生成不定长度的随机字符串：

public class RandomUtils {    public static final String allChar="0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";    public static String generateString(){        StringBuffer sb = new StringBuffer();        Random random = new Random();        int x = random.nextInt(47);        for (int i = 0; i < x; i++) {            sb.append(allChar.charAt(random.nextInt(allChar.length())));        }        return sb.toString();    }        //test code    public static void main(String[] args) {        for (int i = 0; i < 20; i++) {            System.out.println(generateString());        }            }}

生成指定长度的字符串：

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