csRandomAlphaNumericString.java

//http://stackoverflow.com/a/19597101/617028
private static char[] VALID_CHARACTERS =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456879".toCharArray();

// cs = cryptographically secure
public static String csRandomAlphaNumericString(int numChars) {
    SecureRandom srand = new SecureRandom();
    Random rand = new Random();
    char[] buff = new char[numChars];

    for (int i = 0; i < numChars; ++i) {
        // reseed rand once you've used up all available entropy bits
        if ((i % 10) == 0) {
            rand.setSeed(srand.nextLong()); // 64 bits of random!
        }
        buff[i] = VALID_CHARACTERS[rand.nextInt(VALID_CHARACTERS.length)];
    }
    return new String(buff);
}