java实现base64编码和解码

编码规则：1、将3个字节转为4个字节，即每6个字节一组，高位补0

                   2、原字节长度不是3的整数倍，结果串末尾使用=号补齐

以下是使用java代码的是实现

public class Base64 {   /**     * byte数组中单个字节对应的字符      * */    private static final char[] intToBase64 = {        'A','B','C','D','E','F','G','H','I','J',        'K','L','M','N','O','P','Q','R','S','T',        'U','V','W','X','Y','Z','a','b','c','d',        'f','g','h','i','j','k','l','m','n','o',        'p','q','r','s','t','u','v','w','x','y',        'z','0','1','2','3','4','5','6','7','8',        '9','+','/'    };        /**     * 字符ASCII码对应intToBase64中的位置          * */         private static final int[] base64Toint = {        -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,     -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,     -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,     -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,     -1,-1,-1,62,-1,-1,-1,63,52,53,     54,55,56,57,58,59,60,61,-1,-1,     -1,-1,-1,-1,-1,0 ,1 ,2 ,3 , 4,     5 ,6 ,7 ,8 ,9 ,10,11,12,13,14,     15,16,17,18,19,20,21,22,23,24,     25,-1,-1,-1,-1,-1,-1,26,27,28,     29,30,31,32,33,34,35,36,37,38,     39,40,41,42,43,44,45,46,47,48,     49,50,51         }             /**          * base64加密          * 1、将byte数据每3个字节为一组，转成4个字节，单个字节上高位补0          * 2、每个字节上获得的数字从intToBase64中找到要转成的对应字符          * 3、判断分组是否是3的整数倍，如果不是，末尾使用=补齐          * */         public static String encode(byte[] a) {     int totalLen = a.length;     int groupNum = a.length/3;     int lastGroup = totalLen - groupNum*3;     int index = 0;     StringBuffer result = new StringBuffer();     for (int i = 0; i < groupNum ; i++) {     int first = a[index++] & 0xff;     int second = a[index++] & 0xff;     int third = a[index++] & 0xff;     result.append(intToBase64[first >> 2]);     result.append(intToBase64[(first << 4) & 0x3f | second >> 4]);     result.append(intToBase64[(second << 2) & 0x3f | third >> 6]);     result.append(intToBase64[third & 0x3f]);     }     if(lastGroup != 0) {     int first = a[index++] & 0xff;     result.append(intToBase64[first >> 2]);     if(lastGroup == 1) {     result.append(intToBase64[(first << 4) & 0x3f]);     result.append("==");     }else{     int second = a[index++] & 0xff;     result.append(intToBase64[(first << 4) & 0x3f | second >> 4]);     result.append(intToBase64[(second << 2) & 0x3f]);     result.append("=");     }     }     return result.toString();    }    /**     * base64解密      * 1、将字符串每4个字符一组，分别找到字符在intToBase64中对应的位置      * 2、将每个位置的高2位舍弃，即生成3个字节的byte      * 3、判断字符串末尾是否没填充过，是的话将填充部分舍弃，原部分按以上规则生成byte      * */     public static byte[] decode(String s) {     int strlen = s.length();     int groupNum = strlen / 4;     if(groupNum * 4 != strlen) {     throw new IllegalArgumentException("String length must be a multiple of 4.");     }     int lastMissingNum = 0;     int numFullGroup = groupNum;     if(strlen != 0) {     if(s.charAt(strlen - 1) == '=') {     lastMissingNum ++;     numFullGroup --;     }     if(s.charAt(strlen - 2) == '=') {     lastMissingNum ++;     }     }     byte[] result = new byte[groupNum*3 - lastMissingNum];     int charIndex = 0;     int resultIndex = 0;     for (int i = 0; i < numFullGroup; i++) {     int char0 = base64Toint(s.charAt(charIndex++));     int char1 = base64Toint(s.charAt(charIndex++));     int char2 = base64Toint(s.charAt(charIndex++));     int char3 = base64Toint(s.charAt(charIndex++));     result[resultIndex++] = (byte)(char0 << 2 | char1 >> 4);     result[resultIndex++] = (byte)(char1 << 4 | char2 >> 2);     result[resultIndex++] = (byte)(char2 << 6 | char3);     } if(lastMissingNum != 0) { int char0 = base64Toint(s.charAt(charIndex++)); int char1 = base64Toint(s.charAt(charIndex++)); result[resultIndex++] = (byte)(char0 << 2 | char1 >> 4); if(lastMissingNum == 1) { int char2 = base64Toint(s.charAt(charIndex++)); result[resultIndex++] = (byte)(char1 << 4 | char2 >> 2); } } return result;     }     /**     * 根据字符查找在intToBase64中的位置     **/     private static int base64Toint(char c) {int index = base64Toint[c];if(index < 0) {throw new IllegalArgumentException("Illegal character " + c);}return index;}}