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编码可用于在HTTP环境下传递较长的标识信息。例如，在Java Persistence系统Hibernate中，就采用了Base64来将一个较长的唯一标识符（一般为128-bit的UUID）编码为一个字符串，用作HTTP表单和HTTP GET URL中的参数。在其他应用程序中，也常常需要把二进制数据编码为适合放在URL（包括隐藏表单域）中的形式。此时，采用Base64编码具有不可读性，即所编码的数据不会被人用肉眼所直接看到。

package com.hq.coder;

/**
 * Base64加密解密类
 * 
 * @author jianggujin
 * 
 */
public class Base64
{
   private final int BASELENGTH = 128;
   private final int LOOKUPLENGTH = 64;
   private final int TWENTYFOURBITGROUP = 24;
   private final int EIGHTBIT = 8;
   private final int SIXTEENBIT = 16;
   private final int FOURBYTE = 4;
   private final int SIGN = -128;
   private final char PAD = '=';
   private final byte[] base64Alphabet = new byte[BASELENGTH];
   private final char[] lookUpBase64Alphabet = new char[LOOKUPLENGTH];

   public Base64()
   {
      for (int i = 0; i < BASELENGTH; ++i)
      {
         base64Alphabet[i] = -1;
      }
      for (int i = 'Z'; i >= 'A'; i--)
      {
         base64Alphabet[i] = (byte) (i - 'A');
      }
      for (int i = 'z'; i >= 'a'; i--)
      {
         base64Alphabet[i] = (byte) (i - 'a' + 26);
      }

      for (int i = '9'; i >= '0'; i--)
      {
         base64Alphabet[i] = (byte) (i - '0' + 52);
      }

      base64Alphabet['+'] = 62;
      base64Alphabet['/'] = 63;

      for (int i = 0; i <= 25; i++)
      {
         lookUpBase64Alphabet[i] = (char) ('A' + i);
      }

      for (int i = 26, j = 0; i <= 51; i++, j++)
      {
         lookUpBase64Alphabet[i] = (char) ('a' + j);
      }

      for (int i = 52, j = 0; i <= 61; i++, j++)
      {
         lookUpBase64Alphabet[i] = (char) ('0' + j);
      }
      lookUpBase64Alphabet[62] = (char) '+';
      lookUpBase64Alphabet[63] = (char) '/';
   }

   private boolean isWhiteSpace(char octect)
   {
      return (octect == 0x20 || octect == 0xd || octect == 0xa || octect == 0x9);
   }

   private boolean isPad(char octect)
   {
      return (octect == PAD);
   }

   private boolean isData(char octect)
   {
      return (octect < BASELENGTH && base64Alphabet[octect] != -1);
   }

   /**
    * 编码
    * 
    * @param binaryData
    * @return
    */
   public String encode(byte[] binaryData)
   {

      if (binaryData == null)
      {
         return null;
      }

      int lengthDataBits = binaryData.length * EIGHTBIT;
      if (lengthDataBits == 0)
      {
         return "";
      }

      int fewerThan24bits = lengthDataBits % TWENTYFOURBITGROUP;
      int numberTriplets = lengthDataBits / TWENTYFOURBITGROUP;
      int numberQuartet = fewerThan24bits != 0 ? numberTriplets + 1
            : numberTriplets;
      char encodedData[] = null;

      encodedData = new char[numberQuartet * 4];

      byte k = 0, l = 0, b1 = 0, b2 = 0, b3 = 0;

      int encodedIndex = 0;
      int dataIndex = 0;

      for (int i = 0; i < numberTriplets; i++)
      {
         b1 = binaryData[dataIndex++];
         b2 = binaryData[dataIndex++];
         b3 = binaryData[dataIndex++];

         l = (byte) (b2 & 0x0f);
         k = (byte) (b1 & 0x03);

         byte val1 = ((b1 & SIGN) == 0) ? (byte) (b1 >> 2)
               : (byte) ((b1) >> 2 ^ 0xc0);
         byte val2 = ((b2 & SIGN) == 0) ? (byte) (b2 >> 4)
               : (byte) ((b2) >> 4 ^ 0xf0);
         byte val3 = ((b3 & SIGN) == 0) ? (byte) (b3 >> 6)
               : (byte) ((b3) >> 6 ^ 0xfc);

         encodedData[encodedIndex++] = lookUpBase64Alphabet[val1];
         encodedData[encodedIndex++] = lookUpBase64Alphabet[val2 | (k << 4)];
         encodedData[encodedIndex++] = lookUpBase64Alphabet[(l << 2) | val3];
         encodedData[encodedIndex++] = lookUpBase64Alphabet[b3 & 0x3f];
      }

