java - How to determine if a String contains invalid encoded characters

Question

Usage scenario

We have implemented a webservice that our web frontend developers use (via a php api) internally to display product data. On the website the user enters something (i.e. a query string). Internally the web site makes a call to the service via the api.

Note: We use restlet, not tomcat

Original Problem

Firefox 3.0.10 seems to respect the selected encoding in the browser and encode a url according to the selected encoding. This does result in different query strings for ISO-8859-1 and UTF-8.

Our web site forwards the input from the user and does not convert it (which it should), so it may make a call to the service via the api calling a webservice using a query string that contains german umlauts.

I.e. for a query part looking like

    ...v=abc?def

if "ISO-8859-1" is selected, the sent query part looks like

...v=abc%E4def

but if "UTF-8" is selected, the sent query part looks like

...v=abc%C3%A4def

Desired Solution

As we control the service, because we've implemented it, we want to check on server side wether the call contains non utf-8 characters, if so, respond with an 4xx http status

Current Solution In Detail

Check for each character ( == string.substring(i,i+1) )

1. if character.getBytes()[0] equals 63 for '?'
2. if Character.getType(character.charAt(0)) returns OTHER_SYMBOL

Code

protected List< String > getNonUnicodeCharacters( String s ) {
  final List< String > result = new ArrayList< String >();
  for ( int i = 0 , n = s.length() ; i < n ; i++ ) {
    final String character = s.substring( i , i + 1 );
    final boolean isOtherSymbol = 
      ( int ) Character.OTHER_SYMBOL
       == Character.getType( character.charAt( 0 ) );
    final boolean isNonUnicode = isOtherSymbol 
      && character.getBytes()[ 0 ] == ( byte ) 63;
    if ( isNonUnicode )
      result.add( character );
  }
  return result;
}

Question

Will this catch all invalid (non utf encoded) characters? Does any of you have a better (easier) solution?

Note: I checked URLDecoder with the following code

final String[] test = new String[]{
  "v=abc%E4def",
  "v=abc%C3%A4def"
};
for ( int i = 0 , n = test.length ; i < n ; i++ ) {
    System.out.println( java.net.URLDecoder.decode(test[i],"UTF-8") );
    System.out.println( java.net.URLDecoder.decode(test[i],"ISO-8859-1") );
}

This prints:

v=abc?def
v=abc?def
v=abc?def
v=abc?¤def

and it does not throw an IllegalArgumentException sigh

Answer 1

I asked the same question,

Handling Character Encoding in URI on Tomcat

I recently found a solution and it works pretty well for me. You might want give it a try. Here is what you need to do,

1. Leave your URI encoding as Latin-1. On Tomcat, add URIEncoding="ISO-8859-1" to the Connector in server.xml.
2. If you have to manually URL decode, use Latin1 as charset also.
3. Use the fixEncoding() function to fix up encodings.

For example, to get a parameter from query string,

  String name = fixEncoding(request.getParameter("name"));

You can do this always. String with correct encoding is not changed.

The code is attached. Good luck!

 public static String fixEncoding(String latin1) {
  try {
   byte[] bytes = latin1.getBytes("ISO-8859-1");
   if (!validUTF8(bytes))
    return latin1;   
   return new String(bytes, "UTF-8");  
  } catch (UnsupportedEncodingException e) {
   // Impossible, throw unchecked
   throw new IllegalStateException("No Latin1 or UTF-8: " + e.getMessage());
  }

 }

 public static boolean validUTF8(byte[] input) {
  int i = 0;
  // Check for BOM
  if (input.length >= 3 && (input[0] & 0xFF) == 0xEF
    && (input[1] & 0xFF) == 0xBB & (input[2] & 0xFF) == 0xBF) {
   i = 3;
  }

  int end;
  for (int j = input.length; i < j; ++i) {
   int octet = input[i];
   if ((octet & 0x80) == 0) {
    continue; // ASCII
   }

   // Check for UTF-8 leading byte
   if ((octet & 0xE0) == 0xC0) {
    end = i + 1;
   } else if ((octet & 0xF0) == 0xE0) {
    end = i + 2;
   } else if ((octet & 0xF8) == 0xF0) {
    end = i + 3;
   } else {
    // Java only supports BMP so 3 is max
    return false;
   }

   while (i < end) {
    i++;
    octet = input[i];
    if ((octet & 0xC0) != 0x80) {
     // Not a valid trailing byte
     return false;
    }
   }
  }
  return true;
 }

EDIT: Your approach doesn't work for various reasons. When there are encoding errors, you can't count on what you are getting from Tomcat. Sometimes you get ? or ?. Other times, you wouldn't get anything, getParameter() returns null. Say you can check for "?", what happens your query string contains valid "?" ?

Besides, you shouldn't reject any request. This is not your user's fault. As I mentioned in my original question, browser may encode URL in either UTF-8 or Latin-1. User has no control. You need to accept both. Changing your servlet to Latin-1 will preserve all the characters, even if they are wrong, to give us a chance to fix it up or to throw it away.

The solution I posted here is not perfect but it's the best one we found so far.