在开发Java? Web应用的时候，可能会在服务器端用Java做Base64编码，而在客户端用JavaScript进行解码。这样就要求两边的Base64编码机制保持一致。
使 用Base64编码，可能会碰到各种奇怪情况，甚至怀疑编码有bug。但实际上不是这样的。Base64理论上操作的对象不是字符串而是字节数组。它的原 理就是把ASCII码的255个字符缩小到用64个来表示。具体就是原来三个字节用四个字节表示，编码后长度有一定的增长。
1) 最好一次编码，避免分段编码，确实要分段编码，每一段字节数应该是3的倍数。
长字节流，如果要边读取边编码，每一段必须是3的倍数，否则就可能在还原的时候出乱。一般人喜欢用2的乘方来定义数组，例如 byte[1024]，因为不是3的倍数，可能还原时出错。正确的例子是：
byte[] bs=new byte[3*100] ....inputStream.read(bs)......encode(bs )....
对于字符串，一般要整个一次编码，以避免分段编码出错。
当然，如果你分段编码，还原的时候也是一段一段地还原，那是没有问题的。
2)确保字符串还原的时候按照原来的编码还原。
因为它操作的是字节数组，所以对于GBK编码的汉字和UTF-8编码汉字，经过 Base64编码后结果是不一样的。例如“我们”这两个字如果是GBK编码，转成Base64后就是ztLDxw== ；如果是UTF-8编码，转成Base64后就是5oiR5Lus。
也就是 “我们” ==》? getBytes("GBK") ==> Base64
所以Java这边用什么编码转换，在JavaScript那边就要用什么编码还原。要保证Java和JavaScript通用，我们采用Unicode的编码（JavaScript转成UTF-8、GBK不方便，所以就采用了其本身的Unicode编码），具体如下：
服务器端：
1）用getBytes("Unicode")转成Unicode字节数组。
2） 编码成Base64字符串
3）传送到客户端
客户端：
1）Base64 解码成字节数组
2）按Unicode还原
代码如下（相关的函数看附件）：
Base64.encode(data,"Unicode"); //java 端编码
decode64(data);?? //javascript解码
?
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附一：Java中Base64编码
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package websharp.util;
public class Base64 {
??? private static final byte[] encodingTable = {
??????????? (byte) 'A', (byte) 'B', (byte) 'C', (byte) 'D', (byte) 'E',
??????????? (byte) 'F', (byte) 'G', (byte) 'H', (byte) 'I', (byte) 'J',
??????????? (byte) 'K', (byte) 'L', (byte) 'M', (byte) 'N', (byte) 'O',
??????????? (byte) 'P', (byte) 'Q', (byte) 'R', (byte) 'S', (byte) 'T',
??????????? (byte) 'U', (byte) 'V', (byte) 'W', (byte) 'X', (byte) 'Y',
??????????? (byte) 'Z', (byte) 'a', (byte) 'b', (byte) 'c', (byte) 'd',
??????????? (byte) 'e', (byte) 'f', (byte) 'g', (byte) 'h', (byte) 'i',
??????????? (byte) 'j', (byte) 'k', (byte) 'l', (byte) 'm', (byte) 'n',
??????????? (byte) 'o', (byte) 'p', (byte) 'q', (byte) 'r', (byte) 's',
??????????? (byte) 't', (byte) 'u', (byte) 'v', (byte) 'w', (byte) 'x',
??????????? (byte) 'y', (byte) 'z', (byte) '0', (byte) '1', (byte) '2',
??????????? (byte) '3', (byte) '4', (byte) '5', (byte) '6', (byte) '7',
??????????? (byte) '8', (byte) '9', (byte) '+', (byte) '/'
??????? };
??? private static final byte[] decodingTable;
??? static {
??????? decodingTable = new byte[128];
??????? for (int i = 0; i < 128; i++) {
??????????? decodingTable[i] = (byte) -1;
??????? }
??????? for (int i = 'A'; i <= 'Z'; i++) {
??????????? decodingTable[i] = (byte) (i - 'A');
??????? }
??????? for (int i = 'a'; i <= 'z'; i++) {
??????????? decodingTable[i] = (byte) (i - 'a' + 26);
??????? }
??????? for (int i = '0'; i <= '9'; i++) {
??????????? decodingTable[i] = (byte) (i - '0' + 52);
??????? }
??????? decodingTable['+'] = 62;
??????? decodingTable['/'] = 63;
??? }
??? public static byte[] encode(byte[] data,int offset) {
??????? byte[] bytes;
??????? int realCount=data.length-offset;
??????? int modulus = realCount % 3;
??????? if (modulus == 0) {
??????????? bytes = new byte[(4 * realCount) / 3];
??????? } else {
??????????? bytes = new byte[4 * ((realCount / 3) + 1)];
??????? }
??????? int dataLength = (data.length - modulus);
??????? int a1;
??????? int a2;
??????? int a3;
??????? for (int i = offset, j = 0; i < dataLength; i += 3, j += 4) {
??????????? a1 = data[i] & 0xff;
??????????? a2 = data[i + 1] & 0xff;
??????????? a3 = data[i + 2] & 0xff;
??????????? bytes[j] = encodingTable[(a1 >>> 2) & 0x3f];
??????????? bytes[j + 1] = encodingTable[((a1 << 4) | (a2 >>> 4)) & 0x3f];
??????????? bytes[j + 2] = encodingTable[((a2 << 2) | (a3 >>> 6)) & 0x3f];
??????????? bytes[j + 3] = encodingTable[a3 & 0x3f];
??????? }
??????? int b1;
??????? int b2;
??????? int b3;
??????? int d1;
