Used as part of the support for classpath {@link org.apache.tapestry5.Asset}s, to convert the Asset's {@link org.apache.tapestry5.ioc.Resource} to a URL that can be accessed by the client. The asset path, within theclasspath, is converted into a shorter virtual path. The term "alias" here is generally referred to as "virtual folder" elsewhere.
Service configuration is a map from folder aliases (short names) to complete paths. Names should not start or end end with a slash. Generally, an alias should be a single name (not contain a slash). Paths should also not start or end with a slash. An example mapping would be
mylib to
com/example/mylib.
As originally envisioned, this service would simply optimize classpath assets, allowing the URL path for such assets to be shortened (and have a version number added, important for caching); thus the word "alias" makes sense ... it was responsible for creating an "alias" URL shorter than the default "classpath" URL.
Starting in Tapestry 5.2, this changed; all classpath assets must be "aliased" to a shorter URL path. Any URL that can not be shortened is now rejected. This simplifies creating new libraries, but also helps with security concerns, as it limits which portions of the classpath can ever be exposed to the user agent.
Tapestry automatically contributes a number of mappings: for the application root package itself (as alias "app") and for each library (via {@link ComponentClassResolver#getFolderToPackageMapping()});