[java]Base64

    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 charsprivate 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 arraysstatic {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 charactersbase64Data = discardNonBase64(base64Data);// handle the edge case, so we don't have to worry about it// laterif (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 base64Dataint encodedIndex = 0;int dataIndex = 0;{// this sizes the output array properly - rlwint lastData = base64Data.length;// ignore the '=' paddingwhile (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 3cQlb3 = 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 bitencodedDataLength = (numberTriplets + 1) * 4;} else {// 16 or 8 bitencodedDataLength = 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 == 0if (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 groupsdataIndex = 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();}}