详细解析C#多线程同步事件及等待句柄
最近捣鼓了一下多线程的同步问题,发现其实c#关于多线程同步事件处理还是很灵活,这里主要写一下,自己测试的一些代码,涉及到了autoresetevent 和 manualresetevent,当然还有也简要提了一下system.threading.waithandle.waitone 、system.threading.waithandle.waitany和system.threading.waithandle.waitall ,下面我们一最初学者的角度来看,多线程之间的同步。
假设有这样的一个场景,主线程开了一个子线程,让子线程等着,等主线程完成了某件事情时再通知子线程去往下执行,这里关键就在于这个怎让子线程等着,主线程怎通知子线程,一般情况下我们不难想到用一个公共变量,于是咱们就有了下面的代码:
using system; using system.collections.generic; using system.text; using system.threading; namespace autoreseteventtest { class class1 { static bool flag = true; static void dowork() { console.writeline(" worker thread started, now waiting on event..."); while (flag) { } console.writeline(" worker thread reactivated, now exiting..."); } static void main() { console.writeline("main thread starting worker thread..."); thread t = new thread(dowork); t.start(); console.writeline("main thrad sleeping for 1 second..."); thread.sleep(1000); console.writeline("main thread signaling worker thread..."); flag = false; } } }
虽然目的达到了,但是看着这代码就纠结,下面该是我们的主角上场了,autoresetevent 和 manualresetevent,关于这两者我们暂且认为是差不多了,稍后我会介绍他们的不同,这里以autoresetevent为例,其实很多官方的说法太过于抽象,这里通俗地讲,可以认为autoresetevent就是一个公共的变量(尽管它是一个事件),创建的时候可以设置为false,然后在要等待的线程使用它的waitone方法,那么线程就一直会处于等待状态,只有这个autoresetevent被别的线程使用了set方法,也就是要发通知的线程使用了它的set方法,那么等待的线程就会往下执行了,set就是发信号,waitone是等待信号,只有发了信号,等待的才会执行。如果不发的话,waitone后面的程序就永远不会执行。好下面看用autoresetevent改造上面的程序:
using system; using system.collections.generic; using system.text; using system.threading; namespace autoreseteventtest { class class2 { static autoresetevent mevent=new autoresetevent(false); //static manualresetevent mevent = new manualresetevent(false); static void dowork() { console.writeline(" worker thread started, now waiting on event..."); mevent.waitone(); console.writeline(" worker thread reactivated, now exiting..."); } static void main() { console.writeline("main thread starting worker thread..."); thread t = new thread(dowork); t.start(); console.writeline("main thrad sleeping for 1 second..."); thread.sleep(1000); console.writeline("main thread signaling worker thread..."); mevent.set(); } } }
这时代码是不是清爽多了,这里其实你还会看到,把上面的autoresetevent换成manualresetevent也是没有问题的,那么它两之间的区别是什么呢?个人认为它们最大的区别在于,无论何时,只要 autoresetevent 激活线程,它的状态将自动从终止变为非终止。相反,manualresetevent 允许它的终止状态激活任意多个线程,只有当它的 reset 方法被调用时才还原到非终止状态。开下面的代码:
using system; using system.collections.generic; using system.text; using system.threading; namespace autoreseteventtest { class class3 { static autoresetevent mevent = new autoresetevent(false); //static manualresetevent mevent = new manualresetevent(false); static void dowork() { console.writeline(" worker thread started, now waiting on event..."); for (int i = 0; i < 3; i++) { mevent.waitone(); //mevent.reset(); console.writeline(" worker thread reactivated, now exiting..."); } } static void main() { console.writeline("main thread starting worker thread..."); thread t = new thread(dowork); t.start(); for (int i = 0; i < 3; i++) { thread.sleep(1000); console.writeline("main thread signaling worker thread..."); mevent.set(); } } } }
如果你想仅仅把autoresetevent换成manualresetevent的话,你发现输出就会乱套了,为什么呢?
