Java 和JavaScript 真正通用的Base64編碼詳解

Java 和JavaScript Base64編碼

在開發Java Web應用的時候，可能會在服務器端用Java做Base64編碼，而在客戶端用JavaScript進行解碼。這樣就要求兩邊的Base64編碼機制保持一致。

使用Base64編碼，可能會碰到各種奇怪情況，甚至懷疑編碼有bug。但實際上不是這樣的。 Base64理論上操作的對像不是字符串而是字節數組。它的原理就是把ASCII碼的255個字符縮小到用64個來表示。具體就是原來三個字節用四個字節表示，編碼後長度有一定的增長。

1) 最好一次編碼，避免分段編碼，確實要分段編碼，每一段字節數應該是3的倍數。

長字節流，如果要邊讀取邊編碼，每一段必須是3的倍數，否則就可能在還原的時候出亂。一般人喜歡用2的乘方來定義數組，例如byte[1024]，因為不是3的倍數，可能還原時出錯。正確的例子是：

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

對於字符串，一般要整個一次編碼，以避免分段編碼出錯。

當然，如果你分段編碼，還原的時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候按照原來的編碼還原。

因為它操作的是字節數組，所以對於GBK編碼的漢字和UTF-8編碼漢字，經過Base64編碼後結果是不一樣的。例如“我們”這兩個字如果是GBK編碼，轉成Base64後就是ztLDxw== ；如果是UTF-8編碼，轉成Base64後就是5oiR5Lus。

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

所以Java這邊用什麼編碼轉換，在JavaScript那邊就要用什麼編碼還原。要保證Java和JavaScript通用，我們採用Unicode的編碼（JavaScript轉成UTF-8、GBK不方便，所以就採用了其本身的Unicode編碼），具體如下：

服務器端：

1）用getBytes("Unicode")轉成Unicode字節數組。

2） 編碼成Base64字符串

3）傳送到客戶端

客戶端：

1）Base64 解碼成字節數組

2）按Unicode還原

代碼如下（相關的函數看附件）：

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")轉完後會在前頭加上兩字節”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 AZ, az, 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 AZ, az, 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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