java - 观察者，可观察和可运行。为什么同步块(synchronized block)丢失监视器？

Question

我正在尝试将类用作 Observer 和 Observable。此类将作为线程运行。在 run() 方法中，线程将等待并在获取事件线程后得到通知。有示例代码:

public class Runner {

    public static void main(String[] args) {
        MyThread mt = new MyThread();
        Controller c = new Controller();
        mt.addObserver(c);
        c.addObserver(mt);
        Thread t = new Thread(mt);
        t.start();
    }

}


public class MyThread extends Observable implements Observer, Runnable {

    static private Integer op = 0;

    public void run() {
      synchronized (this) {
        while (true) {
          op++;
          System.out.println(op + " Thread started");
          super.setChanged();
          super.notifyObservers(new Object());
          op++;
          System.out.println(op + " Thread send event");
          try {
            op++;
            System.out.println(op + " Thread wait");
            this.wait();
          } catch (InterruptedException e) {
            e.printStackTrace();
          }
        }
      }
    }

    @Override
    public void update(Observable arg0, Object arg1) {
      op++;
      System.out.println(op + " Thread got event");
      synchronized (this) {
        op++;
        System.out.println(op + " We are in synchronized block!");
        this.notify();
      }
    }

}


public class Controller extends Observable implements Observer {

  public void update(Observable arg0, Object arg1) {
    System.out.println("Controller get and send event");
    super.setChanged();
    super.notifyObservers(new Object());
  }

}

得到的输出是:

1 Thread started
Controller get and send event
2 Thread got event
3 We are in synchronized block!
4 Thread send event
5 Thread wait

线程保持锁定状态。预期输出:

1 Thread started
Controller get and send event
2 Thread got event
3 Thread send event
4 Thread wait
5 We are in synchronized block!

怎么了？为什么我在监视器释放之前进入同步块(synchronized block)？附言我有一个想法，问题是将观察者添加到 MyThread 对象，我可以将观察者添加到 Thread 对象吗？但是我该怎么做呢？

Answer 1

好吧，我认为您遇到的主要问题是 synchronized 关键字类似于可重入锁 (http://docs.oracle.com/javase/tutorial/essential/concurrency/locksync.html)。

这意味着，当您在 MyThread 的 run 方法中时，您正在通知 Controller。这个然后调用你的 MyThread 的 update 进入 synchronized block (因为它是可重入的)并完成这个方法。之后，Controller.update 方法返回并且您的 MyThread.run 方法的其余部分继续，因此卡在 this.wait() 上。