Java 和 JavaScript 真正通用的Base64编码详解

 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: /* 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>

  <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>

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