javawatchservice

is Java WatchService per JVM or can an app start multiple?

FileSystems.getDefault().newWatchService();

Does that create a new watchService or just give a singleton object?

it says:

@return a new watch service

Some post I read made me think there might only be 1 and so if you do a .take() or .poll() one thread might get another threads watchKey event. I thought maybe I need to use one but now I am not so sure because I do have 2 or so running and they seemed to work but I have not explicitly tested it.

Anyone know if you can use multiple watchServices in one app?

Solution

  • Going by the source code here it seems that each time it is called it returns a new WatchService

    class WindowsFileSystem
    extends FileSystem
{
..............................
..............................
@Override
    public WatchService newWatchService()
        throws IOException
    {
        return new WindowsWatchService(this);
    }
}

    this here refers to the WindowsFileSystem object ( I am checking this on Windows JVM ) which is static variable and hence single per JVM 

    public final class FileSystems {
    private FileSystems() {
    }
.................
.................
public static FileSystem getDefault() {
        return DefaultFileSystemHolder.defaultFileSystem;
    }

}

    So we can conclude that the FileSystem object is global, however WatchService can be as many as created - all having reference to the global FileSystem.