Some applications need to distinguish between a "real" file and a link/shortcut/alias to a file. Java.io.File does not have an isLink() function, but it appears that links can be distinguished in pure java in at least some cases.

Here's what I understand of the link/alias/shortcut behavior on different operating systems:

• Windows: shortcuts are in fact just files that are treated specially by the file browser. Thus, no issue here.
• WindowsNT: The operating system supports a version of links, but there is no way to create them without writing a program(?). No issue here in most circumstances.
• Macintosh (pre OSX): I do not know if an alias can point to a folder, nor how aliases appear to a java program. Email me if you know this.
• Unix (hard or non-symbolic) links: I do not know if there is any way to detect these. Nor do I understand what they're good for or who uses them...
• Unix symbolic links:

  For a link that actually points to something (either a file or a directory), the absolute path is the path through the link, whereas the canonical path is the path the link references.

  Dangling links appear as files of size zero, and generate a FileNotFoundException when you try to open them. Unfortunately non-existent files have the same behavior (size zero - why did't file.length() throw an exception if the file doesn't exist, rather than returning length zero?).

  The routine below appears to detect Unix/Linux symbolic links, but it also returns true for non-existent files. Note that if a directory is a link, all the contents of this directory will appear as symbolic links as well.

  public static boolean isLink(File file)
  {
    try {
      if (!file.exists())
	return true;
      else
	{
	  String cnnpath = file.getCanonicalPath();
	  String abspath = file.getAbsolutePath();
	  return !abspath.equals(cnnpath);
	}
    }
    catch(IOException ex) {
      System.err.println(ex);
      return true;
    }
  } //isLink

Update 2006 Tom O'Neill at google looked into this matter and found that there is still no complete solution to this problem in java.