Prime calculation formula does not shows prime number till 100000 it show till 9973

long i = 0;
int primeNumberCounter = 1;
long upperLimit = 100000;
PrintWriter writer = resp.getWriter();
while (++i <= upperLimit) {
    long i1 = (long) Math.ceil(Math.sqrt(i));
    boolean isPrimeNumber = false;
    while (i1 > 1) {
        if ((i != i1) && (i % i1 == 0)) {
            isPrimeNumber = false;
            break;
        } else if (!isPrimeNumber) {
            isPrimeNumber = true;
        }
        --i1;
    }

    if (isPrimeNumber) {
        writer.write(String.valueOf(i));
        writer.write("\n");
        ++primeNumberCounter;
    }
}

I deployed above code on google app engine.I am getting value till 9973 not 99991 which must be last prime number.Thanx any help appreciated

via Chebli Mohamed