2-4节,为什么我的实际输出始终为20000

class Req1 implements Runnable{
static Req1 req1 = new Req1();
static int i=0;
@Override
public void run() {
for(int j = 0;j<10000;j++){
i++;
}
}
public static void main(String[] args) throws InterruptedException {
Thread thread1 = new Thread(req1);
Thread thread2 = new Thread(req1);
thread1.start();
thread2.start();
thread1.join();
thread2.join();
System.out.println(i);
}
}