Base64

public final class Base64
{
   
    /** Base64编码表。*/
    private static char[] base64Code =
    {
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V',
        'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q',
        'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '+', '/',
    };
   
    /** Base64解码表。*/
    private static byte[] base64Decode =
    {
        -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1, //注意两个63，为兼容SMP，
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        -1,
        NumberConstants.BYTE_62,
        -1,
        NumberConstants.BYTE_63,
        -1,
        NumberConstants.BYTE_63, //“/”和“-”都翻译成63。
        NumberConstants.BYTE_52, NumberConstants.BYTE_53, NumberConstants.BYTE_54,
        NumberConstants.BYTE_55, NumberConstants.BYTE_56, NumberConstants.BYTE_57,
        NumberConstants.BYTE_58, NumberConstants.BYTE_59, NumberConstants.BYTE_60,
        NumberConstants.BYTE_61, -1, -1, -1, 0, -1, -1, -1,
        0,
        NumberConstants.BYTE_1,
        NumberConstants.BYTE_2,
        NumberConstants.BYTE_3,
        NumberConstants.BYTE_4,
        NumberConstants.BYTE_5,
        NumberConstants.BYTE_6,
        NumberConstants.BYTE_7,
        NumberConstants.BYTE_8,
        NumberConstants.BYTE_9,
        NumberConstants.BYTE_10,
        NumberConstants.BYTE_11,
        NumberConstants.BYTE_12,
        NumberConstants.BYTE_13,
        NumberConstants.BYTE_14, //注意两个0：
        NumberConstants.BYTE_15, NumberConstants.BYTE_16, NumberConstants.BYTE_17,
        NumberConstants.BYTE_18, NumberConstants.BYTE_19, NumberConstants.BYTE_20,
        NumberConstants.BYTE_21,
        NumberConstants.BYTE_22,
        NumberConstants.BYTE_23,
        NumberConstants.BYTE_24,
        NumberConstants.BYTE_25,
        -1,
        -1,
        -1,
        -1,
        -1, //“A”和“=”都翻译成0。
        -1, NumberConstants.BYTE_26, NumberConstants.BYTE_27, NumberConstants.BYTE_28,
        NumberConstants.BYTE_29, NumberConstants.BYTE_30, NumberConstants.BYTE_31,
        NumberConstants.BYTE_32, NumberConstants.BYTE_33, NumberConstants.BYTE_34,
        NumberConstants.BYTE_35, NumberConstants.BYTE_36, NumberConstants.BYTE_37,
        NumberConstants.BYTE_38, NumberConstants.BYTE_39, NumberConstants.BYTE_40,
        NumberConstants.BYTE_41, NumberConstants.BYTE_42, NumberConstants.BYTE_43,
        NumberConstants.BYTE_44, NumberConstants.BYTE_45, NumberConstants.BYTE_46,
        NumberConstants.BYTE_47, NumberConstants.BYTE_48, NumberConstants.BYTE_49,
        NumberConstants.BYTE_50, NumberConstants.BYTE_51, -1, -1, -1, -1, -1,
    };
   
    /**
     * 构造方法私有化，防止实例化。
     */
    private Base64()
    {
    }
   
    /**
     * Base64编码。将字节数组中字节3个一组编码成4个可见字符。
     *
     * @param b 需要被编码的字节数据。
     * @return 编码后的Base64字符串。
     */
    public static String encode(byte[] b)
    {
        int code = 0;
       
        //按实际编码后长度开辟内存，加快速度
        StringBuffer sb =
            new StringBuffer(((b.length - 1) / NumberConstants.NUM_3) << NumberConstants.NUM_2
                + NumberConstants.NUM_4);
       
