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:

Answer 1

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

Answer 2

This is probably not the most effective way, but shows how to do it using Java 8 pipelines:

private static String sanitizeFileName(String name) {
    return name
            .chars()
            .mapToObj(i -> (char) i)
            .map(c -> Character.isWhitespace(c) ? '_' : c)
            .filter(c -> Character.isLetterOrDigit(c) || c == '-' || c == '_')
            .map(String::valueOf)
            .collect(Collectors.joining());
}

The solution could be improved by creating custom collector which uses StringBuilder, so you do not have to cast each light-weight character to a heavy-weight string.

Answer 3

For those looking for a general solution, these might be common critera:

• The filename should resemble the string.
• The encoding should be reversible where possible.
• The probability of collisions should be minimized.

To achieve this we can use regex to match illegal characters, percent-encode them, then constrain the length of the encoded string.

private static final Pattern PATTERN = Pattern.compile("[^A-Za-z0-9_\\-]");

private static final int MAX_LENGTH = 127;

public static String escapeStringAsFilename(String in){

    StringBuffer sb = new StringBuffer();

    // Apply the regex.
    Matcher m = PATTERN.matcher(in);

    while (m.find()) {

        // Convert matched character to percent-encoded.
        String replacement = "%"+Integer.toHexString(m.group().charAt(0)).toUpperCase();

        m.appendReplacement(sb,replacement);
    }
    m.appendTail(sb);

    String encoded = sb.toString();

    // Truncate the string.
    int end = Math.min(encoded.length(),MAX_LENGTH);
    return encoded.substring(0,end);
}

Patterns

The pattern above is based on a conservative subset of allowed characters in the POSIX spec.

If you want to allow the dot character, use:

private static final Pattern PATTERN = Pattern.compile("[^A-Za-z0-9_\\-\\.]");

Just be wary of strings like "." and ".."

If you want to avoid collisions on case insensitive filesystems, you'll need to escape capitals:

private static final Pattern PATTERN = Pattern.compile("[^a-z0-9_\\-]");

Or escape lower case letters:

private static final Pattern PATTERN = Pattern.compile("[^A-Z0-9_\\-]");

Rather than using a whitelist, you may choose to blacklist reserved characters for your specific filesystem. E.G. This regex suits FAT32 filesystems:

private static final Pattern PATTERN = Pattern.compile("[%\\.\"\\*/:<>\\?\\\\\\|\\+,\\.;=\\[\\]]");

Length

On Android, 127 characters is the safe limit. Many filesystems allow 255 characters.

If you prefer to retain the tail, rather than the head of your string, use:

// Truncate the string.
int start = Math.max(0,encoded.length()-MAX_LENGTH);
return encoded.substring(start,encoded.length());

Decoding

To convert the filename back to the original string, use:

URLDecoder.decode(filename, "UTF-8");

Limitations

Because longer strings are truncated, there is the possibility of a name collision when encoding, or corruption when decoding.