Is comparing `getAbsolutePath()` and `getCanonicalPath()` a reliable way to determine if a directory in Java 1.6 is a symbolic link?

Determining Symbolic Links in Java 1.6

In a Java program operating on Unix systems, distinguishing between actual directories and symbolic links is essential. This question explores a method for identifying symbolic links to directories using a specific condition.

Question:

In the context of a DirectoryWalker class, can the following approach be used to accurately determine if a known directory instance represents a symbolic link:

<code class="java">if (file.getAbsolutePath().equals(file.getCanonicalPath())) {
    // real directory ---> do normal stuff      
}
else {
    // possible symbolic link ---> do link stuff
}</code>

Answer:

While the approach provided is a common technique to identify possible symbolic links, it cannot be considered reliable for the following reasons:

• Potential False Positives: A mismatch between the absolute and canonical paths can occur even for actual directories, such as mounted network drives.
• Platform Dependence: The behavior of file paths can vary across different operating systems and file systems.

Instead of relying on the absolute and canonical paths of the file itself, it is recommended to use the canonical path of the parent directory. This approach is more accurate in identifying symbolic links.

Here's an example from Apache Commons that implements this technique:

<code class="java">public static boolean isSymlink(File file) throws IOException {
  if (file == null)
    throw new NullPointerException("File must not be null");
  File canon;
  if (file.getParent() == null) {
    canon = file;
  } else {
    File canonDir = file.getParentFile().getCanonicalFile();
    canon = new File(canonDir, file.getName());
  }
  return !canon.getCanonicalFile().equals(canon.getAbsoluteFile());
}</code>

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