使用Base64編碼，可能會(huì)碰到各種奇怪情況，甚至懷疑編碼有bug。但實(shí)際上不是這樣的。Base64理論上操作的對(duì)象不是字符串而是字節(jié)數(shù)組。它的原理就是把ASCII碼的255個(gè)字符縮小到用64個(gè)來表示。具體就是原來三個(gè)字節(jié)用四個(gè)字節(jié)表示，編碼后長度有一定的增長。

1) 最好一次編碼，避免分段編碼，確實(shí)要分段編碼，每一段字節(jié)數(shù)應(yīng)該是3的倍數(shù)。

長字節(jié)流，如果要邊讀取邊編碼，每一段必須是3的倍數(shù)，否則就可能在還原的時(shí)候出亂。一般人喜歡用2的乘方來定義數(shù)組，例如 byte[1024]，因?yàn)椴皇?的倍數(shù)，可能還原時(shí)出錯(cuò)。正確的例子是：

byte[] bs=new byte[3*100] ....inputStream.read(bs)......encode(bs )....

對(duì)于字符串，一般要整個(gè)一次編碼，以避免分段編碼出錯(cuò)。

當(dāng)然，如果你分段編碼，還原的時(shí)候也是一段一段地還原，那是沒有問題的。

2)確保字符串還原的時(shí)候按照原來的編碼還原。

因?yàn)樗僮鞯氖亲止?jié)數(shù)組，所以對(duì)于GBK編碼的漢字和UTF-8編碼漢字，經(jīng)過 Base64編碼后結(jié)果是不一樣的。例如“我們”這兩個(gè)字如果是GBK編碼，轉(zhuǎn)成Base64后就是ztLDxw== ；如果是UTF-8編碼，轉(zhuǎn)成Base64后就是5oiR5Lus。

也就是 “我們” ==》 getBytes("GBK") ==> Base64

所以Java這邊用什么編碼轉(zhuǎn)換，在JavaScript那邊就要用什么編碼還原。要保證Java和JavaScript通用，我們采用Unicode的編碼（JavaScript轉(zhuǎn)成UTF-8、GBK不方便，所以就采用了其本身的Unicode編碼），具體如下：

服務(wù)器端：

1）用getBytes("Unicode")轉(zhuǎn)成Unicode字節(jié)數(shù)組。

2）編碼成Base64字符串

3）傳送到客戶端

客戶端：

1）Base64 解碼成字節(jié)數(shù)組

2）按Unicode還原

代碼如下（相關(guān)的函數(shù)看附件）：

Base64.encode(data,"Unicode"); //java 端編碼

decode64(data); //javascript解碼

附一：Java中Base64編碼

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")轉(zhuǎn)完后會(huì)在前頭加上兩字節(jié)”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 = "我們"; 
    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編碼

<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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