??????? int d2;
??????? switch (modulus) {
??????? case 0: /* nothing left to do */
??????????? break;
??????? case 1:
??????????? d1 = data[data.length - 1] & 0xff;
??????????? b1 = (d1 >>> 2) & 0x3f;
??????????? b2 = (d1 << 4) & 0x3f;
??????????? bytes[bytes.length - 4] = encodingTable[b1];
??????????? bytes[bytes.length - 3] = encodingTable[b2];
??????????? bytes[bytes.length - 2] = (byte) '=';
??????????? bytes[bytes.length - 1] = (byte) '=';
??????????? break;
??????? case 2:
??????????? d1 = data[data.length - 2] & 0xff;
??????????? d2 = data[data.length - 1] & 0xff;
??????????? b1 = (d1 >>> 2) & 0x3f;
??????????? b2 = ((d1 << 4) | (d2 >>> 4)) & 0x3f;
??????????? b3 = (d2 << 2) & 0x3f;
??????????? bytes[bytes.length - 4] = encodingTable[b1];
??????????? bytes[bytes.length - 3] = encodingTable[b2];
??????????? bytes[bytes.length - 2] = encodingTable[b3];
??????????? bytes[bytes.length - 1] = (byte) '=';
??????????? break;
??????? }
??????? return bytes;
??? }
??? public static byte[] decode(byte[] data) {
??????? byte[] bytes;
??????? byte b1;
??????? byte b2;
??????? byte b3;
??????? byte b4;
??????? data = discardNonBase64Bytes(data);
??????? if (data[data.length - 2] == '=') {
??????????? bytes = new byte[(((data.length / 4) - 1) * 3) + 1];
??????? } else if (data[data.length - 1] == '=') {
??????????? bytes = new byte[(((data.length / 4) - 1) * 3) + 2];
??????? } else {
??????????? bytes = new byte[((data.length / 4) * 3)];
??????? }
??????? for (int i = 0, j = 0; i < (data.length - 4); i += 4, j += 3) {
??????????? b1 = decodingTable[data[i]];
??????????? b2 = decodingTable[data[i + 1]];
??????????? b3 = decodingTable[data[i + 2]];
??????????? b4 = decodingTable[data[i + 3]];
??????????? bytes[j] = (byte) ((b1 << 2) | (b2 >> 4));
??????????? bytes[j + 1] = (byte) ((b2 << 4) | (b3 >> 2));
??????????? bytes[j + 2] = (byte) ((b3 << 6) | b4);
??????? }
??????? if (data[data.length - 2] == '=') {
??????????? b1 = decodingTable[data[data.length - 4]];
??????????? b2 = decodingTable[data[data.length - 3]];
??????????? bytes[bytes.length - 1] = (byte) ((b1 << 2) | (b2 >> 4));
??????? } else if (data[data.length - 1] == '=') {
??????????? b1 = decodingTable[data[data.length - 4]];
??????????? b2 = decodingTable[data[data.length - 3]];
??????????? b3 = decodingTable[data[data.length - 2]];
??????????? bytes[bytes.length - 2] = (byte) ((b1 << 2) | (b2 >> 4));
??????????? bytes[bytes.length - 1] = (byte) ((b2 << 4) | (b3 >> 2));
??????? } else {
??????????? b1 = decodingTable[data[data.length - 4]];
??????????? b2 = decodingTable[data[data.length - 3]];
??????????? b3 = decodingTable[data[data.length - 2]];
??????????? b4 = decodingTable[data[data.length - 1]];
??????????? bytes[bytes.length - 3] = (byte) ((b1 << 2) | (b2 >> 4));
??????????? bytes[bytes.length - 2] = (byte) ((b2 << 4) | (b3 >> 2));
??????????? bytes[bytes.length - 1] = (byte) ((b3 << 6) | b4);
??????? }
??????? return bytes;
??? }
??? public static byte[] decode(String data) {
??????? byte[] bytes;
??????? byte b1;
??????? byte b2;
??????? byte b3;
??????? byte b4;
??????? data = discardNonBase64Chars(data);
??????? if (data.charAt(data.length() - 2) == '=') {
??????????? bytes = new byte[(((data.length() / 4) - 1) * 3) + 1];
??????? } else if (data.charAt(data.length() - 1) == '=') {
??????????? bytes = new byte[(((data.length() / 4) - 1) * 3) + 2];
??????? } else {
??????????? bytes = new byte[((data.length() / 4) * 3)];
??????? }
??????? for (int i = 0, j = 0; i < (data.length() - 4); i += 4, j += 3) {
??????????? b1 = decodingTable[data.charAt(i)];
??????????? b2 = decodingTable[data.charAt(i + 1)];
