Java线程练习: Access control to a limited resource not working

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我遇到这样的情况:有一家比萨店，有不同数量的顾客。这家比萨店在开业前随机准备数量的比萨饼，并且只有 10 个座位。每个客户都是一个线程。我需要让他们在有空位的情况下同时吃饭。其他人则等待，直到有免费套餐(以及准备好的比萨饼)。我尝试过这个方案，但是这样每个顾客都是单独吃饭。只有当它吃完后，其他顾客才能吃。正确的解决方案是什么？

Pizzeria 的这个方法由每个 Customer 线程调用:

public boolean eatPizza(Integer nPizzas){
        if(freeSeats< 1){
            try {
                wait();
            } catch (InterruptedException ex) {
            }
        }
        if(preparedPizzas == 0)    //Pizzas terminated for this evening
            return false;

        freeSeats--;     
        if(nPizzas > preparedPizzas)
            nPizzas = preparedPizzas;  //if he wants more pizzas than avaiable ones he have to settle
        preparedPizzas-= nPizzas;
        try {
            Thread.sleep(2000);  //Time for eat
        } catch (InterruptedException ex) {
        }
        freeSeats++;
        servedCustomers++;
        servedPizzas+= nPizzas;
        if(preparedPizzas> 0){
            notify();      
        }
        else{
            notifyAll(); //Pizzas are terminated for this evening
        }
        return true;
    }

客户线程:

public void run(){
        int wait = (int)(Math.random() * 5000);
        try {
            Thread.sleep(wait);
        } catch (InterruptedException ex) {
        }
        nPizzas = (int)(Math.random() * 2 + 1);  //He eats 1 or 2 pizzas random
        if(pizzeria.eatPizza(nPizzas))
            fireEatenEvent();
    }

感谢您的帮助

Answer 1

我使用 ExecutorService 创建了一个简单的示例和一个BlockingQueue 。这样您就不必自己处理任何锁。

public class Restaurant {

    public static void main(String[] args) throws Exception {
        final Random rnd = new Random();
        // The queue from which the pizzas are taken
        final BlockingQueue<Pizza> pizzas = new LinkedBlockingQueue<>();
        // The threadpool that represents the seats of the restaurant
        // 10 customers are handled at one time
        final ExecutorService seats = Executors.newFixedThreadPool(10);
        final int nPizzas = rnd.nextInt(50);
        final int nCustomers = rnd.nextInt(100);

        System.out.println("There are " + nPizzas + " pizzas for " + nCustomers + " customers.");

        // Put some pizzas into the queue
        for (int i = 1; i < nPizzas + 1; i++) {
            pizzas.put(new Pizza("Pizza " + i));
        }

        // Marks the end of the queue
        pizzas.put(new PoisonPizza());

        // Create some customers and send them to the restaurant
        for (int i = 1; i < nCustomers + 1; i++) {
            seats.submit(new Customer("Customer " + i, pizzas));
        }

        // Knock off when all customers are gone
        seats.shutdown();
    }

    private static class Pizza {

        private String name;

        public Pizza() {}

        public Pizza(final String name) {
            this.name = name;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

    }

    private static class PoisonPizza extends Pizza {

        public PoisonPizza() {
            super("PoisonPizza");
        }

    }

    private static class Customer implements Runnable {

        private String name;

        private BlockingQueue<Pizza> pizzas;

        public Customer() {}

        public Customer(final String name, final BlockingQueue<Pizza> pizzas) {
            this.name = name;
            this.pizzas = pizzas;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public BlockingQueue<Pizza> getPizzas() {
            return pizzas;
        }

        public void setPizzas(final BlockingQueue<Pizza> pizzas) {
            this.pizzas = pizzas;
        }

        @Override
        public void run() {
            try {
                // Order a pizza
                final Pizza pizza = pizzas.take();

                // Leave the restaurant if all pizzas have been aten
                if (pizza instanceof PoisonPizza) {
                    pizzas.put(pizza);
                    System.out.println(name + ": I'm so hungry.");

                    return;
                }

                // Eat the pizza
                System.out.println(name + ": " + pizza.getName() + " was delicious!");
            } catch (InterruptedException e) {
                return;
            }
        }

    }

}