看了api还是没有搞清楚什么意思！

import java.util.concurrent.*;
class SemaphoreEx1{
    public static void main(String arg[]){
        Semaphore sem = new Semaphore(0);
        new TA(sem).start();
        new TB(sem).start();
    }
}
class TA extends Thread{
    Semaphore sem;
    public TA(Semaphore s){
        sem = s;
    }
    public void run(){
        System.out.println("TA going first");
        sem.release();
    }
}
class TB extends Thread{
    Semaphore sem;
    public TB(Semaphore s){
        sem = s;
    }
    public void run(){
        try{
            sem.acquire();
        }
        catch(InterruptedException e){
        }
        System.out.println("TB going second");
    }
}
初始化是0，怎么TA能获得信号呢？