      if (fewerThan24bits == EIGHTBIT)
      {
         b1 = binaryData[dataIndex];
         k = (byte) (b1 & 0x03);
         byte val1 = ((b1 & SIGN) == 0) ? (byte) (b1 >> 2)
               : (byte) ((b1) >> 2 ^ 0xc0);
         encodedData[encodedIndex++] = lookUpBase64Alphabet[val1];
         encodedData[encodedIndex++] = lookUpBase64Alphabet[k << 4];
         encodedData[encodedIndex++] = PAD;
         encodedData[encodedIndex++] = PAD;
      }
      else if (fewerThan24bits == SIXTEENBIT)
      {
         b1 = binaryData[dataIndex];
         b2 = binaryData[dataIndex + 1];
         l = (byte) (b2 & 0x0f);
         k = (byte) (b1 & 0x03);

         byte val1 = ((b1 & SIGN) == 0) ? (byte) (b1 >> 2)
               : (byte) ((b1) >> 2 ^ 0xc0);
         byte val2 = ((b2 & SIGN) == 0) ? (byte) (b2 >> 4)
               : (byte) ((b2) >> 4 ^ 0xf0);

         encodedData[encodedIndex++] = lookUpBase64Alphabet[val1];
         encodedData[encodedIndex++] = lookUpBase64Alphabet[val2 | (k << 4)];
         encodedData[encodedIndex++] = lookUpBase64Alphabet[l << 2];
         encodedData[encodedIndex++] = PAD;
      }

      return new String(encodedData);
   }

   /**
    * 解码
    * 
    * @param encoded
    * @return
    */
   public byte[] decode(String encoded)
   {

      if (encoded == null)
      {
         return null;
      }

      char[] base64Data = encoded.toCharArray();
      int len = removeWhiteSpace(base64Data);

      if (len % FOURBYTE != 0)
      {
         return null;
      }

      int numberQuadruple = (len / FOURBYTE);

      if (numberQuadruple == 0)
      {
         return new byte[0];
      }

      byte decodedData[] = null;
      byte b1 = 0, b2 = 0, b3 = 0, b4 = 0;
      char d1 = 0, d2 = 0, d3 = 0, d4 = 0;

      int i = 0;
      int encodedIndex = 0;
      int dataIndex = 0;
      decodedData = new byte[(numberQuadruple) * 3];

      for (; i < numberQuadruple - 1; i++)
      {

         if (!isData((d1 = base64Data[dataIndex++]))
               || !isData((d2 = base64Data[dataIndex++]))
               || !isData((d3 = base64Data[dataIndex++]))
               || !isData((d4 = base64Data[dataIndex++])))
         {
            return null;
         }

         b1 = base64Alphabet[d1];
         b2 = base64Alphabet[d2];
         b3 = base64Alphabet[d3];
         b4 = base64Alphabet[d4];

         decodedData[encodedIndex++] = (byte) (b1 << 2 | b2 >> 4);
         decodedData[encodedIndex++] = (byte) (((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf));
         decodedData[encodedIndex++] = (byte) (b3 << 6 | b4);
      }

      if (!isData((d1 = base64Data[dataIndex++]))
            || !isData((d2 = base64Data[dataIndex++])))
      {
         return null;// if found "no data" just return null
      }

      b1 = base64Alphabet[d1];
      b2 = base64Alphabet[d2];

      d3 = base64Data[dataIndex++];
      d4 = base64Data[dataIndex++];
      if (!isData((d3)) || !isData((d4)))
      {
         if (isPad(d3) && isPad(d4))
         {
            if ((b2 & 0xf) != 0)
            {
               return null;
            }
            byte[] tmp = new byte[i * 3 + 1];
            System.arraycopy(decodedData, 0, tmp, 0, i * 3);
            tmp[encodedIndex] = (byte) (b1 << 2 | b2 >> 4);
            return tmp;
         }
         else if (!isPad(d3) && isPad(d4))
         {
            b3 = base64Alphabet[d3];
            if ((b3 & 0x3) != 0)// last 2 bits should be zero
            {
               return null;
            }
            byte[] tmp = new byte[i * 3 + 2];
            System.arraycopy(decodedData, 0, tmp, 0, i * 3);
            tmp[encodedIndex++] = (byte) (b1 << 2 | b2 >> 4);
            tmp[encodedIndex] = (byte) (((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf));
            return tmp;
         }
         else
         {
            return null;
         }
      }
      else
      {
         b3 = base64Alphabet[d3];
         b4 = base64Alphabet[d4];
         decodedData[encodedIndex++] = (byte) (b1 << 2 | b2 >> 4);
         decodedData[encodedIndex++] = (byte) (((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf));
         decodedData[encodedIndex++] = (byte) (b3 << 6 | b4);

      }

      return decodedData;
   }

   private int removeWhiteSpace(char[] data)
   {
      if (data == null)
      {
         return 0;
      }

      int newSize = 0;
      int len = data.length;
      for (int i = 0; i < len; i++)
      {
         if (!isWhiteSpace(data[i]))
         {
            data[newSize++] = data[i];
         }
      }
      return newSize;
   }

   public static void main(String[] args)
   {
      String data = "BASE64Demo";
      Base64 base64 = new Base64();
      String encode = base64.encode(data.getBytes());
      System.out.println(encode);
      System.out.println(new String(base64.decode(encode)));
   }
}

运行结果：
QkFTRTY0RGVtbw==
BASE64Demo