FULL PRODUCT VERSION :
java version "1.8.0_65"
Java(TM) SE Runtime Environment (build 1.8.0_65-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.65-b01, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Linux debian 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt25-2 (2016-04-08) x86_64 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
An attempt to open a FileChannel or AsynchronousFileChannel e.g. using FileChannel.open(path, StandardOpenOption.WRITE) fails if path points to a file on a gvfs mounted WebDav share.

When using the terminal to inspect the share ls -l shows that the files are writable by the user running the java program. In addition using a FileOutputStream to write data to the file is possible.

When mounting the same WebDav share using mount -t davfs [...] channels can be opened.

When doing FileChannel.open(path, StandardOpenOption.READ) this works so seem to be releated to the WRITE option.

I am note sure if this applies to all gvfs mounts or only WebDav shares. I confirmed this for a WebDav share.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1 Start a webdav server which may afterwards be reachable e.g. at http://localhost:42232/a
2 Use "gvfs-mount dav:localhost:42232/a" to mount the drive.
3 Create a file "test" in this share using e.g. a text editor.
4 Create a path to the file (gvfs mounted to /run/user/1000/gvfs/dav:host=localhost,port=42232,ssl=false,prefix=%2Fa in my example)
5 Use this path to create a file channel with the WRITE open option specified

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Because writing is in general possible, e.g. using FileOutputStream, I would expect it to create a writable FileChannel as an alternativ an UnsupportedOperationException as specified in the javadoc of open could be thrown.
ACTUAL -
A FileSystemException (not UnsupportedOperationException) with message "Operation not supported" is thrown.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
java.nio.file.FileSystemException: /run/user/1000/gvfs/dav:host=localhost,port=42232,ssl=false,prefix=%2Fa/test: Operation not supported
at sun.nio.fs.UnixException.translateToIOException(UnixException.java:91)
at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:102)
at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:107)
at sun.nio.fs.UnixFileSystemProvider.newAsynchronousFileChannel(UnixFileSystemProvider.java:196)
at java.nio.channels.AsynchronousFileChannel.open(AsynchronousFileChannel.java:248)
at java.nio.channels.AsynchronousFileChannel.open(AsynchronousFileChannel.java:301)
[...]



REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
private final String root = "/run/user/1000/gvfs/dav:host=localhost,port=42232,ssl=false,prefix=%2Fa";
private final Path test = Paths.get(root + "/test");

@Test
public void testCreateChannelForWriting() throws IOException {
try (FileChannel channel = FileChannel.open(test, StandardOpenOption.WRITE)) {

}
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Mount the filesystem using mount -t davfs [...] instead of gvfs. This is possible in some cases but not always an option because the mount process used by third party software can not always be influenced.