Check if file is in (sub)directory

Question

I would like to check whether an existing file is in a specific directory or a subdirectory of that.

I have two File objects.

File dir;
File file;

Both are guaranteed to exist. Let's assume

dir  = /tmp/dir 
file = /tmp/dir/subdir1/subdir2/file.txt

I want this check to return true

For now i am doing the check this way:

String canonicalDir = dir.getCanonicalPath() + File.separator;
boolean subdir = file.getCanonicalPath().startsWith(canonicalDir);

This seems to work with my limited tests, but i am unsure whether this might make problems on some operating systems. I also do not like that getCanonicalPath() can throw an IOException which i have to handle.

Is there a better way? Possibly in some library?

Thanks

Answer 1

In addition to the asnwer from rocketboy, use getCanonicalPath() instad of getAbsolutePath() so \dir\dir2\..\file is converted to \dir\file:

    boolean areRelated = file.getCanonicalPath().contains(dir.getCanonicalPath() + File.separator);
    System.out.println(areRelated);

or

boolean areRelated = child.getCanonicalPath().startsWith(parent.getCanonicalPath() + File.separator);

Do not forget to catch any Exception with try {...} catch {...}.

NOTE: You can use FileSystem.getSeparator() instead of File.separator. The 'correct' way of doing this will be to get the getCanonicalPath() of the directory that you are going to check against as a String, then check if ends with a File.separator and if not then add File.separator to the end of that String, to avoid double slashes. This way you skip future odd behaviours if Java decides to return directories with a slash in the end or if your directory string comes from somewhere else than Java.io.File.

NOTE2: Thanx to @david for pointing the File.separator problem.

Answer 2

I would create a small utility method:

public static boolean isInSubDirectory(File dir, File file) {

    if (file == null)
        return false;

    if (file.equals(dir))
        return true;

    return isInSubDirectory(dir, file.getParentFile());
}