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。
## 一、编码规则
Base64编码的思想是是采用64个基本的ASCII码字符对数据进行重新编码。它将需要编码的数据拆分成字节数组。以3个字节为一组。按顺序排列24 位数据，再把这24位数据分成4组，即每组6位。再在每组的的最高位前补两个0凑足一个字节。这样就把一个3字节为一组的数据重新编码成了4个字节。当所要编码的数据的字节数不是3的整倍数，也就是说在分组时最后一组不够3个字节。这时在最后一组填充1到2个0字节。并在最后编码完成后在结尾添加1到2个 “=”。
例：将对ABC进行BASE64编码：
1、首先取ABC对应的ASCII码值。A（65）B（66）C（67）；
2、再取二进制值A（01000001）B（01000010）C（01000011）；
3、然后把这三个字节的二进制码接起来（010000010100001001000011）；
4、 再以6位为单位分成4个数据块,并在最高位填充两个0后形成4个字节的编码后的值，（00010000）（00010100）（00001001）（00000011），其中蓝色部分为真实数据；
5、再把这四个字节数据转化成10进制数得（16）（20）（9）（3）；
6、最后根据BASE64给出的64个基本字符表，查出对应的ASCII码字符（Q）（U）（J）（D），这里的值实际就是数据在字符表中的索引。
注：BASE64字符表：ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/
## 二、解码规则
解码过程就是把4个字节再还原成3个字节再根据不同的数据形式把字节数组重新整理成数据。
## 三、java程序
~~~
import java.io.ByteArrayOutputStream;
import java.io.IOException;
public class Base64 {
// Create arrays to hold the base64 characters and a
// lookup for base64 chars
private static byte[] base64Alphabet;
/**
* The base length.
*/
static final int BASELENGTH = 255;
/**
* Chunk separator per RFC 2045 section 2.1.
*
* @see <a href="http://www.ietf.org/rfc/rfc2045.txt">RFC 2045 section 2.1</a>
*/
static final byte[] CHUNK_SEPARATOR = "\r\n".getBytes();
/**
* Chunk size per RFC 2045 section 6.8.
*
* <p>
* The {@value} character limit does not count the trailing CRLF, but counts
* all other characters, including any equal signs.
* </p>
*
* @see <a href="http://www.ietf.org/rfc/rfc2045.txt">RFC 2045 section 6.8</a>
*/
static final int CHUNK_SIZE = 76;
/**
* Used to calculate the number of bits in a byte.
*/
static final int EIGHTBIT = 8;
/**
* Used to get the number of Quadruples.
*/
static final int FOURBYTE = 4;
private static byte[] lookUpBase64Alphabet;
/**
* Lookup length.
*/
static final int LOOKUPLENGTH = 64;
/**
* Byte used to pad output.
*/
static final byte PAD = (byte) '=';
/**
* Used to test the sign of a byte.
*/
static final int SIGN = -128;
/**
* Used when encoding something which has fewer than 24 bits.
*/
static final int SIXTEENBIT = 16;
/**
* Used to determine how many bits data contains.
*/
static final int TWENTYFOURBITGROUP = 24;
// Populating the lookup and character arrays
static {
lookUpBase64Alphabet = new byte[LOOKUPLENGTH];
base64Alphabet = new byte[BASELENGTH];
for (int i = 0; i < BASELENGTH; i++) {
base64Alphabet[i] = (byte) -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] = (byte) ('A' + i);
}
for (int i = 26, j = 0; i <= 51; i++, j++) {
lookUpBase64Alphabet[i] = (byte) ('a' + j);
}
for (int i = 52, j = 0; i <= 61; i++, j++) {
lookUpBase64Alphabet[i] = (byte) ('0' + j);
}
lookUpBase64Alphabet[62] = (byte) '+';
lookUpBase64Alphabet[63] = (byte) '/';
}
/**
* Decodes Base64 data into octects
*
* @param base64Data
* Byte array containing Base64 data
* @return Array containing decoded data.
