discussed in detail in Yahoo's Performance Best Practices.
To handle this problem in Tapestry 5.1 and earlier, you should map your library assets to a versioned folder. This can be accomplished using another contribution from the HappyModule, this time to the ClasspathAssetAliasManager service whose configuration maps a virtual folder underneath /assets to a package:
With this in place, and the library and applications rebuilt and redeployed, the URL for happy.jpg becomes /happyapp/assets/happylib/1.0/components/happy.jpg. This is shorter, but also incorporates a version number ("1.0") that can be changed in a later release.
That's it! Autoloading plus the virtual folders for components and for assets takes care of all the issues related to components. Just build your JARs, setup the JAR Manifest, and drop them into your applications.