base64convert.txt

base64编码的原理就是将每三个字节扩展为四个字节,高位补零。

public class Base64Converter 
     ////////////////////////////////////////////////////////////////////// 
     ////////////////////////////////////////////////////////////////////// 
     { 

     public static final char [ ] alphabet = { 
       'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', // 0 to 7 
       'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', // 8 to 15 
       'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', // 16 to 23 
       'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', // 24 to 31 
       'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', // 32 to 39 
       'o', 'p', 'q', 'r', 's', 't', 'u', 'v', // 40 to 47 
       'w', 'x', 'y', 'z', '0', '1', '2', '3', // 48 to 55 
       '4', '5', '6', '7', '8', '9', '+', '/' }; // 56 to 63 

     ////////////////////////////////////////////////////////////////////// 
     ////////////////////////////////////////////////////////////////////// 

     public static String encode ( String s ) 
     ////////////////////////////////////////////////////////////////////// 
     { 
       return encode ( s.getBytes ( ) ); 
     } 

     public static String encode ( byte [ ] octetString ) 
     ////////////////////////////////////////////////////////////////////// 
     { 
       int bits24; 
       int bits6; 

       char [ ] out 
         = new char [ ( ( octetString.length - 1 ) / 3 + 1 ) * 4 ]; 

       int outIndex = 0; 
       int i = 0; 

       while ( ( i + 3 ) <= octetString.length ) 
{ 
// store the octets 
bits24 = ( octetString [ i++ ] & 0xFF ) << 16; 
bits24 |= ( octetString [ i++ ] & 0xFF ) << 8; 
bits24 |= ( octetString [ i++ ] & 0xFF ) << 0; 

bits6 = ( bits24 & 0x00FC0000 ) >> 18; 
         out [ outIndex++ ] = alphabet [ bits6 ]; 
         bits6 = ( bits24 & 0x0003F000 ) >> 12; 
         out [ outIndex++ ] = alphabet [ bits6 ]; 
         bits6 = ( bits24 & 0x00000FC0 ) >> 6; 
         out [ outIndex++ ] = alphabet [ bits6 ]; 
         bits6 = ( bits24 & 0x0000003F ); 
         out [ outIndex++ ] = alphabet [ bits6 ]; 
       } 

       if ( octetString.length - i == 2 ) 
       { 
         // store the octets 
         bits24 = ( octetString [ i ] & 0xFF ) << 16; 
bits24 |= ( octetString [ i + 1 ] & 0xFF ) << 8; 

bits6 = ( bits24 & 0x00FC0000 ) >> 18; 
         out [ outIndex++ ] = alphabet [ bits6 ]; 
         bits6 = ( bits24 & 0x0003F000 ) >> 12; 
         out [ outIndex++ ] = alphabet [ bits6 ]; 
         bits6 = ( bits24 & 0x00000FC0 ) >> 6; 
         out [ outIndex++ ] = alphabet [ bits6 ]; 

         // padding 
         out [ outIndex++ ] = '='; 
       } 
       else if ( octetString.length - i == 1 ) 
       { 
         // store the octets 
         bits24 = ( octetString [ i ] & 0xFF ) << 16; 

bits6 = ( bits24 & 0x00FC0000 ) >> 18; 
         out [ outIndex++ ] = alphabet [ bits6 ]; 
         bits6 = ( bits24 & 0x0003F000 ) >> 12; 
         out [ outIndex++ ] = alphabet [ bits6 ]; 

         // padding 
         out [ outIndex++ ] = '='; 
         out [ outIndex++ ] = '='; 
       } 

       return new String ( out ); 
     }