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Base64是什么：

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（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编码不仅比较简短，同时也具有不可读性，即所编码的数据不会被人用肉眼所直接看到

简介

　　标准的Base64并不适合直接放在URL里传输，因为URL编码器会把标准Base64中的“/”和“+”字符变为形如“%XX”的形式，而这些“%”号在存入数据库时还需要再进行转换，因为ANSI SQL中已将“%”号用作通配符。 　　为解决此问题，可采用一种用于URL的改进Base64编码，它不在末尾填充'='号，并将标准Base64中的“+”和“/”分别改成了“*”和“-”，这样就免去了在URL编解码和数据库存储时所要作的转换，避免了编码信息长度在此过程中的增加，并统一了数据库、表单等处对象标识符的格式。 　　另有一种用于正则表达式的改进Base64变种，它将“+”和“/”改成了“!”和“-”，因为“+”,“*”以及前面在IRCu中用到的“[”和“]”在正则表达式中都可能具有特殊含义。 　　此外还有一些变种，它们将“+/”改为“_-”或“._”（用作编程语言中的标识符名称）或“.-”（用于XML中的Nmtoken）甚至“_:”（用于XML中的Name）。 　　Base64要求把每三个8Bit的字节转换为四个6Bit的字节（3*8 = 4*6 = 24），然后把6Bit再添两位高位0，组成四个8Bit的字节，也就是说，转换后的字符串理论上将要比原来的长1/3。 规则 　　关于这个编码的规则： 　　①.把3个字符变成4个字符.. 　　②每76个字符加一个换行符.. 　　③.最后的结束符也要处理.. 　　这样说会不会太抽象了？不怕，我们来看一个例子： 　　转换前 aaaaaabb ccccdddd eeffffff 　　转换后 00aaaaaa 00bbcccc 00ddddee 00ffffff 　　应该很清楚了吧？上面的三个字节是原文，下面的四个字节是转换后的Base64编码，其前两位均为0。 　　转换后，我们用一个码表来得到我们想要的字符串（也就是最终的Base64编码），这个表是这样的：（摘自RFC2045）

java代码示例：

public final class Base64 {	static private final int BASELENGTH = 255;	static private final int LOOKUPLENGTH = 64;	static private final int TWENTYFOURBITGROUP = 24;	static private final int EIGHTBIT = 8;	static private final int SIXTEENBIT = 16;	static private final int SIXBIT = 6;	static private final int FOURBYTE = 4;	static private final int SIGN = -128;	static private final char PAD = '=';	static private final boolean fDebug = false;	static final private byte[] base64Alphabet = new byte[BASELENGTH];	static final private char[] lookUpBase64Alphabet = new char[LOOKUPLENGTH];	static {		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) '/';	}	protected static boolean isWhiteSpace(char octect) {		return (octect == 0x20 || octect == 0xd || octect == 0xa || octect == 0x9);	}	protected static boolean isPad(char octect) {		return (octect == PAD);	}	protected static boolean isData(char octect) {		return (base64Alphabet[octect] != -1);	}	protected static boolean isBase64(char octect) {		return (isWhiteSpace(octect) || isPad(octect) || isData(octect));	}	/**	 * Encodes hex octects into Base64	 * 	 * @param binaryData	 *            Array containing binaryData	 * @return Encoded Base64 array	 */	public static 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;		int numberLines = (numberQuartet - 1) / 19 + 1;		char encodedData[] = null;		encodedData = new char[numberQuartet * 4 + numberLines];		byte k = 0, l = 0, b1 = 0, b2 = 0, b3 = 0;		int encodedIndex = 0;		int dataIndex = 0;		int i = 0;		if (fDebug) {			System.out.println("number of triplets = " + numberTriplets);		}		for (int line = 0; line < numberLines - 1; line++) {			for (int quartet = 0; quartet < 19; quartet++) {				b1 = binaryData[dataIndex++];				b2 = binaryData[dataIndex++];				b3 = binaryData[dataIndex++];				if (fDebug) {					System.out.println("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);				if (fDebug) {					System.out.println("val2 = " + val2);					System.out.println("k4 = " + (k << 4));					System.out.println("vak = " + (val2 | (k << 4)));				}				encodedData[encodedIndex++] = lookUpBase64Alphabet[val1];				encodedData[encodedIndex++] = lookUpBase64Alphabet[val2						| (k << 4)];				encodedData[encodedIndex++] = lookUpBase64Alphabet[(l << 2)						| val3];				encodedData[encodedIndex++] = lookUpBase64Alphabet[b3 & 0x3f];				i++;			}			encodedData[encodedIndex++] = 0xa;		}		for (; i < numberTriplets; i++) {			b1 = binaryData[dataIndex++];			b2 = binaryData[dataIndex++];			b3 = binaryData[dataIndex++];			if (fDebug) {				System.out.println("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);			if (fDebug) {				System.out.println("val2 = " + val2);				System.out.println("k4 = " + (k << 4));				System.out.println("vak = " + (val2 | (k << 4)));			}			encodedData[encodedIndex++] = lookUpBase64Alphabet[val1];			encodedData[encodedIndex++] = lookUpBase64Alphabet[val2 | (k << 4)];			encodedData[encodedIndex++] = lookUpBase64Alphabet[(l << 2) | val3];			encodedData[encodedIndex++] = lookUpBase64Alphabet[b3 & 0x3f];		}		// form integral number of 6-bit groups		if (fewerThan24bits == EIGHTBIT) {			b1 = binaryData[dataIndex];			k = (byte) (b1 & 0x03);			if (fDebug) {				System.out.println("b1=" + b1);				System.out.println("b1<<2 = " + (b1 >> 2));			}			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;		}		encodedData[encodedIndex] = 0xa;		return new String(encodedData);	}	/**	 * Decodes Base64 data into octects	 * 	 * @param binaryData	 *            Byte array containing Base64 data	 * @return Array containind decoded data.	 */	public static byte[] decode(String encoded) {		if (encoded == null)			return null;		char[] base64Data = encoded.toCharArray();		// remove white spaces		int len = removeWhiteSpace(base64Data);		if (len % FOURBYTE != 0) {			return null;// should be divisible by four		}		int numberQuadruple = (len / FOURBYTE);		if (numberQuadruple == 0)			return new byte[0];		byte decodedData[] = null;		byte b1 = 0, b2 = 0, b3 = 0, b4 = 0, marker0 = 0, marker1 = 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;// if found "no data" just 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))) {// Check if they are PAD characters			if (isPad(d3) && isPad(d4)) { // Two PAD e.g. 3c[Pad][Pad]				if ((b2 & 0xf) != 0)// last 4 bits should be zero					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)) { // One PAD e.g. 3cQ[Pad]				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;// an error like "3c[Pad]r", "3cdX", "3cXd", "3cXX"							// where X is non data			}		} else { // No PAD e.g 3cQl			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;	}	/**	 * remove WhiteSpace from MIME containing encoded Base64 data.	 * 	 * @param data	 *            the byte array of base64 data (with WS)	 * @return the new length	 */	protected static int removeWhiteSpace(char[] data) {		if (data == null)			return 0;		// count characters that's not whitespace		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) {		System.out.println(encode("中华人民共和国".getBytes()));	}}

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