Dynamic loading of FileSystemProvider instances

[Alan Bateman]

Christian Schlichtherle wrote:
> Hi Alan,
>
> we recently discussed that I have a need to determine the NIO.2
> FileSystemProvider instances at run time. 
I think that discussion was more that there isn't a method to add to the 
list of installed providers at runtime (the list returned by 
FileSystemProvider.installedProviders is created at startup and there 
isn't a method to register/add to it). You can of course use 
ServiceLoader.load(FileSystemProvider.class, loader) to locate 
additional providers yourself.

> Because this is not possible, you
> suggested I should introduce one VFS scheme for my TrueZIP
> TFileSystemProvider. 
That suggestion wasn't exactly related but rather that if you are 
developing a virtual file system then you need to choose a (singular) 
URI scheme in order to deploy as a provider. That provider can of course 
delegate to other providers, which I think is what you are doing.

> For example, the NIO.2 Path URI "tzp:/archive.zip"
> should get identified by my TrueZIP TFilesystemProvider and internally
> rewritten to the TrueZIP FsPath URI "zip:file:/archive.zip!/".
>
> While this is certainly possible, it still limits my implementation: The
> issue is that I need to determine a TrueZIP FsMountPoint URI for rewriting
> the NIO.2 Path URI. In the example above, this is "file:/". However, on
> Windows, it could be "file:/C:/" or "file:/D:/" or whatever. In TrueZIP, it
> could even be "http://somehost/" or "http://someotherhost/base/" or
> whatever. It could even be an entirely different scheme, depending on the
> file system driver.
>
> To overcome this, I need a way to determine the NIO.2 FileSystemProvider
> instances at run time. To make minimal impact on the current NIO.2 API, I
> suggest the following quick draft:
>
> <code>
> package java.nio.file.spi;
>
> public interface FileSystemProviderCatalogue extends
> Iterable<FileSystemProvider> {
> }
> </code>
>
> You could then simply use java.util.ServiceLoader to locate
> FileSystemProviderCatalogue instances on the run time class path while I
> could happily implement the iterator to figure the TFileSystemProvider
> instances I need.
>   
I don't think I fully understand it. If there are providers on the class 
path then they should be returned by the 
FileSystemProvider.installedProviders method.

-Alan