java - 同一对象上的同步块(synchronized block)是否真的阻止其他线程执行该 block ？

Question

嗨，我试图理解等待通知，我在代码中看到了这种行为，即两个线程在同一对象上的同步块(synchronized block)内打印语句。

public class WaitNotifyExample {

/**
 * @param args
 */
public static void main(String[] args) {
    Message msg = new Message("process it");
    Waiter waiter = new Waiter(msg);
    new Thread(waiter,"waiter").start();

    Waiter waiter1 = new Waiter(msg);
    new Thread(waiter1, "waiter1").start();

    Notifier notifier = new Notifier(msg);
    new Thread(notifier, "notifier").start();
    //System.out.println("All the threads are started");

}

}

class Message {
private String msg;

public Message(String str){
    this.msg=str;
}

public String getMsg() {
    return msg;
}

public void setMsg(String str) {
    this.msg=str;
}

}

class Waiter implements Runnable{

private Message msg;

public Waiter(Message m){
    this.msg=m;
}

@Override
public void run() {
    String name = Thread.currentThread().getName();
    synchronized (msg) {
        try{
            System.out.println(name+" waiting to get notified at time:"+System.currentTimeMillis());
            msg.wait();
        }catch(InterruptedException e){
            e.printStackTrace();
        }
        System.out.println(name+" waiter thread got notified at time:"+System.currentTimeMillis());
        //process the message now
        System.out.println(name+" processed: "+msg.getMsg());
    }
}

}
class Notifier implements Runnable {

private Message msg;

public Notifier(Message msg) {
    this.msg = msg;
}

@Override
public void run() {
    String name = Thread.currentThread().getName();
    System.out.println(name+" started");
    try {
        Thread.sleep(1000);
        synchronized (msg) {
            msg.setMsg(name+" Notifier work done");
            //msg.notify();
            msg.notifyAll();
        }
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

}

}

这是程序的输出:

    -waiter waiting to get notified at time:1393849891481      -notifier started      -waiter1 waiting to get notified at time:1393849891483      -waiter1 waiter thread got notified at time:1393849892483      -waiter1 processed: notifier Notifier work done      -waiter waiter thread got notified at time:1393849892483  waiter processed: notifier Notifier work done

Answer 1

阅读有关 Object.wait 的 Javadoc:

The thread releases ownership of this monitor and waits until another thread notifies threads waiting on this object's monitor

当线程在 msg.wait 内阻塞时，它不拥有 msg 的监视器。任何其他线程都可以自由获取它。