Java等待并通知: IllegalMonitorStateException

Question

我不完全理解wait和notify(Object)是如何工作的，结果我被迫瘦身将我的尝试记入以下代码部分。

Main.java:

import java.util.ArrayList;

class Main
{
  public static Main main = null;

  public static int numRunners = 4;
  public static ArrayList<Runner> runners = null;

  public static void main(String[] args)
  {
    main = new Main();
  }

  Main()
  {
    runners = new ArrayList<Runner>(numRunners);

    for (int i = 0; i < numRunners; i++)
    {
      Runner r = new Runner();
      runners.add(r);
      new Thread(r).start();
    }

    System.out.println("Runners ready.");
    notifyAll();
  }
}

Runner.java:

class Runner implements Runnable
{
  public void run()
  {
    try
    {
      Main.main.wait();
    } catch (InterruptedException e) {}
    System.out.println("Runner away!");
  }
}

目前，我在调用 Main.main.wait(); 时收到 IllegalMonitorStateException，但我不明白为什么。据我所知，我需要同步 Runner.run，但这样做时我假设它只会通知一个线程，而我的想法是通知所有线程。

我查看了java.util.concurrent，但找不到合适的替代品(也许我只是错过了一些东西)。

Answer 1

除非当前线程拥有该对象的监视器，否则您无法在对象上wait()。为此，您必须对其进行同步:

class Runner implements Runnable
{
  public void run()
  {
    try
    {
      synchronized(Main.main) {
        Main.main.wait();
      }
    } catch (InterruptedException e) {}
    System.out.println("Runner away!");
  }
}

同样的规则也适用于 notify()/notifyAll()。

Javadocs for wait()提到这一点:

This method should only be called by a thread that is the owner of this object's monitor. See the notify method for a description of the ways in which a thread can become the owner of a monitor.

Throws:

IllegalMonitorStateException – if the current thread is not the owner of this object's monitor.

来自 notify() :

A thread becomes the owner of the object's monitor in one of three ways:

• By executing a synchronized instance method of that object.
• By executing the body of a synchronized statement that synchronizes on the object.
• For objects of type Class, by executing a synchronized static method of that class.