??????????? b3 = decodingTable[data.charAt(i + 2)];
??????????? b4 = decodingTable[data.charAt(i + 3)];
??????????? bytes[j] = (byte) ((b1 << 2) | (b2 >> 4));
??????????? bytes[j + 1] = (byte) ((b2 << 4) | (b3 >> 2));
??????????? bytes[j + 2] = (byte) ((b3 << 6) | b4);
??????? }
??????? if (data.charAt(data.length() - 2) == '=') {
??????????? b1 = decodingTable[data.charAt(data.length() - 4)];
??????????? b2 = decodingTable[data.charAt(data.length() - 3)];
??????????? bytes[bytes.length - 1] = (byte) ((b1 << 2) | (b2 >> 4));
??????? } else if (data.charAt(data.length() - 1) == '=') {
??????????? b1 = decodingTable[data.charAt(data.length() - 4)];
??????????? b2 = decodingTable[data.charAt(data.length() - 3)];
??????????? b3 = decodingTable[data.charAt(data.length() - 2)];
??????????? bytes[bytes.length - 2] = (byte) ((b1 << 2) | (b2 >> 4));
??????????? bytes[bytes.length - 1] = (byte) ((b2 << 4) | (b3 >> 2));
??????? } else {
??????????? b1 = decodingTable[data.charAt(data.length() - 4)];
??????????? b2 = decodingTable[data.charAt(data.length() - 3)];
??????????? b3 = decodingTable[data.charAt(data.length() - 2)];
??????????? b4 = decodingTable[data.charAt(data.length() - 1)];
??????????? bytes[bytes.length - 3] = (byte) ((b1 << 2) | (b2 >> 4));
??????????? bytes[bytes.length - 2] = (byte) ((b2 << 4) | (b3 >> 2));
??????????? bytes[bytes.length - 1] = (byte) ((b3 << 6) | b4);
??????? }
??????? for(int i=0;i<bytes.length;i++) System.out.println(","+bytes[i]);
??????? return bytes;
??? }
??? private static byte[] discardNonBase64Bytes(byte[] data) {
??????? byte[] temp = new byte[data.length];
??????? int bytesCopied = 0;
??????? for (int i = 0; i < data.length; i++) {
??????????? if (isValidBase64Byte(data[i])) {
??????????????? temp[bytesCopied++] = data[i];
??????????? }
??????? }
??????? byte[] newData = new byte[bytesCopied];
??????? System.arraycopy(temp, 0, newData, 0, bytesCopied);
??????? return newData;
??? }
??? private static String discardNonBase64Chars(String data) {
??????? StringBuffer sb = new StringBuffer();
??????? int length = data.length();
??????? for (int i = 0; i < length; i++) {
??????????? if (isValidBase64Byte((byte) (data.charAt(i)))) {
??????????????? sb.append(data.charAt(i));
??????????? }
??????? }
??????? return sb.toString();
??? }
??? private static boolean isValidBase64Byte(byte b) {
??????? if (b == '=') {
??????????? return true;
??????? } else if ((b < 0) || (b >= 128)) {
??????????? return false;
??????? } else if (decodingTable[b] == -1) {
??????????? return false;
??????? }
??????? return true;
??? }
??? public static String? encode(String data,String charset)throws Exception
??? {
??????? // byte[] result =? (data.getBytes("Unicode"));
???????? if(data==null || data.length()==0) return data;
???????? int offset=0;
???????? // getBytes("unicode")转完后会在前头加上两字节”FE“
???????? byte[] result=encode (data.getBytes(charset),offset);
???????? StringBuffer sb=new StringBuffer(result.length);
???????? for (int i=0;i<result.length;i++)?? sb.append((char)result[i]);
???????? return sb.toString();
??? }
??? public static String? decode(String data,String charset)throws Exception
??? {
??????? if(data==null || data.length()==0) return data;
??????? return new String(Base64.decode(data),charset);
??? }
??? public static void main(String[] args) throws Exception {
??????? String data = "我们";
??????? String data1=encode(data,"Unicode");
??????? String data2=decode(data1,"Unicode");
??????? System.out.println(data);
??????? System.out.println(data1);
??????? System.out.println(data2);
??? }
}
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附二：JavaScript中Base64编码
<html>
<head>
