首先，不同编码的文本，是根据文本的前两个字节来定义其编码格式的。定义如下：

  ANSI：　　　　　　　　无格式定义；

  Unicode： 　　　　　　前两个字节为FFFE；

  Unicode big endian：　前两字节为FEFF；　 

  UTF-8：　 　　　　　　前两字节为EFBB；　

  知道了各种编码格式的区别，写代码就容易了.

1 package charset; 2  3 import java.io.BufferedInputStream; 4 import java.io.File; 5 import java.io.FileInputStream; 6  7 public class demo { 8     public static void main(String[] args) { 9         String txtCharset=get_charset(new File("e:/1.TXT"));10         System.out.println(txtCharset);11     }12     public static String get_charset(File file) {13         String charset = "GBK";14         byte[] first3Bytes = new byte[3];15         try {16             boolean checked=false;17             BufferedInputStream bis = new BufferedInputStream(18                     new FileInputStream(file));19             bis.mark(0);20             int read = bis.read(first3Bytes, 0, 3);21             if (read == -1)22                 return charset;23             if (first3Bytes[0] == (byte) 0xFF && first3Bytes[1] == (byte) 0xFE) {24                 charset = "UTF-16LE";25                 checked = true;26             } else if (first3Bytes[0] == (byte) 0xFE27                     && first3Bytes[1] == (byte) 0xFF) {28                 charset = "UTF-16BE";29                 checked = true;30             } else if (first3Bytes[0] == (byte) 0xEF31                     && first3Bytes[1] == (byte) 0xBB32                     && first3Bytes[2] == (byte) 0xBF) {33                 charset = "UTF-8";34                 checked = true;35             }36             bis.reset();37             if (!checked) {38                 // int len = 0;39                 int loc = 0;40 41                 while ((read = bis.read()) != -1) {42                     loc++;43                     if (read >= 0xF0)44                         break;45                     if (0x80 <= read && read <= 0xBF) // 单独出现BF以下的，也算是GBK46                         break;47                     if (0xC0 <= read && read <= 0xDF) {48                         read = bis.read();49                         if (0x80 <= read && read <= 0xBF) // 双字节 (0xC0 - 0xDF)50                                                             // (0x8051                                                             // - 0xBF),也可能在GB编码内52                             continue;53                         else54                             break;55                     } else if (0xE0 <= read && read <= 0xEF) {
   // 也有可能出错，但是几率较小56                         read = bis.read();57                         if (0x80 <= read && read <= 0xBF) {58                             read = bis.read();59                             if (0x80 <= read && read <= 0xBF) {60                                 charset = "UTF-8";61                                 break;62                             } else63                                 break;64                         } else65                             break;66                     }67                 }68                 // System.out.println( loc + " " + Integer.toHexString( read )69                 // );70             }71 72             bis.close();73         } catch (Exception e) {74             e.printStackTrace();75         }76 77         return charset;78     }79 }