An absolute path may sometimes contain aliases, shadows, symbolic links and shortcuts as opposed to canonical paths, which refer to actual files/directories that these point to. Canonicalizing file names makes it much easier to verify a path, directory, or file name by making it easier to compare names.
Non-Compliant Code Example
In this example, the user inputs a part of the path as a command line argument. Let argv[1] be "java" where /tmp/java is a symbolic link that points to another file in some directory. On Unix, the getAbsolutePath() method includes /tmp/java (name of the symbolic link) in the path that it returns. On the other hand, on Windows and Macintosh systems, this behavior is not observed. The symbolic link is fully resolved in this case leading to implementation defined behavior.
public static void main(String[] args) {
try {
File f = new File("/tmp/" + args[1]);
String absPath = f.getAbsolutePath();
}
catch(IOException ie) {}
}
Compliant Solution
Use the getCanonicalPath() method wherever possible since it resolves the aliases, shortcuts or symbolic links across all platforms. The value of the alias is not included in the returned value. Moreover, relative references like the double period (..) are also removed. The getCanonicalPath() method throws a security exception when used within applets since it reveals too much information about the host machine. The getCanonicalFile() method (Java 2) behaves like getCanonicalPath() but returns a new File object instead of a String.
public static void main(String[] args) {
try {
File f = new File("/tmp/" + args[1]);
String canonicalPath = f.getCanonicalPath();
}
catch(IOException ie) {}
}
References
Java I/O, by Elliotte Rusty Harold