<title>base64 Encoding/Decoding</title>
</head>
<script type="text/javascript"><!--
var keyStr = "ABCDEFGHIJKLMNOP" +
???????????? "QRSTUVWXYZabcdef" +
???????????? "ghijklmnopqrstuv" +
???????????? "wxyz0123456789+/" +
???????????? "=";
function encode64(input) {
?? input = unicodetoBytes(input);
?? var output = "";
?? var chr1, chr2, chr3 = "";
?? var enc1, enc2, enc3, enc4 = "";
?? var i = 0;
?? do {
????? chr1 = input[i++];
????? chr2 = input[i++];
????? chr3 = input[i++];
????? enc1 = chr1 >> 2;
????? enc2 = ((chr1 & 3) << 4) | (chr2 >> 4);
????? enc3 = ((chr2 & 15) << 2) | (chr3 >> 6);
????? enc4 = chr3 & 63;
????? if (isNaN(chr2)) {
???????? enc3 = enc4 = 64;
????? } else if (isNaN(chr3)) {
???????? enc4 = 64;
????? }
????? output = output +
???????? keyStr.charAt(enc1) +
???????? keyStr.charAt(enc2) +
???????? keyStr.charAt(enc3) +
???????? keyStr.charAt(enc4);
????? chr1 = chr2 = chr3 = "";
????? enc1 = enc2 = enc3 = enc4 = "";
?? } while (i < input.length);
?? return output;
}
function decode64(input) {
?? var output = "";
?? var chr1, chr2, chr3 = "";
?? var enc1, enc2, enc3, enc4 = "";
?? var i = 0;
?? // remove all characters that are not A-Z, a-z, 0-9, +, /, or =
?? var base64test = /[^A-Za-z0-9/+///=]/g;
?? if (base64test.exec(input)) {
????? alert("There were invalid base64 characters in the input text./n" +
??????????? "Valid base64 characters are A-Z, a-z, 0-9, '+', '/', and '='/n" +
??????????? "Expect errors in decoding.");
?? }
?? input = input.replace(/[^A-Za-z0-9/+///=]/g, "");
??? output=new Array();
?? do {
????? enc1 = keyStr.indexOf(input.charAt(i++));
????? enc2 = keyStr.indexOf(input.charAt(i++));
????? enc3 = keyStr.indexOf(input.charAt(i++));
????? enc4 = keyStr.indexOf(input.charAt(i++));
????? chr1 = (enc1 << 2) | (enc2 >> 4);
????? chr2 = ((enc2 & 15) << 4) | (enc3 >> 2);
????? chr3 = ((enc3 & 3) << 6) | enc4;
????? output.push(chr1);
????? if (enc3 != 64) {
???????? output.push(chr2);
????? }
????? if (enc4 != 64) {
???????? output.push(chr3);
????? }
????? chr1 = chr2 = chr3 = "";
????? enc1 = enc2 = enc3 = enc4 = "";
?? } while (i < input.length);
?? return bytesToUnicode(output);
}
? function unicodetoBytes(s)
?? {
????? var result=new Array();
????? if(s==null || s=="") return result;
????? result.push(255); // add "FE" to head
????? result.push(254);
????? for(var i=0;i<s.length;i++)
????? {
??????? var c=s.charCodeAt(i).toString(16);
??????? if(c.length==1) i="000"+c;
??????? else if(c.length==2) c="00"+c;
??????? else if(c.length==3) c="0"+c;
??????? var var1=parseInt( c.substring(2),16);
??????? var var2=parseInt( c.substring(0,2),16);
??????? result.push( var1);
??????? result.push(var2) ;
????? }
????? return result;
?? }
?? function bytesToUnicode(bs)
?? {
???? var result="";
????? var offset=0;
????? if(bs.length>=2 && bs[0]==255 && bs[1]==254) offset=2;? // delete "FE"
????? for(var i=offset;i<bs.length;i+=2)
????? {
??????????? var code=bs[i]+(bs[i+1]<<8);
??????????? result+=String.fromCharCode(code);
????? }
????? return result;
?? }
//-->
</script>
<body>
<form name="base64Form">
?? Type in the message you want to encode in base64, or paste<br>
?? base64 encoded text into the text field, select Encode or Decode, <br>
?? and click the button!<br>
?? <textarea name="theText" cols="40" rows="6"></textarea><br>
?? <input type="button" name="encode" value="Encode to base64"
????? onClick="document.base64Form.theText.value=encode64(document.base64Form.theText.value);">
?? <input type="button" name="decode" value="Decode from base64"
????? onClick="document.base64Form.theText.value=decode64(document.base64Form.theText.value);">
</form>
</body>
</html>