Android userspace filesystem driver on non-rooted device?

I had the same need some time ago and came to the point where I had to admit that's not possible at all (mostly). It shouldn't be a problem to build libfuse for android, also the Java wrapper is no problem.

The real problem is that most of the systems I have seen aren't build with fuse support buildin nor do they provide modules which may be loaded (which would require root, but anyway).

You can easily find out if fuse is enabled by reading /proc/filesystems. It should list fuse, otherwise you will need root. But it's very likely that most android devices are build without fuse support.

My other ideas were to use another filesystem to "fake" something like fuse. This may be possible with nfs or some other network filesystem where you can implement the server by yourself. This whould enable you to write a fake fuse but I don't think it's worth it.

Edit:And even if many devices would have fuse support buildin chances are high they wouldn't let you mount it as a user, you would need root access as your app wouldn't have the privileges to mount fuse filesystems.

Luminger almost has the right idea. Here's what I found on my Galaxy S3 (on Verizon):

The appropriate fuse stuff does exist in /proc/filesystems. So something must be using it, quite possibly system. However, when I attempt to execute 'fusermount', I get "Cannot execute -- Permission denied."

So it looks like all the FUSE stuff is there, but I've got no idea whether I could actually use it directly (dropbox? sshfs?) without rooting the device.

It depends on your implementation. What it sounds like to me is you want to have a physical file(s) that represent a full user space file system. Perhaps similar to a mounted .iso. If this is the case, I cannot think of any reason for needing root: You simply create/install the file somewhere such as /sdcard/ and "mount" your file system on top of it.

That said, if you want said file system to be accessible from other applications that you do not control, you'll need root as your application will be running in a Android sandbox otherwise.