Java – improve code that breaks down faster?

I want to finish this code faster It returns all the factors (prime) of a long number If longnumber is specific, it obviously takes a few minutes to execute@ H_ 301_ 7@

int div = 2;
String factors = "";

while (longNumer != 1)
{
    if (longNumer % div == 0)
    {
        longNumer /= div;
        factors += div + " ";
    }
    else { div++; }
}

//longNumber = 10,gives: 2 5. 
//longNumber = 150,gives: 3 5 7.
//longNumber = 10523,gives: 17 619.

Solution

For large numbers, you can use the sieve of Eratosthenes algorithm to first find prime numbers up to sqrt (n), and then you must check whether these prime numbers are factors