BASE64String 的解码实现

 public byte[] decode(String encodeString){
        
        if(encodeString == null || (encodeString.length() == 0) || (encodeString.length()) % 4 != 0){
            throw new IllegalArgumentException("传入的参数不是正确的base64字符串");
        }
        int encodeLength = encodeString.length();
        byte decodeByte[] = new byte[encodeLength /4 * 3];
        int  indexPos = 0;
    
        char c    = encodeString.charAt(0);
        int  code = 0;
        int  pos  = 0;
        boolean invalidChar = false;
        
        for( int j = 0; c != '=' && j < encodeLength;  j++){
            pos = pos % 3;
            switch (pos){
                case 0 :
                    code = getCharacterPos(c) << 2;
                    c =  encodeString.charAt(j + 1);
                    if(c != '='){
                        code = code | (getCharacterPos(c) >> 4);
                    }
                    invalidChar = false; 
                    break;
                case 1 :
                    code = getCharacterPos(c) << 4;
                    c =  encodeString.charAt(j + 1);
                    if(code == 0 && c == '='){
                        invalidChar = true;
                    }else  if(c != '='){
                        code = code | getCharacterPos(c)>>2;
                    }
                    break;
                case 2 :
                    code = getCharacterPos(c)<< 6;
                    j++;
                    c =  encodeString.charAt(j);
                    if(c == '='){
                        invalidChar = true;
                    }else{
                        invalidChar = false;
                        code = code | getCharacterPos(c);
                    }                  
                    if( j + 1 < encodeLength){
                        c = encodeString.charAt(j + 1);
                    }
                    break;
            }
            pos ++;
            if( ! invalidChar){                
                decodeByte[indexPos] = (byte)code;
                indexPos ++ ;
            }
        }
        byte [] tmp ;
        if(encodeLength > indexPos){
             tmp = new byte[indexPos];
             for(int i = 0; i < indexPos; i++){
                 tmp[i] = decodeByte[i];
             }
        }else{
            tmp = decodeByte;
        }
       
        return tmp;
    }