This article is based on, and contains excerpts from, the book Pro JSF: Building Rich Internet Components by Jonas Jacobi and John Fallows, published by Apress. Book is available on fine bookstores and Amazon.

In our previous article - "Rich Internet Components with JavaServer Faces" (JDJ, Vol. 10, issue 11) - we discussed how JavaServer Faces can fulfill new presentation requirements without sacrificing application developer productivity building Rich Internet Applications (RIA). We discussed how JSF component writers can utilize technologies, such as AJAX and Mozilla XUL, to provide application developers with rich, interactive, and reusable components.

 To use AJAX and Mozilla XUL with JSF, component writers have to make sure to provide any resource files needed by these technologies, such as images, style sheets, or scripts. The standard approach to providing resource files for a JSF component library is to serve them directly out of the Web application root file system. These resources are usually packaged in an archive (such as a ZIP file) and shipped separately from the JSF component library.

This article introduces a new open source project - Weblets - which can be found on the java.net Website (http://weblets.dev.java.net). The goal of this open source project is to provide JSF component writers with a facility that can serve resource files out of a Java archive (JAR), rather than serving them from the Web application root file system. Unlike traditional Web applications, which have statically configured URL mappings defined in web.xml, there is a need for dynamic configuration of URL mappings, based on the presence of a component library JAR. In essence, Weblets provide developers with an easy way to package Web application resources in the same Java archive (JAR) that their implementation code resides in.

Resource Loading
Let's assume that we have a JSF component that needs to have a JavaScript file, myScript.js, served to the client. This JavaScript file is used by the component to provide some level of richness when interacted with by the end user. This JavaScript file is traditionally served by the Web application via a relative path that is hard coded into the actual Renderer code for the JSF component. This requires the application developer to deploy additional resources that are delivered and packaged in a separate archive file, e.g., ZIP, often referred to as "installables."

It is important to note that the JavaServer Faces HTML basic RenderKit does not have any images, styles, or scripts, so there is no standard solution to the Faces resource packaging problem.

The following sample Renderer code illustrates the installables approach to serving a JavaScript file, /myresources/myScript.js, from the Web application root file system.

ViewHandler handler = context.getApplication().getViewHandler();
String resourceURL = handler.getResourceURL(context,"/myresources/myScript.js");
out.startElement("script", null);
out.writeAttribute("type", "text/javascript", null);
out.writeAttribute("src", resourceURL, null);
out.endElement("script");

Although the installables approach is convenient for the JSF component author, it does increase the installation burden on the application developer, who must remember to extract the installables archive each time the component library is upgraded to a new version. Therefore, we need a way to package our additional resources into the same JAR file containing the Renderer classes, simplifying deployment for application developers using our component library.

Using Weblets
The open source Weblets project aims to solve the resource-packaging problem in a generic and extensible way so that it can be leveraged by all JavaServer Faces component writers, while placing only a minimal installation burden on the application developer.

A Weblet acts as a mediator that intercepts requests from the client and uses short Web URLs to serves resources from a JAR file. Unlike the Servlet or Filter approach, a Weblet can be registered and configured inside a JAR, so the component library Renderers, their resource files, and the Weblet configuration file (weblets-config.xml) can all be packaged together in the same JAR. The Weblet Container can be registered just once in the Web application configuration file - web.xml - for all component libraries. There is no need to separately deploy additional installables when the component libraries are upgraded to new versions.

It is important to note that all resources served up by Weblets are internal resources, used only by the Renderer. Any resources, like images, that are provided by the application, are supplied as component attribute values and loaded from the context root as external resources.

Weblet Architecture
Although Weblets were designed to be used by any Web client, the Weblets implementation has been integrated with JavaServer Faces using a custom ViewHandler, WebletsViewHandler, as shown in Figure 1. During rendering of the main JavaServer Faces page, the WebletsViewHandler is responsible for converting Weblet-specific resource URLs into the actual URLs used by the browser to request Weblet-managed resources.

