上傳檔案轉換編碼(using Java)

網頁服務滿常有上傳檔案這個基本功能，偶爾會遇到編碼的問題，這裡分享一下上傳檔案後轉換成utf-8的方法。

主程式邏輯中，先判斷檔案的編碼是什麼，再依照編碼去判斷是否要另外用指定編碼產生新檔案。

String fileEncode = getCharsetByBOM(file);
logger.info("The encoding of upload file : " + fileEncode);
/* Change the encoding if the that was not UTF-8 */
if (!fileEncode.equals("UTF-8")) {
  saveAsNewFile(file, filePath, fileEncode);
  logger.info(String.format("Encoding of upload file: %s -> UTF-8", fileEncode));
} else {
  file.renameTo(new File(filePath));
}

判斷編碼的方法如下

/**
 * Distinguish the encoding of the input file using BOM.
 * @param file
 * @return
 * @throws Exception
 */
private static String getCharsetByBOM(File file) throws Exception {
 BufferedInputStream bin = new BufferedInputStream(new FileInputStream(file));
 int p = (bin.read() << 8) + bin.read();
 String encode = null;
switch (p) {
 case 0xefbb:
  encode = "UTF-8";
  break;
 case 0xfffe:
  encode = "Unicode";
  break;
 case 0xfeff:
  encode = "UTF-16BE";
  break;
 default:
  encode = "BIG5";
 }
 return encode;
}

若指定編碼不是utf-8，則另外產生指定編碼的新檔案。

/**
 * Save the file to certain directory in UTF-8 encoding.
 * @param file - The uploaded file
 * @param filePath - Output file path
 * @param currentEncode - The encoding type of uploaded file
 * @throws Exception
 */
public static void saveAsNewFile(File file, String filePath, String currentEncode) throws Exception {
 InputStream fin = new FileInputStream(file);
 FileOutputStream fout = new FileOutputStream(new File(filePath));
byte[] c = new byte[(int) file.length()];
 while (fin.available() > 0) {
  fin.read(c);
  fout.write(new String(c, currentEncode).getBytes("utf-8"));
 }
 fin.close();
 fout.close();
}