Java 和 Javascript base64编码
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
附一：Java中base64编码
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:
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);
}
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