private static Random randGen = null;
    private static char[] numbersAndLetters = null;

    public static final String randomString(int length) {
             if (length < 1) {
                 return null;
             }
             if (randGen == null) {
                    randGen = new Random();
                    numbersAndLetters = ("0123456789abcdefghijklmnopqrstuvwxyz" +
                       "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ").toCharArray();
                      //numbersAndLetters = ("0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ").toCharArray();
                     }
             char [] randBuffer = new char[length];
             for (int i=0; i<randBuffer.length; i++) {
                 randBuffer[i] = numbersAndLetters[randGen.nextInt(71)];
              //randBuffer[i] = numbersAndLetters[randGen.nextInt(35)];
             }
             return new String(randBuffer);
    }

    //调用此方法randomString(int),int是字符串的长度，即可产生指定长度的随机字符串。