        //进行编码
        for (int i = 0; i < b.length; i++)
        {
            code |=
                (b[i] << (NumberConstants.NUM_16 - i % NumberConstants.NUM_3 * NumberConstants.NUM_8))
                    & (NumberConstants.NUM_0XFF << (NumberConstants.NUM_16 - i % NumberConstants.NUM_3
                        * NumberConstants.NUM_8));
            if (i % NumberConstants.NUM_3 == NumberConstants.NUM_2 || i == b.length - 1)
            {
                sb.append(base64Code[(code & NumberConstants.NUM_0XFC0000) >>> NumberConstants.NUM_18]);
                sb.append(base64Code[(code & NumberConstants.NUM_0X3F000) >>> NumberConstants.NUM_12]);
                sb.append(base64Code[(code & NumberConstants.NUM_0XFC0) >>> NumberConstants.NUM_6]);
                sb.append(base64Code[code & NumberConstants.NUM_0X3F]);
                code = 0;
            }
        }
       
        //对于长度非3的整数倍的字节数组，编码前先补0，编码后结尾处编码用=代替，
        //=的个数和短缺的长度一致，以此来标识出数据实际长度
        if (b.length % NumberConstants.NUM_3 > 0)
        {
            sb.setCharAt(sb.length() - 1, '=');
        }
        if (b.length % NumberConstants.NUM_3 == 1)
        {
            sb.setCharAt(sb.length() - NumberConstants.NUM_2, '=');
        }
        return sb.toString();
    }
   
    /**
     * Base64解码。
     *
     * @param code 用Base64编码的ASCII字符串
     * @return 解码后的字节数据
     */
    public static byte[] decode(String code)
    {
       
        //检查参数合法性
        if (code == null)
        {
            return null;
        }
        int len = code.length();
        if (len % NumberConstants.NUM_4 != 0)
        {
            throw new IllegalArgumentException("Base64 string length must be 4*n");
        }
        if (code.length() == 0)
        {
            return new byte[0];
        }
       
        //统计填充的等号个数
        int pad = 0;
        if (code.charAt(len - 1) == '=')
        {
            pad++;
        }
        if (code.charAt(len - NumberConstants.NUM_2) == '=')
        {
            pad++;
        }
       
        //根据填充等号的个数来计算实际数据长度
        int retLen = len / NumberConstants.NUM_4 * NumberConstants.NUM_3 - pad;
       
        //分配字节数组空间
        byte[] ret = new byte[retLen];
       
        //查表解码
        char ch1, ch2, ch3, ch4;
        int i;
        for (i = 0; i < len; i += NumberConstants.BYTE_4)
        {
            int j = i / NumberConstants.NUM_4 * NumberConstants.NUM_3;
            ch1 = code.charAt(i);
            ch2 = code.charAt(i + 1);
            ch3 = code.charAt(i + NumberConstants.NUM_2);
            ch4 = code.charAt(i + NumberConstants.NUM_3);
            int tmp =
                (base64Decode[ch1] << NumberConstants.NUM_18) | (base64Decode[ch2] << NumberConstants.NUM_12)
                    | (base64Decode[ch3] << NumberConstants.BYTE_6) | (base64Decode[ch4]);
            ret[j] = (byte)((tmp & NumberConstants.NUM_0XFF0000) >> NumberConstants.NUM_16);
            if (i < len - NumberConstants.NUM_4)
            {
                ret[j + NumberConstants.NUM_1] =
                    (byte)((tmp & NumberConstants.NUM_0X00FF00) >> NumberConstants.NUM_8);
                ret[j + NumberConstants.NUM_2] = (byte)((tmp & NumberConstants.NUM_0X0000FF));
            }
            else
            {
                if (j + 1 < retLen)
                {
                    ret[j + 1] = (byte)((tmp & NumberConstants.NUM_0X00FF00) >> NumberConstants.NUM_8);
                }
                if (j + NumberConstants.NUM_2 < retLen)
                {
                    ret[j + NumberConstants.NUM_2] = (byte)((tmp & NumberConstants.NUM_0X0000FF));
                }
            }
        }
        return ret;
    }
}