*/
public static byte[] decodeBase64(byte[] base64Data) {
// RFC 2045 requires that we discard ALL non-Base64 characters
base64Data = discardNonBase64(base64Data);
// handle the edge case, so we don't have to worry about it
// later
if (base64Data.length == 0) {
return new byte[0];
}
int numberQuadruple = base64Data.length / FOURBYTE;
byte decodedData[] = null;
byte b1 = 0, b2 = 0, b3 = 0, b4 = 0, marker0 = 0, marker1 = 0;
// Throw away anything not in base64Data
int encodedIndex = 0;
int dataIndex = 0;
{
// this sizes the output array properly - rlw
int lastData = base64Data.length;
// ignore the '=' padding
while (base64Data[lastData - 1] == PAD) {
if (--lastData == 0) {
return new byte[0];
}
}
decodedData = new byte[lastData - numberQuadruple];
}
for (int i = 0; i < numberQuadruple; i++) {
dataIndex = i * 4;
marker0 = base64Data[dataIndex + 2];
marker1 = base64Data[dataIndex + 3];
b1 = base64Alphabet[base64Data[dataIndex]];
b2 = base64Alphabet[base64Data[dataIndex + 1]];
if (marker0 != PAD && marker1 != PAD) {
// No PAD e.g 3cQl
b3 = base64Alphabet[marker0];
b4 = base64Alphabet[marker1];
decodedData[encodedIndex] = (byte) (b1 << 2 | b2 >> 4);
decodedData[encodedIndex + 1] = (byte) (((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf));
decodedData[encodedIndex + 2] = (byte) (b3 << 6 | b4);
} else if (marker0 == PAD) {
// Two PAD e.g. 3c[Pad][Pad]
decodedData[encodedIndex] = (byte) (b1 << 2 | b2 >> 4);
} else if (marker1 == PAD) {
// One PAD e.g. 3cQ[Pad]
b3 = base64Alphabet[marker0];
decodedData[encodedIndex] = (byte) (b1 << 2 | b2 >> 4);
decodedData[encodedIndex + 1] = (byte) (((b2 & 0xf) << 4) | ((b3 >> 2) & 0xf));
}
encodedIndex += 3;
}
return decodedData;
}
/**
* Discards any characters outside of the base64 alphabet, per the
* requirements on page 25 of RFC 2045 - "Any characters outside of the
* base64 alphabet are to be ignored in base64 encoded data."
*
* @param data
* The base-64 encoded data to groom
* @return The data, less non-base64 characters (see RFC 2045).
*/
static byte[] discardNonBase64(byte[] data) {
byte groomedData[] = new byte[data.length];
int bytesCopied = 0;
for (int i = 0; i < data.length; i++) {
if (isBase64(data[i])) {
groomedData[bytesCopied++] = data[i];
}
}
byte packedData[] = new byte[bytesCopied];
System.arraycopy(groomedData, 0, packedData, 0, bytesCopied);
return packedData;
}
/**
* Discards any whitespace from a base-64 encoded block.
*
* @param data
* The base-64 encoded data to discard the whitespace from.
* @return The data, less whitespace (see RFC 2045).
*/
static byte[] discardWhitespace(byte[] data) {
byte groomedData[] = new byte[data.length];
int bytesCopied = 0;
for (int i = 0; i < data.length; i++) {
switch (data[i]) {
case (byte) ' ':
case (byte) '\n':
case (byte) '\r':
case (byte) '\t':
break;
default:
groomedData[bytesCopied++] = data[i];
}
}
byte packedData[] = new byte[bytesCopied];
System.arraycopy(groomedData, 0, packedData, 0, bytesCopied);
return packedData;
}
/**
* Encodes binary data using the base64 algorithm but does not chunk the
* output.
*
* @param binaryData
* binary data to encode
* @return Base64 characters
*/
public static byte[] encodeBase64(byte[] binaryData) {
return encodeBase64(binaryData, false);
}
/**
* Encodes binary data using the base64 algorithm, optionally chunking the
* output into 76 character blocks.
*
* @param binaryData
* Array containing binary data to encode.
* @param isChunked
* if isChunked is true this encoder will chunk the base64 output
* into 76 character blocks
* @return Base64-encoded data.
*/
public static byte[] encodeBase64(byte[] binaryData, boolean isChunked) {
int lengthDataBits = binaryData.length * EIGHTBIT;
int fewerThan24bits = lengthDataBits % TWENTYFOURBITGROUP;
int numberTriplets = lengthDataBits / TWENTYFOURBITGROUP;
byte encodedData[] = null;
int encodedDataLength = 0;
int nbrChunks = 0;
if (fewerThan24bits != 0) {
// data not divisible by 24 bit
encodedDataLength = (numberTriplets + 1) * 4;
} else {
// 16 or 8 bit
encodedDataLength = numberTriplets * 4;
}
// If the output is to be "chunked" into 76 character
// sections,
// for compliance with RFC 2045 MIME, then it is important to
// allow for extra length to account for the separator(s)
if (isChunked) {
nbrChunks = (CHUNK_SEPARATOR.length == 0 ? 0 : (int) Math
.ceil((float) encodedDataLength / CHUNK_SIZE));
encodedDataLength += nbrChunks * CHUNK_SEPARATOR.length;
}
encodedData = new byte[encodedDataLength];
byte k = 0, l = 0, b1 = 0, b2 = 0, b3 = 0;
int encodedIndex = 0;
int dataIndex = 0;
int i = 0;
int nextSeparatorIndex = CHUNK_SIZE;
int chunksSoFar = 0;
// log.debug("number of triplets = " + numberTriplets);
for (i = 0; i < numberTriplets; i++) {
dataIndex = i * 3;
b1 = binaryData[dataIndex];
b2 = binaryData[dataIndex + 1];
b3 = binaryData[dataIndex + 2];
// log.debug("b1= " + b1 +", b2= " + b2 + ", b3= " + b3);
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];
// log.debug( "val2 = " + val2 );
// log.debug( "k4 = " + (k<<4) );
// log.debug( "vak = " + (val2 | (k<<4)) );
encodedData[encodedIndex + 1] = lookUpBase64Alphabet[val2
| (k << 4)];
encodedData[encodedIndex + 2] = lookUpBase64Alphabet[(l << 2)
| val3];
encodedData[encodedIndex + 3] = lookUpBase64Alphabet[b3 & 0x3f];
encodedIndex += 4;
// If we are chunking, let's put a chunk separator down.
