Legacy java.io.File and new FileSystems

[Michael Hall]

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On Aug 11, 2010, at 7:26 AM, Alan Bateman wrote:


> The API allows the default provider to be replaced and also  
> supports providers that want to layer themselves over the default  
> provider. You'll find the details in the javadoc.
>


Sorry, I'm trying not to take too much time on what may not be seen  
as an important feature. However, this still seems to have  
limitations with regard to legacy code.
If as indicated in the javadoc I start with
- -Djava.nio.file.spi.DefaultFileSystemProvider=<a custom file system  
provider>
and run a simple program that does

new java.io.File("TestFSP.java").exists();

with verbose classloading it shows no nio is involved.
It appears to get java.io.FileSystem and java.io.UnixFileSystem only.
if I change the program to

new java.io.File("TestFSP.java").toPath().exists();

nio does get loaded and my custom FileSystem is passed the exists()  
invocation.

Again, this leaves no way to get unmodified File based legacy code to  
use the new FileSystem API. This doesn't seem to be a limitation of  
the File class. The same system property check that Path apparently  
does somewhere could be done in the java.io.FileSystem.getFileSystem  
invocation? Or even locally File.exists could do a toPath().exists(),  
unless there's some worry this might in some instances change legacy  
behavior in unanticipated and unwanted ways.

Also, I haven't got that far, but assuming it would be possible to  
provide an alternate legacy provider for the 'file' scheme, I might  
not always want it to handle all instances. For example if it was a  
zip provider and the path was my.path.to.zip.and.to.entry my provider  
would want to handle it. If was to a.non.pkzip.file.entirely.txt,  
then I would want to pass it to the default provider. Some way to  
delegate non-handled to the default or some other provider would be  
good? If use of the system property erases the only reference to the  
default platform 'file' provider during launching this wouldn't be  
possible.

Mike Hall        hallmike at att dot net
http://www195.pair.com/mik3hall
http://www195.pair.com/mik3hall/home.html
http://sourceforge.net/projects/macnative




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