Which Operating Systems support native (inotify-like) file watching in Java

Question

The JavaDoc for java.nio.file.WatchService states;

The implementation ... is intended to map directly on to the native file event notification facility where available, or to use a primitive mechanism, such as polling, when a native facility is not available.

I assume this means it will try a lightweight, native mechanism when it can (something like inotify on Linux) and poll if it can not.

Is that correct?

What operating systems are likely or unlikely to provide such a facility? Distro level for Linux would be really useful or can I assume that if the JVM runs on *unix, it will be supported?

Answer 1

This should have probably been a comment, but it's too big to post it as such...

I am looking at jdk-9 sources (could easily be searched in the jdk-8 repo as well), but here is some relevant to your question comments:

   /**
     * Linux implementation of WatchService based on inotify.
     *
     * In summary a background thread polls inotify plus a socket used for the wakeup
     * mechanism. Requests to add or remove a watch, or close the watch service,
     * cause the thread to wakeup and process the request. Events are processed
     * by the thread which causes it to signal/queue the corresponding watch keys.
     */

 class LinuxWatchService  extends AbstractWatchService

And for windows:

/*
 * Win32 implementation of WatchService based on ReadDirectoryChangesW.
 */

class WindowsWatchService extends AbstractWatchService

And so on.. you can find all the available implementations under:

 jdk/src/java.base/{windows|unix|solaris|linux...}/classes/sun/nio/fs/

As what OS actually supports this, it seems like that would require for you to look at the actual distro.

Answer 2

I think that you can find some information in this post Watchservice in windows 7 does not work. There is some note about file system watchers on Windows and MacOSX (author of post says that they are using polling). On Linux file system monitoring is part of a Linux kernel and it's implemented using quite nice notifications mechanism (more info about some of them can be found here)