if (isChunked) {
// this assumes that CHUNK_SIZE % 4 == 0
if (encodedIndex == nextSeparatorIndex) {
System.arraycopy(CHUNK_SEPARATOR, 0, encodedData,
encodedIndex, CHUNK_SEPARATOR.length);
chunksSoFar++;
nextSeparatorIndex = (CHUNK_SIZE * (chunksSoFar + 1))
+ (chunksSoFar * CHUNK_SEPARATOR.length);
encodedIndex += CHUNK_SEPARATOR.length;
}
}
}
// form integral number of 6-bit groups
dataIndex = i * 3;
if (fewerThan24bits == EIGHTBIT) {
b1 = binaryData[dataIndex];
k = (byte) (b1 & 0x03);
// log.debug("b1=" + b1);
// log.debug("b1<<2 = " + (b1>>2) );
byte val1 = ((b1 & SIGN) == 0) ? (byte) (b1 >> 2)
: (byte) ((b1) >> 2 ^ 0xc0);
encodedData[encodedIndex] = lookUpBase64Alphabet[val1];
encodedData[encodedIndex + 1] = lookUpBase64Alphabet[k << 4];
encodedData[encodedIndex + 2] = PAD;
encodedData[encodedIndex + 3] = 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 + 1] = lookUpBase64Alphabet[val2
| (k << 4)];
encodedData[encodedIndex + 2] = lookUpBase64Alphabet[l << 2];
encodedData[encodedIndex + 3] = PAD;
}
if (isChunked) {
// we also add a separator to the end of the final chunk.
if (chunksSoFar < nbrChunks) {
System.arraycopy(CHUNK_SEPARATOR, 0, encodedData,
encodedDataLength - CHUNK_SEPARATOR.length,
CHUNK_SEPARATOR.length);
}
}
return encodedData;
}
/**
* Encodes binary data using the base64 algorithm and chunks the encoded
* output into 76 character blocks
*
* @param binaryData
* binary data to encode
* @return Base64 characters chunked in 76 character blocks
*/
public static byte[] encodeBase64Chunked(byte[] binaryData) {
return encodeBase64(binaryData, true);
}
/**
* Tests a given byte array to see if it contains only valid characters
* within the Base64 alphabet.
*
* @param arrayOctect
* byte array to test
* @return true if all bytes are valid characters in the Base64 alphabet or
* if the byte array is empty; false, otherwise
*/
public static boolean isArrayByteBase64(byte[] arrayOctect) {
arrayOctect = discardWhitespace(arrayOctect);
int length = arrayOctect.length;
if (length == 0) {
// shouldn't a 0 length array be valid base64 data?
// return false;
return true;
}
for (int i = 0; i < length; i++) {
if (!isBase64(arrayOctect[i])) {
return false;
}
}
return true;
}
private static boolean isBase64(byte octect) {
if (octect == PAD) {
return true;
} else if (base64Alphabet[0xff & octect] == -1) {
return false;
} else {
return true;
}
}
/**
* Decodes a byte[] containing containing characters in the Base64 alphabet.
*
* @param pArray
* A byte array containing Base64 character data
* @return a byte array containing binary data
*/
public byte[] decode(byte[] pArray) {
return decodeBase64(pArray);
}
/**
* Encodes a byte[] containing binary data, into a byte[] containing
* characters in the Base64 alphabet.
*
* @param pArray
* a byte array containing binary data
* @return A byte array containing only Base64 character data
*/
public byte[] encode(byte[] pArray) {
return encodeBase64(pArray, false);
}
public static byte[] encodeXml(byte src[]) {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
ByteArrayOutputStream text_os = new ByteArrayOutputStream();
byte tag_left = (byte) '<';
byte tag_right = (byte) '>';
boolean begin = false;
boolean in_tag = false;
for (int i = 0; i < src.length; i++) {
byte b = src[i];
if (begin == false && b == tag_left)
begin = true;
if (b == tag_left)
in_tag = true;
else if (b == tag_right)
in_tag = false;
if (!begin)
continue;
if (in_tag || b == tag_right) {
if (b == tag_left) {
byte[] text = text_os.toByteArray();
try {
baos.write(encodeBase64(text));
} catch (IOException e) {
e.printStackTrace();
}
text_os.reset();
}
baos.write(b);
} else {
text_os.write(b);
}
}
return baos.toByteArray();
}
}
~~~
