int i = (int)(8+Math.random()*(20-8+1)) ;
     String pd=this.getRandomPassword(i);

 

 

public  String getRandomPassword(int len) {
   String result = null;
   while(len>=6){
    result = this.makeRandomPassword(len);
    if (result.matches(".*[a-z]{1,}.*") && result.matches(".*[A-Z]{1,}.*") && result.matches(".*\\d{1,}.*") && result.matches(".*[~!@#$%^&*\\.?]{1,}.*")) {
    return result;
    }
    result = makeRandomPassword(len);
    }
    return "长度不得少于6位!";
    }
  public  String makeRandomPassword(int len){
   char charr[] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890~!@#$%^&*.?".toCharArray();
   StringBuilder sb = new StringBuilder();
   Random r = new Random();
   for (int x = 0; x < len; ++x) {
   sb.append(charr[r.nextInt(charr.length)]);
   }
   return sb.toString();
   }