Java 和 JavaScript 之间真正通用的Base64编码

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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: /* 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);
    }
}

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>

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字符编码详解及由来(GBK,UTF-8) | js数组的详解

2013-01-27 21:36
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