无法在多线程中实现同步

Question

我基本上是在尝试实现一个多人在线预订一辆出租车的真实示例。在我的代码中，我有 3 个类——出租车、客户和服务器。
必须有多个客户（线程）和一个出租车。但我无法做到这一点。每次我创建新客户时，都会创建一个新的出租车实例。
这是出租车类代码-

    public class taxi  {
        boolean BOOKED=false;
         String id;
        void book(){
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            BOOKED=true;
            System.out.println("Customer "+Thread.currentThread().getName()+" BOOKED taxi");
        }

        void release(){
            BOOKED=false;
            System.out.println("Customer "+Thread.currentThread().getName()+" RELEASED taxi");
        }

        void setId(String id){
            this.id=id;
        }

        String getId(){
            return id;
        }

    }

客户类代码-

public class customer extends Thread {
     taxi t=new taxi();
        public  void run(){
            //System.out.println(t.hashCode());
            t.setId(Thread.currentThread().getName());
            System.out.println("Customer "+Thread.currentThread().getName()+" trying to BOOK taxi");
            t.book();
            System.out.println("Customer "+Thread.currentThread().getName()+" is currently USING taxi");

            try {


Thread.sleep(2000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("Customer "+Thread.currentThread().getName()+" RELEASING taxi");
        t.release();
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("taxi used by customer "+Thread.currentThread().getName()+" set id to "+t.getId());
    }



}

服务器类代码-

public class server {

public static void main(String args[]){

         customer A=new customer();
         customer B=new customer();
         customer C=new customer();
         customer D=new customer();
        Thread t=new Thread();
         A.setName("A");
         B.setName("B");
         C.setName("C");
         D.setName("D");

         A.start();
         B.start();
         C.start();
         D.start();




    }


    }

这是我的输出-

Customer B trying to BOOK taxi
Customer D trying to BOOK taxi
Customer A trying to BOOK taxi
Customer C trying to BOOK taxi
Customer B BOOKED taxi
Customer A BOOKED taxi
Customer A is currently USING taxi
Customer D BOOKED taxi
Customer D is currently USING taxi
Customer B is currently USING taxi
Customer C BOOKED taxi
Customer C is currently USING taxi
Customer C RELEASING taxi
Customer C RELEASED taxi
Customer D RELEASING taxi
Customer D RELEASED taxi
Customer A RELEASING taxi
Customer A RELEASED taxi
Customer B RELEASING taxi
Customer B RELEASED taxi
taxi used by customer D set id to D
taxi used by customer C set id to C
taxi used by customer A set id to A
taxi used by customer B set id to B

如您所见，每辆出租车的 ID 不同，而不是相同。
请帮助。

Answer 1

关于您的代码的一些要点：

1. 您应该只创建 Taxi 类的单个实例，并从 Customer 类中删除出租车实例变量，并在服务器类中实例化 Taxi。
2. 将您的客户类更改为在共享出租车上工作。您可以在 Customer 类中创建参数化构造函数来初始化共享出租车。
3. setId 应该在方法 book 中调用，这样出租车的 Id 只能由想要预订出租车的线程更改。

您可以在Taxi类中通过这种方式使用等待/通知机制来实现同步：

public class Taxi {
    Boolean BOOKED = false;
    String id;

    void book() throws InterruptedException {
        synchronized (this) {
            while (BOOKED) {
                this.wait();
            }
            try {
                setId(Thread.currentThread().getName());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            BOOKED = true;
            System.out.println("Customer " + Thread.currentThread().getName() + " BOOKED taxi");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

    void release() throws InterruptedException {
        synchronized (this) {
            BOOKED = false;
            System.out.println("Customer " + Thread.currentThread().getName() + " RELEASED taxi");
            this.notifyAll();
        }
    }

    void setId(String id) throws InterruptedException {
        System.out.println("SETTING ID TO CUSTOMER " + Thread.currentThread().getName());
        this.id = id;
    }

    String getId() {
        return id;
    }
}

客户：

   public class Customer extends Thread {
    Taxi taxi;

    public Customer(Taxi taxi){
        this.taxi = taxi;
    }

    public  void run(){
        //System.out.println(t.hashCode());

        System.out.println("Customer "+Thread.currentThread().getName()+" trying to BOOK taxi");
        try {
            taxi.book();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Customer "+Thread.currentThread().getName()+" is currently USING taxi");

        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("Customer "+Thread.currentThread().getName()+" RELEASING taxi");
        try {
            taxi.release();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

Answer 2

我认为，出租车课不必是一个线程。通过将出租车类作为所有其他客户线程之间的共享资源（具有同步方法的单吨），我们可以在多线程中实现预期的同步。

Answer 3

为什么您每次都在客户中创建新出租车？问题是：taxi t=new taxi();。只需通过构造函数传递出租车或使其成为全局或类似的东西。这样，每个客户都将使用相同的出租车。此外，线程是危险的，所以你应该非常小心它们。你没有任何同步。我建议您不了解如何正确使用线程。我建议您阅读synchronize、volatile、wait、notify、notifyAll 并尝试一些基本示例。我认为前两个在Head First Java 中得到了很好的描述。 题外话：命名变量和方法时应遵循约定。