RandomStringUtil.java

public static final String allChar = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    public static final String letterChar = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    public static final String numberChar = "0123456789";

    public static String generateString(int length)
    {
     StringBuffer sb = new StringBuffer();
     SecureRandom random = new SecureRandom();
     
     for (int i = 0; i < length; i++)
     {
      sb.append(allChar.charAt(random.nextInt(allChar.length())));
     }
    return sb.toString();
    }