After receiving the rendered markup for the main page, the browser downloads each additional resource using a separate request. Each request for a Weblet-managed resource is intercepted by the WebletsPhaseListener, which then asks the WebletContainer to stream the Weblet-managed resource file out of the component library JAR.

The WebletContainer is designed to leverage the browser cache where possible. This improves overall rendering performance by minimizing the total number of requests made for Weblet-managed resource files.

To ensure flexibility and optimization, and avoid collisions with existing Web application resources, Weblets can be configured by application developers to override any default settings provided by the component author.

Using Weblets in a Component library
Weblets are configured using a weblets-config.xml file, which must be stored in the /META-INF directory of the component library JAR. Configuring a Weblet is similar to configuring a Servlet or a Filter. Each Weblet entry in the weblets-config.xml file has a Weblet name, implementation class, and initialization parameters (see Listing 1). The Weblet mapping associates a particular URL pattern with a specific Weblet name, e.g., org.myapp.html. The Weblet name and default URL pattern define the public API for the Weblet-managed resources and should not be modified between releases of your component library, in order to maintain backward compatibility.

Our component library packages resources in the org.myapp.faces.renderer.html.resources Java package and makes them available to the browser using the default URL mapping of /myresources/*.

The PackagedWeblet is a built-in Weblet implementation that can read from a particular Java package using the ClassLoader and stream the result back to the browser. The package initialization parameter tells the PackagedWeblet which Java package to use as a root when resolving Weblet-managed resource requests.

Weblet Versioning
Weblets also has built-in support for versioning of the component library. This is used to allow the browser to cache packaged resources such as myScript.js when possible, preventing unnecessary round-trips to the Web server.

Each time the browser renders a page, it will ensure that all resources used by that page are available. During the initial rendering of the page, the browser populates its cache with the contents of each resource URL by downloading a fresh copy from the Web server. As it does so, the browser records the Last-Modified and Expires timestamps from the response headers. The cached content is said to have expired if the current time is later than the expiration timestamp, or if no expiration timestamp information exists.

On the next render of the same page, the browser checks to see if the locally cached resource has expired. The locally cached copy is reused if it has not expired. Otherwise, a new request is made to the Web server, including the last modified information in the If-Modified-Since request header. The Web server responds by either indicating that the browser cache is still up-to-date, or by streaming the new resource contents back to the browser with updated Last-Modified and Expires timestamps in the response headers.

Weblets use versioning to leverage the browser cache behavior so that packaged resources can be downloaded and cached as efficiently as possible. The browser only needs to check for new updates when the cache has been emptied or when the component library has been upgraded at the Web server.

Listing 2 illustrates the Weblets versioning feature by adding a 1.0 version to our org.myapp.html Weblet.

By specifying a Weblet version, you indicate that the packaged resource is not going to change until the version number changes. Therefore, the version number is included as part of the resource URL determined at runtime by the WebletsViewHandler, e.g., /myresources$1.0/myScript.js. When the WebletContainer services this request, it extracts the version number from the URL and determines that the resource should be cached and never expire. As soon as a new version of the component library is deployed to the Web application, the resource URL created at runtime by the WebletsViewHandler changes, e.g., /myresources$2.0/myScript.js, thus the browser's cached copy of myScript.js for version 1.0 is no longer valid because the URL is different.

During development, the contents of packaged resources can change frequently, so it is important for the browser to keep checking back with the Web server to detect the latest resource URL contents. This check happens by default every time the main Web page is rendered if the Weblet version is omitted from weblets-config.xml.

Alternatively the Weblet configuration allows component authors to append -SNAPSHOT to the version number. For example, 1.0-SNAPSHOT, as shown in the following code, indicates that this file is under development and should behave as though the version number has been omitted.

<?xml version="1.0" encoding="UTF-8" ?>
<weblets-config xmlns="http://weblets.dev.java.net/config" >
   <weblet>
     <weblet-name>org.myapp.html</weblet-name>
     <weblet-class>net.java.dev.weblets.packaged.PackagedWeblet      </weblet-class>
     <weblet-version>1.0-SNAPSHOT</weblet-version>
     ...
   </weblet>
   ...
</weblets-config>