判斷文件的編碼

首先，不同編碼的文本，是根據文本的前兩個字節來定義其編碼格式的。定義如下：

  ANSI：　　　　　　　　無格式定義；
  Unicode： 　　　　　　前兩個字節爲FFFE；
  Unicode big endian：　前兩字節爲FEFF；　 
  UTF-8：　 　　　　　　前兩字節爲EFBB；　

  知道了各種編碼格式的區別，寫代碼就容易了.

public static String get_charset( File file ) {
        String charset = "GBK";
        byte[] first3Bytes = new byte[3];
        try {
            boolean;
            BufferedInputStream bis = new BufferedInputStream( new FileInputStream( file ) );
            bis.mark( 0 );
            int read = bis.read( first3Bytes, 0, 3 );
            if ( read == -1 ) return charset;
            if ( first3Bytes[0] == (byte) 0xFF && first3Bytes[1] == (byte) 0xFE ) {
                charset = "UTF-16LE";
                checked = true;
            }
            else if ( first3Bytes[0] == (byte) 0xFE && first3Bytes[1] == (byte) 0xFF ) {
                charset = "UTF-16BE";
                checked = true;
            }
            else if ( first3Bytes[0] == (byte) 0xEF && first3Bytes[1] == (byte) 0xBB && first3Bytes[2] == (byte) 0xBF ) {
                charset = "UTF-8";
                checked = true;
            }
            bis.reset();
            if ( !checked ) {
            //    int len = 0;
                int loc = 0;

                while ( (read = bis.read()) != -1 ) {
                    loc++;
                    if ( read >= 0xF0 ) break;
                    if ( 0x80 <= read && read <= 0xBF ) // 單獨出現BF以下的，也算是GBK
                    break;
                    if ( 0xC0 <= read && read <= 0xDF ) {
                        read = bis.read();
                        if ( 0x80 <= read && read <= 0xBF ) // 雙字節 (0xC0 - 0xDF) (0x80
                                                                        // - 0xBF),也可能在GB編碼內
                        continue;
                        else break;
                    }
                    else if ( 0xE0 <= read && read <= 0xEF ) {// 也有可能出錯，但是機率較小
                        read = bis.read();
                        if ( 0x80 <= read && read <= 0xBF ) {
                            read = bis.read();
                            if ( 0x80 <= read && read <= 0xBF ) {
                                charset = "UTF-8";
                                break;
                            }
                            else break;
                        }
                        else break;
                    }
                }
                //System.out.println( loc + " " + Integer.toHexString( read ) );
            }

            bis.close();
        } catch ( Exception e ) {
            e.printStackTrace();
        }

        return charset;
    }

From: http://ajava.org/code/I18N/14816.html