有趣的多线程编程（4）——死锁

// DeadLockSample.cs
// 分析一下为什么会发生死锁？

using System;
using System.Threading;

public class Test
{
    static readonly object firstLock = new object();
    static readonly object secondLock = new object();
    
    static void Main()
    {
        new Thread(new ThreadStart(ThreadJob)).Start();
        
        // Wait until we're fairly sure the other thread
        // has grabbed firstLock
        Thread.Sleep(500);
        
        Console.WriteLine ("Locking secondLock");
        lock (secondLock)
        {
            Console.WriteLine ("Locked secondLock");
            Console.WriteLine ("Locking firstLock");
            lock (firstLock)
            {
                Console.WriteLine ("Locked firstLock");
            }
            Console.WriteLine ("Released firstLock");
        }
        Console.WriteLine("Released secondLock");
    }
    
    static void ThreadJob()
    {
        Console.WriteLine ("/t/t/t/tLocking firstLock");
        lock (firstLock)
        {
            Console.WriteLine("/t/t/t/tLocked firstLock");
            // Wait until we're fairly sure the first thread
            // has grabbed secondLock
            Thread.Sleep(1000);
            Console.WriteLine("/t/t/t/tLocking secondLock");
            lock (secondLock)
            {
                Console.WriteLine("/t/t/t/tLocked secondLock");
            }
            Console.WriteLine ("/t/t/t/tReleased secondLock");
        }
        Console.WriteLine("/t/t/t/tReleased firstLock");
    }
}

因应之道，使用Queue和Monitor:

//QueueMonitorThread.cs

using System;
using System.Collections;
using System.Threading;

public class Test
{
    static ProducerConsumer queue;
    
    static void Main()
    {
        queue = new ProducerConsumer();
        new Thread(new ThreadStart(ConsumerJob)).Start();
        
        Random rng = new Random(0);
        for (int i=0; i < 10; i++)
        {
            Console.WriteLine ("Producing {0}", i);
            queue.Produce(i);
            Thread.Sleep(rng.Next(1000));
        }
    }
    
    static void ConsumerJob()
    {
        // Make sure we get a different random seed from the
        // first thread
        Random rng = new Random(1);
        // We happen to know we've only got 10 
        // items to receive
        for (int i=0; i < 10; i++)
        {
            object o = queue.Consume();
            Console.WriteLine ("/t/t/t/tConsuming {0}", o);
            Thread.Sleep(rng.Next(1000));
        }
    }
}

public class ProducerConsumer
{
    readonly object listLock = new object();
    Queue queue = new Queue();

    public void Produce(object o)
    {
        lock (listLock)
        {
            queue.Enqueue(o);
            if (queue.Count==1)
            {
                Monitor.Pulse(listLock);
            }
        }
    }
    
    public object Consume()
    {
        lock (listLock)
        {
            while (queue.Count==0)
            {
                Monitor.Wait(listLock);
            }
            return queue.Dequeue();
        }
    }
}