base64_encode和base64_decode的JAVA实现

base64是网络上最常见的用于传输8bit字节代码的编码方式之一，大家可以查看rfc2045～rfc2049，上面有mime的详细规范。 base64要求把每三个8bit的字节转换为四个6bit的字节（3*8 = 4*6 = 24），然后把6bit再添两位高位0，组成四个8bit的字节，也就是说，转换后的字符串理论上将要比原来的长1/3

php 的函数：base64_encode() 和 base64_decode()

base64的编,解码原理

base64 编码其实是将3个8位字节转换为4个6位字节,( 3*8 = 4*6 = 24 ) 这4个六位字节 其实仍然是8位,只不过高两位被设置为0. 当一个字节只有6位有效时,它的取值空间为0 到 2的6次方减1 即63,也就是说被转换的base64编码的每一个编码的取值空间为(0~63) 。

事实上，0~63之间的ascii码有许多不可见字符，所以应该再做一个映射，映射表为

'a' ~ 'z' ? ascii（0 ~ 25）

'a' ~ 'z' ? ascii（26 ~ 51）

'0' ~ '9' ? ascii（52 ~ 61）

' ' ? ascii（62）

'/' ? ascii（63）

这样就可以将3个8位字节，转换为4个可见字符。

具体的字节拆分方法为：(图（画得不好，领会精神 :-))

aaaaaabb ccccdddd eeffffff    //abcdef其实就是1或0，为了看的清楚就用abcdef代替

~~~~~~~~ ~~~~~~~~ ~~~~~~~~

字节 1 字节 2 字节 3

　　　　||
　　　　\/

00aaaaaa 00bbcccc 00ddddee 00ffffff

注：上面的三个字节位原文，下面四个字节为base64编码，其前两位均为0。

这样拆分的时候，原文的字节数量应该是3的倍数，当这个条件不能满足时，用全零字节

补足，转化时base64编码用=号代替，这就是为什么有些base64编码以一个或两个等号结

束的原因，但等号最多有两个，因为：如果f(origin)代表原文的字节数，f(remain)代

表余数，则

f(remain) = f(origin) mod 3 成立。

所以f(remain)的可能取值为0,1,2.

如果设 n = [f(origin) – f(remain)] / 3

当f(remain) = 0 时，恰好转换为4*n个字节的base64编码。

当f(remain) = 1 时，由于一个原文字节可以拆分为属于两个base64编码的字节，为了

让base64编码是4的倍数，所以应该为补2个等号。

当f(remain) = 2 时，由于两个原文字节可以拆分为属于3个base64编码的字节，同理，

应该补上一个等号。

base64 编码后的字符串末尾会有0到2个等号，这些等号在解码是并不必要，所以可以删除。
在网络get 和 post参数列表的时候，‘+'不能正常传输，可以把它替换成‘|'
这样经过base64编码后的字符串就只有‘|'和‘/‘，所以经过这样处理base64编码的字符串可以作为参数列表的以个参数值来传输

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以下是老外写的一个实现：
package   com.meterware.httpunit;

/******************************************************************************************************************** 
* $id: base64.java,v 1.4 2002/12/24 15:17:17 russgold exp $ 
* 
* copyright (c) 2000-2002 by russell gold 
* 
* permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated 
* documentation files (the "software "), to deal in the software without restriction, including without limitation 
* the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the software, and 
* to permit persons to whom the software is furnished to do so, subject to the following conditions: 
* 
* the above copyright notice and this permission notice shall be included in all copies or substantial portions 
* of the software. 
* 
* the software is provided "as is ", without warranty of any kind, express or implied, including but not limited to 
* the warranties of merchantability, fitness for a particular purpose and noninfringement. in no event shall the 
* authors or copyright holders be liable for any claim, damages or other liability, whether in an action of 
* contract, tort or otherwise, arising from, out of or in connection with the software or the use or other 
* dealings in the software. 
* 
*******************************************************************************************************************/ 

/** 
* a utility class to convert to and from base 64 encoding. 
* 
* @author <a href= "mailto:[e-mail] "> russell gold </a> 
**/ 
 
public class base64 { 		
	final static string encodingchar = "abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz0123456789+/ "; /** 
  * returns the base 64 encoded equivalent of a supplied string. 
  * @param source the string to encode 
  */ 
 public static string encode( string source ) { 
  char[] sourcebytes = getpaddedbytes( source ); 
  int numgroups = (sourcebytes.length + 2) / 3; 
  char[] targetbytes = new char[4]; 
  char[] target = new char[ 4 * numgroups ]; 

  for (int group = 0; group < numgroups; group++) { 
   convert3to4( sourcebytes, group*3, targetbytes ); 
   for (int i = 0; i < targetbytes.length; i++) { 
    target[ i + 4*group ] = encodingchar.charat( targetbytes[i] ); 
   } 
  } 

  int numpadbytes = sourcebytes.length - source.length(); 

  for (int i = target.length-numpadbytes; i < target.length; i++) target[i] = '= '; 
  return new string( target ); 
 } 


 private static char[] getpaddedbytes( string source ) { 
  char[] converted = source.tochararray(); 
  int requiredlength = 3 * ((converted.length+2) /3); 
  char[] result = new char[ requiredlength ]; 
  system.arraycopy( converted, 0, result, 0, converted.length ); 
  return result; 
 } 


 private static void convert3to4( char[] source, int sourceindex, char[] target ) { 
  target[0] = (char) ( source[ sourceindex ] > > > 2); 
  target[1] = (char) (((source[ sourceindex ] & 0x03) < < 4) | (source[ sourceindex+1 ] > > > 4)); 
  target[2] = (char) (((source[ sourceindex+1 ] & 0x0f) < < 2) | (source[ sourceindex+2 ] > > > 6)); 
  target[3] = (char) ( source[ sourceindex+2 ] & 0x3f); 
 } 


 /** 
  * returns the plaintext equivalent of a base 64-encoded string. 
  * @param source a base 64 string (which must have a multiple of 4 characters) 
  */ 
 public static string decode( string source ) { 
  if (source.length()%4 != 0) throw new runtimeexception( "valid base64 codes have a multiple of 4 characters " ); 
  int numgroups = source.length() / 4; 
  int numextrabytes = source.endswith( "== " ) ? 2 : (source.endswith( "= " ) ? 1 : 0); 
  byte[] targetbytes = new byte[ 3*numgroups ]; 
  byte[] sourcebytes = new byte[4]; 
  for (int group = 0; group < numgroups; group++) { 
   for (int i = 0; i < sourcebytes.length; i++) { 
    sourcebytes[i] = (byte) math.max( 0, encodingchar.indexof( source.charat( 4*group+i ) ) ); 
   } 
   convert4to3( sourcebytes, targetbytes, group*3 ); 
  } 
  return new string( targetbytes, 0, targetbytes.length - numextrabytes ); 
 } 


 private static void convert4to3( byte[] source, byte[] target, int targetindex ) { 
  target[ targetindex ] = (byte) (( source[0] < < 2) | (source[1] > > > 4)); 
  target[ targetindex+1 ] = (byte) (((source[1] & 0x0f) < < 4) | (source[2] > > > 2)); 
  target[ targetindex+2 ] = (byte) (((source[2] & 0x03) < < 6) | (source[3])); 
 } 

}