噢,忘了.这是代码
public class Base64Codec {
protected static byte[] _encode_map = {
'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', '+', '/' };
protected static byte _decode_map[] = new byte[128];
static {
for (int i = 0; i < _encode_map.length; i++) {
_decode_map[_encode_map[i]] = (byte)i;
}
} private Base64Codec () {
} public final static byte[] encode (byte[] data) {
if (data == null) {
return (null);
} byte dest[] = new byte[((data.length + 2) / 3) * 4];
int i, j;
int data_len = data.length - 2;
for (i = 0, j = 0; i < data_len; i += 3) {
dest[j++] = _encode_map[(data[i] >>> 2) & 077];
dest[j++] = _encode_map[(data[i + 1] >>> 4) & 017 |(data[i] << 4) & 077];
dest[j++] = _encode_map[(data[i + 2] >>> 6) & 003 |(data[i + 1] << 2) & 077];
dest[j++] = _encode_map[data[i + 2] & 077];
}
if (i < data.length) {
dest[j++] = _encode_map[(data[i] >>> 2) & 077];
if (i < data.length-1) {
dest[j++] = _encode_map[(data[i + 1] >>> 4) & 017 |(data[i] << 4) & 077];
dest[j++] = _encode_map[(data[i + 1] << 2) & 077];
}
else {
dest[j++] = _encode_map[(data[i] << 4) & 077];
}
}
for ( ; j < dest.length; j++) {
dest[j] = (byte)'=';
}
return (dest);
} public final static byte[] decode (byte[] data) {
if (data == null)return (null);
int ending = data.length;
if (ending < 1) return (null);
while (data[ending - 1] == '=')
ending--;
byte dest[] = new byte[ending - data.length / 4];
for (int i = 0; i < data.length; i++) {
data[i] = _decode_map[data[i]];
} int i, j;
int dest_len = dest.length - 2;
for (i = 0, j = 0; j < dest_len; i += 4, j += 3) {
dest[j] = (byte) (((data[i] << 2) & 255) |((data[i + 1] >>> 4) & 003));
dest[j + 1] = (byte) (((data[i + 1] << 4) & 255) |((data[i + 2] >>> 2) & 017));
dest[j + 2] = (byte) (((data[i + 2] << 6) & 255) |(data[i + 3] & 077));
} if (j < dest.length) {
dest[j] = (byte) (((data[i] << 2) & 255) |((data[i + 1] >>> 4) & 003));
} j++;
if (j < dest.length) {
dest[j] = (byte) (((data[i + 1] << 4) & 255) |((data[i + 2] >>> 2) & 017));
}
return (dest);
}
}
public class Base64Codec {
protected static byte[] _encode_map = {
'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', '+', '/' };
protected static byte _decode_map[] = new byte[128];
static {
for (int i = 0; i < _encode_map.length; i++) {
_decode_map[_encode_map[i]] = (byte)i;
}
} private Base64Codec () {
} public final static byte[] encode (byte[] data) {
if (data == null) {
return (null);
} byte dest[] = new byte[((data.length + 2) / 3) * 4];
int i, j;
int data_len = data.length - 2;
for (i = 0, j = 0; i < data_len; i += 3) {
dest[j++] = _encode_map[(data[i] >>> 2) & 077];
dest[j++] = _encode_map[(data[i + 1] >>> 4) & 017 |(data[i] << 4) & 077];
dest[j++] = _encode_map[(data[i + 2] >>> 6) & 003 |(data[i + 1] << 2) & 077];
dest[j++] = _encode_map[data[i + 2] & 077];
}
if (i < data.length) {
dest[j++] = _encode_map[(data[i] >>> 2) & 077];
if (i < data.length-1) {
dest[j++] = _encode_map[(data[i + 1] >>> 4) & 017 |(data[i] << 4) & 077];
dest[j++] = _encode_map[(data[i + 1] << 2) & 077];
}
else {
dest[j++] = _encode_map[(data[i] << 4) & 077];
}
}
for ( ; j < dest.length; j++) {
dest[j] = (byte)'=';
}
return (dest);
} public final static byte[] decode (byte[] data) {
if (data == null)return (null);
int ending = data.length;
if (ending < 1) return (null);
while (data[ending - 1] == '=')
ending--;
byte dest[] = new byte[ending - data.length / 4];
for (int i = 0; i < data.length; i++) {
data[i] = _decode_map[data[i]];
} int i, j;
int dest_len = dest.length - 2;
for (i = 0, j = 0; j < dest_len; i += 4, j += 3) {
dest[j] = (byte) (((data[i] << 2) & 255) |((data[i + 1] >>> 4) & 003));
dest[j + 1] = (byte) (((data[i + 1] << 4) & 255) |((data[i + 2] >>> 2) & 017));
dest[j + 2] = (byte) (((data[i + 2] << 6) & 255) |(data[i + 3] & 077));
} if (j < dest.length) {
dest[j] = (byte) (((data[i] << 2) & 255) |((data[i + 1] >>> 4) & 003));
} j++;
if (j < dest.length) {
dest[j] = (byte) (((data[i + 1] << 4) & 255) |((data[i + 2] >>> 2) & 017));
}
return (dest);
}
}
<<<,<<是左移多少位,>>>,>>是右移多少位
为什么要这样呢?
我知道base64是把3 bytes encodes into 4 characters
并且 each output line is at most 76 characters.
但为什么要象上面的程序那样呢?
能解释一下吗?
谢谢了