.Net线程同步的n个方法(一)
本文主要描述在C#中线程同步的方法。线程的基本概念在上一章中已经介绍过了,网上资料也很多就不再赘述了。直接接入主题,在多线程开发的应用中,线程同步是不可避免的。在.Net框架中,实现线程同步主要通过以下的几种方式来实现,在MSDN的线程指南中已经讲了几种,本文结合作者实际中用到的方式一起说明一下。
1. 维护自由锁(InterLocked)实现同步
2. 监视器(Monitor)和互斥锁(lock)
3. 读写锁(ReadWriteLock)
4. 系统内核对象
1) 互斥(Mutex), 信号量(Semaphore), 事件(AutoResetEvent/ManualResetEvent)
2) 线程池
除了以上的这些对象之外实现线程同步的还可以使用Thread.Join方法。这种方法比较简单,当你在第一个线程运行时想等待第二个线程执行结果,那么你可以让第二个线程Join进来就可以了。
@_@自由锁(InterLocked)
对一个32位的整型数进行递增和递减操作来实现锁,有人会问为什么不用++或--来操作。因为在多线程中对锁进行操作必须是原子的,而++和--不具备这个能力。因此Increment(ref counter)这样的操作是线程安全的。InterLocked类还提供了两个另外的函数Exchange, CompareExchange用于实现交换和比较交换。Exchange操作会将新值设置到变量中并返回变量的原来值:int oVal = InterLocked.Exchange(ref val, 1)。
@_@监视器(Monitor)
在MSDN中对Monitor的描述是: Monitor 类通过向单个线程授予对象锁来控制对对象的访问。
Monitor类是一个静态类因此你不能通过实例化来得到类的对象。Monitor的成员可以查看MSDN,基本上Monitor的效果和lock是一样的,通过加锁操作Enter设置临界区,完成操作后使用Exit操作来释放对象锁。不过相对的来说Monitor的功能更强,Moniter可以进行测试锁的状态,因此你可以控制对临界区的访问选择,等待or离开, 而且Monitor还可以在释放锁之前通知指定的对象,更重要的是使用Monitor可以跨越方法来操作。Monitor提供的方法很少就只有获取锁的方法Enter, TryEnter;释放锁的方法Wait, Exit;还有消息通知方法Pulse, PulseAll。经典的Monitor操作是这样的
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其中Names是一个List<string>, 这里有一个小技巧,如果你想声明整个方法为线程同步可以使用方法属性
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对于Monitor的使用有一个方法是比较诡异的,那就是Wait方法。在MSDN中对Wait的描述是:
释放对象上的锁以便允许其他线程锁定和访问该对象。
这里提到的是先释放锁,那么显然我们需要先得到锁,否则调用Wait会出现异常,所以我们必须在Wait前面调用Enter方法或其他获取锁的方法如lock,这点很重要。对应Enter方法Monitor给出来另一种实现TryEnter。这两种方法的主要区别在与是否阻塞当前线程,Enter方法在当获取不到锁时,会阻塞当前线程直到得到锁。不过缺点是如果永远得不到锁那么程序就会进入死锁状态,我们可以采用Wait来解决,在调用Wait时加入超时时限就可以。
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@_@ 互斥锁(lock)
lock关键字是实现线程同步的比较简单的方式,其实就是设置一个临界区。在lock之后的{...}区块为一个临界区,在进入临界区是加互斥锁离开临界区时释放互斥锁。MSDN对lock关键字的描述是:
lock 关键字可将语句块标记为临界区,方法是获取给定对象的互斥锁,执行语句,然后释放该锁。
具体例子如下:
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对lock的使用有几点建议:对实例锁定lock(this),对静态变量锁定lock(typeof(val))。lock的对象访问权限最好是private,否则会出现失去访问控制现象。
@_@读写锁(ReadWriteLock)
读写锁的出现主要是在很多情况下,我们读资源的操作要多于写资源的操作。但是如果每次只对资源赋予一个线程的访问权限显然是低效的,读写锁的优势是同时可以有多个线程对同一资源进行读操作。因此在读操作比写操作多很多,并且写操作的时间很短的情况下使用读写锁是比较有效率的。读写锁是一个非静态类所以你在使用前需要先声明一个读写锁对象:
static private ReaderWriterLock _rwlock = new ReaderWriterLock();
读写锁是通过调用AcquireReaderLock,ReleaseReaderLock,AcquireWriterLock,ReleaseWriterLock来完成读锁和写锁控制的。
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如果你想在读的时候插入写操作请使用UpgradeToWriterLock和DowngradeFromWriterLock来进行操作,而不是释放读锁。
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这里有一点要注意的就是读锁和写锁的超时等待时间间隔的设置,通常情况下设置写锁的等待超时要比读锁的长,否则会经常发生写锁等待失败的情况。
由于关于线程同步内容比较长,我将它分成两部分来写。下一篇我们将讨论利用内核对象进行线程同步。