public class RandomStrCreator {

    private static final String[] tmp = { "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U",
            "V", "W", "X", "Y", "Z", "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v",
            
 
"w", "x", "y", "z", "0", "1", "2", "3", "4", "5", "6", "7", "8", "9" };//,"+","/"

    private static Random rand = new Random();

    public static String createRandomStr(int i) {
        StringBuilder sb = new StringBuilder();
        for (int j = 0; j < i; j++) {
            sb.append(tmp[rand.nextInt(tmp.length)]);
        }
        
 
return sb.toString();
    }

    public static void main(String[] args) {
        long currentTimeMillis = System.currentTimeMillis();
        String str = createRandomStr(128);
        System.out.println(str);
        System.out.println(System.currentTimeMillis() - currentTimeMillis);

    }

}

結果:

6DAcWg7ilbxbH0WLUWhSScHxSN7AZr065KobhOnB4tIqLNdtgHsA4uzGg91W2jEqfhWn8KjpcuRAHhkSjdSawjkUVefzqCQcTUMBPVe0psnutAHxSh74I1PJiWFkHVxl

0