Uniquely identify file in Java

Question

Im on Linux and my Java application is not intended to be portable.

I'm looking for a way to identify a file uniquely in Java. I can make use of statfs syscall since the pair (f_fsid, ino) uniquely identifies a file (not only across a file system) as specified here: http://man7.org/linux/man-pages/man2/statfs.2.html

The question is if it is possible extract fsid from Java directly so I can avoid writing JNI function?

inode can be extracted with NIO, but how about fsid? inode and fsid comes from different structure and are operated by different syscalls...

Pardon my ignorance, but wouldn't a file system path (e.g. /home/user/file.txt) identify the file? — Karol Dowbecki
– Karol Dowbecki, Commented Dec 26, 2018 at 18:23
@KarolDoabecki Unfortunately identifying simply by file path are not tolerable to renaming and is prone to race conditions in case we are operating on files concurrently. — Some Name
– Some Name, Commented Dec 26, 2018 at 18:24
if (and this is a big if) you need to go the native route and depending on your performance requirements I would suggest using JNA over JNI because JNA doesn't require that you write a native support library. I was surprised by how easy it was to use JNA. github.com/java-native-access/jna — HairOfTheDog
– HairOfTheDog, Commented Dec 26, 2018 at 18:40

HairOfTheDog · Accepted Answer · 2018-12-26 18:24:59Z

4

This java example demonstrates how to get the unix inode number of a file.

import java.nio.file.*;
import java.nio.file.attribute.*;

public class MyFile {

  public static void main(String[] args) throws Exception  {

    BasicFileAttributes attr = null;
    Path path = Paths.get("MyFile.java");

    attr = Files.readAttributes(path, BasicFileAttributes.class);

    Object fileKey = attr.fileKey();
    String s = fileKey.toString();
    String inode = s.substring(s.indexOf("ino=") + 4, s.indexOf(")"));
    System.out.println("Inode: " + inode);
  }
}

The output

$ java MyFile
Inode: 664938

$ ls -i MyFile.java 
664938 MyFile.java

credit where credit is due: https://www.javacodex.com/More-Examples/1/8

answered Dec 26, 2018 at 18:24

HairOfTheDog

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8 Comments

Some Name Over a year ago

If we are operating across the same file system it seems to work fine. But the inode of two files can be the same (even unlikely) if they are in some different directories which are on different file systems... cant they?

Some Name Over a year ago

Or this inode includes fsid as well...?

HairOfTheDog Over a year ago

@SomeName How about the FileStore which you can access via Files.getFileStore? docs.oracle.com/javase/8/docs/api/java/nio/file/FileStore.html docs.oracle.com/javase/8/docs/api/java/nio/file/…

Some Name Over a year ago

Looks like this is exactly what I need. Thanks!

Holger Over a year ago

@SomeName it’s not really clear at which point using Java code or using native code really is required. Java NIO has a way to uniquely identify files (the object returned by fileKey() does that) and it also has a watch service abstraction. Note that the code of the answer goes great lengths to destroy the uniqueness of the file key. It first converts it to a String, then extracts the not-unique inode from it. If you look at the original string representation, you’ll see something like "(dev=xxx,inode=yyy)", clearly hinting at the fact that the key object does uniquely identify the file.

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Jonathan Rosenne · Accepted Answer · 2018-12-26 18:30:26Z

2

I would suggest the GIT method of hashing the file contents. This is proof against copying and renaming.

Java is supposed to be platform independent so using Unix specific methods may not be what you want.

answered Dec 26, 2018 at 18:30

Jonathan Rosenne

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2 Comments

Some Name Over a year ago

This is the exact solution Im currently using... But unfortunately it was proven to not has enough flexibility we need.

Jonathan Rosenne Over a year ago

@Some Name could you please elaborate?

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