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Java : How to determine the correct charset encoding of a stream
User will upload a CSV file to the server, server need to check if the CSV file is encoded as UTF-8. If so need to inform user, (s)he uploaded a wrong encoding file. The problem is how to detect the file user uploaded is UTF-8 encoding? The back end is written in Java. So anyone get the suggestion?
Well, you can't. You could show kind of a "preview" (or should I say review?) with some sample data from the file so the user can check if it looks okay. Perhaps with the possibility of selecting different encoding options to help determine the correct one.
You can try and guess the encoding using a 3rd party library, for example: http://glaforge.free.fr/wiki/index.php?wiki=GuessEncoding
At least in the general case, there's no way to be certain what encoding is used for a file -- the best you can do is a reasonable guess based on heuristics. You can eliminate some possibilities, but at best you're narrowing down the possibilities without confirming any one. For example, most of the ISO 8859 variants allow any byte value (or pattern of byte values), so almost any content could be encoded with almost any ISO 8859 variant (and I'm only using "almost" out of caution, not any certainty that you could eliminate any of the possibilities).
You can, however, make some reasonable guesses. For example, a file that start out with the three characters of a UTF-8 encoded BOM (EF BB BF), it's probably safe to assume it's really UTF-8. Likewise, if you see sequences like: 110xxxxx 10xxxxxx, it's a pretty fair guess that what you're seeing is encoded with UTF-8. You can eliminate the possibility that something is (correctly) UTF-8 enocded if you ever see a sequence like 110xxxxx 110xxxxx. (110xxxxx is a lead byte of a sequence, which must be followed by a non-lead byte, not another lead byte in properly encoded UTF-8).