线程同步，使用 notifyAll()

Question

这是 Bruce Eckel 的 Thinking in JAVA 3rd Edition，Revision 4.0，第 13 章（练习 10）中的练习。我应该有一个像厨师一样下订单并通知服务员的班级，以及两个等待订单准备好的服务员，然后接受它。

这是我目前所拥有的：

class Order {
    private static int i = 0;
    private int count = i++;
    public Order() {
        if(count == 10) {
            System.out.println("Out of food, closing.");
            System.exit(0);
        }
    }
    public String toString() {
            return "Order " + count;
    }
}
class WaitPerson extends Thread {
    public  Restaurant restaurant;
    private int waitPersonId;
    public WaitPerson(Restaurant r, int waitPersonId) {
        restaurant = r;
        this.waitPersonId = waitPersonId;
        start();
    }
    public void run() {
        Order currentOrder;
        while(true) {

            while(restaurant.order == null) {

                synchronized(Restaurant.waitpersonsList) {
                    try {
                        System.out.println("Waitperson" + this.waitPersonId + " is waiting...");
                        wait();
                        System.out.println("WaitPerson" + this.waitPersonId + " is attempting to get " + restaurant.order);
                    }
                    catch(InterruptedException e) {
                        throw new RuntimeException(e);
                    }
                }
            }

            currentOrder = restaurant.order;
            System.out.println("Waitperson" + this.waitPersonId + " got " + currentOrder);
            restaurant.order = null;

            try {
                sleep(3000); //waitperson is busy for 3 seconds
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            System.out.println("Waitperson" + this.waitPersonId + " delivered " + currentOrder);

        }
    }
}
class Chef extends Thread {
    private Restaurant restaurant;
    private int ChefId;
    public Chef(Restaurant r, int ChefId) {
        restaurant = r;
        this.ChefId = ChefId;
        start();
    }
    public void run() {
        while(true) {
            if(restaurant.order == null) {
                restaurant.order = new Order();
                System.out.println("Chef" + this.ChefId + ": Order up! ");
                synchronized(Restaurant.waitpersonsList) {
                    Restaurant.waitpersonsList.notifyAll();
                }
            }
            try {
                sleep(100);
            }
            catch(InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }
}

class Restaurant {
    public Order order;
    public static List<WaitPerson> waitpersonsList = Collections.synchronizedList(new ArrayList<WaitPerson>());
}
public class Main {
    public static void main(String[] args) {
        Restaurant r = new Restaurant();
        WaitPerson wp1 = new WaitPerson(r, 1);
        WaitPerson wp2 = new WaitPerson(r, 1);
        Restaurant.waitpersonsList.add(wp1);
        Restaurant.waitpersonsList.add(wp2);
        Chef chef = new Chef(r, 1);
    }
}

一切正常，直到 WaitPerson 到达他应该等待（）的部分，直到他得到通知。然后我得到一个IllegalMonitorStateException，这意味着线程正在等待对象的监视器而不拥有指定的监视器。不过还是想不通。

Answer 1

您的问题是您已在 Restaurant.waitpersonsList 上进行了同步，但随后在 WaitPerson 上调用了等待。您必须在您调用 wait、notify 或 notifyAll 的任何对象上进行同步

             synchronized(Restaurant.waitpersonsList) {
                try {
                    System.out.println("Waitperson" + this.waitPersonId + " is waiting...");
                    wait();
                    System.out.println("WaitPerson" + this.waitPersonId + " is attempting to get " + restaurant.order);
                }
                catch(InterruptedException e) {
                    throw new RuntimeException(e);
                }
            }

Answer 2

您正在等待一个对象并通知另一个对象！

如果您在 Restaurant.waitpersonsList 上进行同步，那么您也需要等待它，而引发异常的 wait() 正在尝试等待 WaiterList 对象本身。