Excalibur Source Resolving - Overview

A very common problem is resolving a source, which means finding a source using a URI.

The source resolver of Avalon Excalibur is a component helping you in this task. It resolves sources from a given URI. The URI can use all available protocols of the JRE. In addition own protocols can be plugged-in. So using the standard protocols like HTTP, FTP or file can be handled in the same way, like dealing with custom, self-build protocols such as myxmldatabase://root/documents/test.xml.

The main advantage in comparisson to the mechanisms provided by the JRE is that the source resolver can be used without any problems with web application servers. Each web application can use it's own configured version of this component avoiding any possible conflicts between these applications.

The architecture of this package is simple but powerful. The main component is the SourceResolver. It is used to resolve any URI. If the SourceResolver can resolve the protocol of the URI, it returns a Source object. This Source object is an abstraction of the underlying resource. This resource can be accessed by a provided InputStream.

Own protocols can be configured using the SourceFactory interface. Whenever the SourceResolver finds a protocol that it can't handle by itself, it gets a role selector for a SourceFactory and tries to get a component with the role name of the protocol. If such a factory exists, the source creation is passed on to this factory.

The Source object is handled similar to Avalon components. After it has been used it must be released using the SourceResolver. The SourceResolver in turn passed on the release of the object to the SourceFactory that created it.

The Source object is a lightwight object which can be extended with several interface. For example the XMLizable interface from the XML package to generate SAX events from the Source. Or the Monitorable interface from the monitor package to monitor the resource.

For caching purposes the Source object offers a SourceValidity object which can be used in addition to the system ID of the Source to verify if a cache contains a valid version of the Source object.

You can see the source resolving in action in the Apache Cocoon project.