class FooBar {
private int n;

public FooBar(int n) {
    this.n = n;
}

Semaphore foo = new Semaphore(1);
Semaphore bar = new Semaphore(0);

public void foo(Runnable printFoo) throws InterruptedException {
    for (int i = 0; i < n; i++) {
        foo.acquire();
        printFoo.run();
        bar.release();
    }
}

public void bar(Runnable printBar) throws InterruptedException {
    for (int i = 0; i < n; i++) {
        bar.acquire();
        printBar.run();
        foo.release();
    }
}

}
老师，这是我在leetcode上面看到的一道题，这个Semaphore bar = new Semaphore(0)是什么用法，0个信号量？起什么特殊作用？