java - 多线程生产者消费者问题，出现了空值，问题出在哪里？

Question

多线程生产者消费者问题， producer生产电影（两种）， customer 观看电影

---

Film.java

package Film;

public class Film {
    private String name;
    private String actor;
    private boolean flag = false;

    
    
    public synchronized void set(String name, String actor)
            throws InterruptedException {
        
        if(!flag){
            this.wait();
        }

        this.setName(name);
        Thread.sleep(1000);
        this.setActor(actor);

        flag = false;
        this.notify();
    }
    
    public synchronized void get() throws InterruptedException{
        
        if(flag){
            this.wait();
        }
        Thread.sleep(1000);
        System.out.println("3"+this.getName()+"-->"+this.getActor());
        flag=true;
        this.notify();
    }

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

    public void setActor(String actor) {
        this.actor = actor;
    }

    public String getName() {
        return name;
    }

    public String getActor() {
        return actor;
    }
}

Producer.java

package Film;

public class Producer implements Runnable {

    private Film film ;

    public Producer(Film film) {
        super();
        this.film = film;
    }

    @Override
    public void run() {
        // TODO Auto-generated method stub

        for (int i = 1; i < 10; i++) {

            if (0==i%2) {
                try {
                    System.out.println("1-produce film " + i);

                    film.set("film1", "actor1");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            } else {
                try {
                    System.out.println("1-produce film " + i);
                    film.set("film2", "actor2");

                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

            }

        }

    }

}

Customer.java

public class Customer implements Runnable {
    private Film film ;
    
    public Customer(Film film){
        this.film=film;
    
    }
    public void run() {
        
        for(int x=1;x<10;x++){
            try {
                Thread.sleep(1000);
                film.get();
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            
        }

    }

}

MainTest.java

package Film;

public class MainTest {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Film film = new Film();
        Producer pro = new Producer(film);
        Customer cus = new Customer(film);
        new Thread(pro).start();
        new Thread(cus).start();

    }

}

输出如下

1-produce film 1
3null-->null
1-produce film 2
3film2-->actor2
1-produce film 3
3film1-->actor1
1-produce film 4
3film2-->actor2
1-produce film 5
3film1-->actor1
1-produce film 6
3film2-->actor2
1-produce film 7
3film1-->actor1
1-produce film 8
3film2-->actor2
1-produce film 9
3film1-->actor1

请问第二行为什么会出现空值？

Answer 1

在 Film 這個程式碼裡面，flag初始化為false，
看你的 Film class 中的set， get 方法，必須先執行過 get 之後 才能執行 set。

所以我覺得 File class 中 的get，set方法開頭的flag判斷應該是寫反了吧！

Answer 2

雷雷