How can I safely encode a string in Java to use as a filename ?

Question

I'm receiving a string from an external process. I want to use that String to make a filename, and then write to that file. Here's my code snippet to do this:

    String s = ... // comes from external source
    File currentFile = new File(System.getProperty("user.home"), s);
    PrintWriter currentWriter = new PrintWriter(currentFile);

If s contains an invalid character, such as '/' in a Unix-based OS, then a java.io.FileNotFoundException is (rightly) thrown.

How can I safely encode the String so that it can be used as a filename?

Edit: What I'm hoping for is an API call that does this for me.

I can do this:

    String s = ... // comes from external source
    File currentFile = new File(System.getProperty("user.home"), URLEncoder.encode(s, "UTF-8"));
    PrintWriter currentWriter = new PrintWriter(currentFile);

But I'm not sure whether URLEncoder it is reliable for this purpose.

Answer 1

You could remove the invalid chars ( '/', '\', '?', '*') and then use it.

Answer 2

My suggestion is to take a "white list" approach, meaning don't try and filter out bad characters. Instead define what is OK. You can either reject the filename or filter it. If you want to filter it:

String name = s.replaceAll("\W+", "");

What this does is replaces any character that isn't a number, letter or underscore with nothing. Alternatively you could replace them with another character (like an underscore).

The problem is that if this is a shared directory then you don't want file name collision. Even if user storage areas are segregated by user you may end up with a colliding filename just by filtering out bad characters. The name a user put in is often useful if they ever want to download it too.

For this reason I tend to allow the user to enter what they want, store the filename based on a scheme of my own choosing (eg userId_fileId) and then store the user's filename in a database table. That way you can display it back to the user, store things how you want and you don't compromise security or wipe out other files.

You can also hash the file (eg MD5 hash) but then you can't list the files the user put in (not with a meaningful name anyway).

Answer 3

If you want the result to resemble the original file, SHA-1 or any other hashing scheme is not the answer. Instead you want something like this.

char fileSep = '/'; // ... or do this portably.
char escape = '%'; // ... or some other legal char.
String s = ...
int len = s.length();
StringBuilder sb = new StringBuilder(len);
for (int i = 0; i < len; i++) {
    char ch = s.charAt(i);
    if (ch < ' ' || ch >= 0x7F || ch == fileSep || ... // add other illegal chars
        || (ch == '.' && i == 0) // we don't want to collide with "." or ".."!
        || ch == escape) {
        sb.append(escape);
        if (ch < 0x10) {
            sb.append('0');
        }
        sb.append(Integer.toHexString(ch));
    } else {
        sb.append(ch);
    }
}
File currentFile = new File(System.getProperty("user.home"), sb.toString());
PrintWriter currentWriter = new PrintWriter(currentFile);

This solution gives a reversible encoding (with no collisions) where the encoded strings resemble the original strings in most cases. I'm assuming that you are using 8-bit characters.

URLEncoder has the disadvantage that it encodes a whole lot of legal file name characters.

Edit: If you want a not-guaranteed-to-be-reversible solution, then simply remove the 'bad' characters rather than replacing them with and escape sequence.

Edit 2: Addressed collisions with "." and ".." directory entries.