> -----Original Message----- > From: Dominique Devienne [mailto:[EMAIL PROTECTED] > Sent: Friday, 22 September 2006 11:44 AM > To: Ant Developers List > Subject: Re: Resource.getURL() > > > Just for reference I have quite a bit of experience working with > > custom protocol handlers. > > OK, that's better than my casual experience with them. How do > you install them? What does it require in terms of JVM > startup arguments?
There are two primary mechanisms: * declaring a URLStreamHandlerFactory * declaring the 'java.protocol.handler.pkgs' system property > Does that play well with application containers that often embed Ant? The first option (the URLStreamHandlerFactory) can cause problems in that only one factory may be declared and conflicts can arise with applications that assume they have control over this resource. The second option is more flexible in that the system property value can be set within an application and any existing declarations can be maintained because arguments are separated by the '|' character (i.e. Ant can read in the value of the property and supplement that value with any additional protocol package names that are declared). An example of the declaration of the 'java.protocol.handler.pkgs' system property is: -Djava.protocol.handler.pkgs=net.dpml.transit In this example the property value identifies a package path which will be used to resolve a protocol handler. In the case of the Transit resource management system there are a number of protocols including 'artifact', 'link', and 'local'. When a URL is constructed the protocol is identified (e.g. file:, http:, or in the case of Transit - artifact:). The JVM uses the combination of package path and protocol name to locate a protocol handler using the convention: [package-path].[protocol-name].Handler In the case of the DPML Transit artifact protocol [1] and handler [2] this translates to the class: net.dpm.transit.artifact.Handler Something to keep in mind is that the creation of a URL is handled inside the system classloader. As such when the JVM attempts to locate handler classes, it is limited to classes that exist within the system classloader or system extensions. Application normally deal with this constraint be ensuring that the jar file containing protocol handlers is included in system classloader or is installed as a JVM extension - however, an alternative is to deploy a JVM with a mutable system classloader. > > In the context of Ant this subject will inevitably link back to the > > subject of system classloader mutation. Enabling the > > declaration of > > custom protocol handlers as a part of system classloader expansion > > would significantly simply things for applications such as Depot > > (which makes extensive use of custom handlers). In fact the > > combination of system classloader mutation plus support for > custom URL > > handlers would contribute to a significant potential > simplification of > > Depot's extensions to the Ant project model and task some > task implementations. > > You're loosing me a bit here ;-) As far as I understand them, > Ant's new Resource type is kind of a duplicate to URLs to get > access to some content (stream), but without the "location" > part. There's already a URLResource I believe, so whatever > URL the JVM already supports can be hooked up to a resource. > Why would Ant need to "mess" with custom URL handlers? Custom handlers allow the introduction of alternative resource identification and retrieval mechanisms. For example - data could be referenced using a query structure expressed as a url and the handler could use the url to interrogate a remote database. Another example is multiple remote data sources (as is the case of the 'artifact' protocol). In these example the custom URL enables the removal of logic concerning resource type and associated retrieval mechanics from application code. This results in greater portability and easier long-term maintenance. > I guess my position on these handlers depends on the ease to > install them to a running JVM, and the actual use cases > having them would solves. --DD I'll need to get myself more up-to-date with the resource class and its usage in Ant to answer this properly (which I'll do in the next few days). Cheers, Steve. [1] artifact protocol http://www.dpml.net/transit/technical/artifact.html [2] artifact protocol handler javadoc and source http://www.dpml.net/api/dpml/1.0.2/net/dpml/transit/artifact/Handler.html --------------------------------------------------------------------- To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED]
