public static FileReference mockFileReferenceForUnitTesting(File file) {
        if (! file.exists())
            throw new IllegalArgumentException("File '" + file.getAbsolutePath() + "' does not exist.");
        return new FileReference(file.getPath());
    }
An immutable file reference.

@author Tony Vaagenes
config-lib/src/main/java/com/yahoo/config/FileReference.java