File id for keeping track of file changes in Java?

问题:

I'm trying to find a way to keep track of files even when they are moved or renamed in the file system.

One idea I had was to use the new UserDefinedFileAttributeView in Java 7 and create a custom file attribute as a sort of custom id. I thought that this might work on different platforms (Windows and Mac primarily). But I can't get it to work. Even trying the example on this page - http://docs.oracle.com/javase/tutorial/essential/io/fileAttr.html - When I get to this line:

UserDefinedFileAttributeView view = Files.getFileAttributeView(file, UserDefinedFileAttributeView.class);

I only get a null value for the view variable, and then the program stops with a nullpointer exception.

I then found that there is a perhaps easier way to do this for Mac at least: use the BasicFileAttributes fileKey attribute. I tried this, and the fileKey seems to stay the same even if I move the file or rename it. However, it also says that this functionality is platform dependent, and I remember reading somewhere that it doesn't work on Windows...

So first of all, is the fileKey method a stable way of doing this on Mac? And if so, what can I do for the same functionality for Windows? Anyone know why I get null on the UserDefinedFileAttributeView? Because if I could get that to work it should be cross-platform I guess.

This is not something that needs to be super robust and scalable, it is just a small helper application I'm developing, but it needs to be at least reliable in identifying files when moved or renamed...

回答1:

I tried the Oracle example on a Windows XP computer. There was a very minor bug in the code example, but other than this, the code worked fine -- at least on Windows XP. Hopefully it would also work on Linux etc, but I personally have only tried it on Windows XP.

public static void main(String args[])
        throws Exception
{
    Path target = Paths.get("C:\\mytemp\\Something.txt");
    Files.createFile(target);
    UserDefinedFileAttributeView view = Files.getFileAttributeView(target, UserDefinedFileAttributeView.class);
    view.write("user.mimetype", Charset.defaultCharset().encode("text/html"));
    String name = "user.mimetype";
    ByteBuffer buf = ByteBuffer.allocate(view.size(name));
    view.read(name, buf);
    buf.flip();
    String value = Charset.defaultCharset().decode(buf).toString();
    System.out.println("value="+value);

Just to be sure the attribute was not just being read from the view, I also ran the same code using a 2nd view. This also worked...

public static void main(String args[])
        throws Exception
{
    Path target = Paths.get("C:\\mytemp\\SomethingDifferent.txt");
    Files.createFile(target);
    UserDefinedFileAttributeView view = Files.getFileAttributeView(target, UserDefinedFileAttributeView.class);
    view.write("user.mimetype", Charset.defaultCharset().encode("text/html"));
    String name = "user.mimetype";

    UserDefinedFileAttributeView view2 = Files.getFileAttributeView(target, UserDefinedFileAttributeView.class);
    ByteBuffer buf = ByteBuffer.allocate(view2.size(name));
    view2.read(name, buf);
    buf.flip();
    String value = Charset.defaultCharset().decode(buf).toString();
    System.out.println("value="+value);


}

It would be great if such custom file attributes work across all the major platforms, as such custom file attributes are incredibly handy in some situations. Hope they do.

回答2:

This is not implemented on the OSX version of Java. This bug is still open: https://bugs.openjdk.java.net/browse/JDK-8030048

This bug was closed, https://bugs.openjdk.java.net/browse/JDK-8040830, with a reference to use a 3rd party workaround (which I haven't tried): https://github.com/IsNull/xattrj