Java fails in moving (renaming) a file when the resulting file is on another filesystem

Question

A program we have erred when trying to move files from one directory to another. After much debugging I located the error by writing a small utility program that just moves a file from one directory to another (code below). It turns out that while moving files around on the local filesystem works fine, trying to move a file to another filesystem fails.

Why is this? The question might be platform specific - we are running Linux on ext3, if that matters.

And the second question; should I have been using something else than the renameTo() method of the File class? It seems as if this just works on local filesystems.

Tests (run as root):

touch /tmp/test/afile

java FileMover /tmp/test/afile /root/
The file move was successful

touch /tmp/test/afile

java FileMover /tmp/test/afile /some_other_disk/
The file move was erroneous

Code:

import java.io.File;

public class FileMover {
    public static void main(String arguments[] ) throws Exception {
        boolean success;
        File file = new File(arguments[0]);
        File destinationDir = new File(arguments[1]);
        File destinationFile = new File(destinationDir,file.getName() );
        success = file.renameTo(destinationFile);
        System.out.println("The file move was " + (success?"successful":"erroneous"));
    }
}

Answer 1

Java 7 and above

Use Files.move(Path source, Path target, CopyOption... opts).

Note that you must not provide the ATOMIC_MOVE option when moving files between file systems.

Java 6 and below

From the docs of File.renameTo:

[...] The rename operation might not be able to move a file from one filesystem to another [...]

The obvious workaround would be to copy the file "manually" by opening a new file, write the content to the file, and delete the old file.

You could also try the FileUtils.moveFile method from Apache Commons.

Answer 2

Javadoc to the rescue:

Many aspects of the behavior of this method are inherently platform-dependent: The rename operation might not be able to move a file from one filesystem to another, it might not be atomic, and it might not succeed if a file with the destination abstract pathname already exists. The return value should always be checked to make sure that the rename operation was successful.

Note that the Files class defines the move method to move or rename a file in a platform independent manner.