假如有autoevent.waitone()和manualevent.waitone(),当线程得到信号后都得以继续执行。差别就在调用后,autoevent.waitone()每次只允许一个线程进入,当某个线程得到信号(也就是有其他线程调用了autoevent.set()方法后)后,autoevent会自动又将信号置为不发送状态,则其他调用waitone的线程只有继续等待,也就是说,autoevent一次只唤醒一个线程。而manualevent则可以唤醒多个线程,当某个线程调用了set方法后,其他调用waitone的线程获得信号得以继续执行,而manualevent不会自动将信号置为不发送,也就是说,除非手工调用了manualevent.reset()方法,否则manualevent将一直保持有信号状态,manualevent也就可以同时唤醒多个线程继续执行。
在上面代码中,如果将autoresetevent换成manualresetevent的话,只要要在waitone后面做下reset,就会达到同样的效果。
之后咱们再来个简单的例子:
using system; using system.collections.generic; using system.text; using system.threading; namespace autoreseteventtest { class class4 { public static autoresetevent mevent = new autoresetevent(false); public static void trmain() { thread tr = thread.currentthread; console.writeline("thread: waiting for an event"); mevent.waitone(); console.writeline("thread: got an event"); for (int x = 0; x < 10; x++) { thread.sleep(1000); console.writeline(tr.name + ": " + x); } } static void main(string[] args) { thread thrd1 = new thread(new threadstart(trmain)); thrd1.name = "thread1"; thrd1.start(); for (int x = 0; x < 10; x++) { thread.sleep(900); console.writeline("main:" + x); if (5 == x) mevent.set(); } while (thrd1.isalive) { thread.sleep(1000); console.writeline("main: waiting for thread to stop"); } } } }
是不是更有感觉了?之后咱来看看另外几个东东:
system.threading.waithandle.waitone 使线程一直等待,直到单个事件变为终止状态;
system.threading.waithandle.waitany 阻止线程,直到一个或多个指示的事件变为终止状态;
system.threading.waithandle.waitall 阻止线程,直到所有指示的事件都变为终止状态。
然后再来个例子,以waitall使用为例:
using system; using system.collections.generic; using system.text; using system.threading; namespace autoreseteventtest { class other { static void main(string[] args) { random randomgenerator = new random(); autoresetevent[] resets=new autoresetevent[5]; for (int i = 0; i < 5; i++) { resets[i] = new autoresetevent(false); int wtime = randomgenerator.next(10)+1; worker w = new worker(wtime, resets[i]); thread thrd1 = new thread(new threadstart(w.work)); thrd1.start(); } waithandle.waitall(resets); console.writeline("all worker done - main exiting."); } } public class worker { public string name; public int wtime; public autoresetevent mevent; public worker(int w, autoresetevent m) { name = w.tostring(); wtime = w * 1000; mevent = m; } public void work() { console.writeline(name + " worker thread waiting for " + wtime + "...."); thread.sleep(wtime); console.writeline(name + " worker thread back..."); mevent.set(); } } }
简单来说就是,开了5个线程,每个线程随机休眠若干秒,都完成后通知主线程退出,这里就开了一个autoresetevent数组,主线程就waithandle.waitall(resets) ,子线程休眠完后就set1个autoresetevent,最后都set完后,主线程就会往下执行。最后最后再来个买书付款取货的例子,加深理解:
using system; using system.collections.generic; using system.text; using system.threading; namespace autoreseteventtest { class program { const int numiterations = 10; static autoresetevent myresetevent = new autoresetevent(false); static autoresetevent changeevent = new autoresetevent(false); //static manualresetevent myresetevent = new manualresetevent(false); //static manualresetevent changeevent = new manualresetevent(false); static int number; //这是关键资源 static void main() { thread paymoneythread = new thread(new threadstart(paymoneyproc)); paymoneythread.name = "付钱线程"; thread getbookthread = new thread(new threadstart(getbookproc)); getbookthread.name = "取书线程"; paymoneythread.start(); getbookthread.start(); for (int i = 1; i <= numiterations; i++) { console.writeline("买书线程:数量{0}", i); number = i; //signal that a value has been written. myresetevent.set(); //changeevent.set(); thread.sleep(10); } paymoneythread.abort(); getbookthread.abort(); } static void paymoneyproc() { while (true) { myresetevent.waitone(); //myresetevent.reset(); console.writeline("{0}:数量{1}", thread.currentthread.name, number); changeevent.set(); } } static void getbookproc() { while (true) { changeevent.waitone(); //changeevent.reset(); console.writeline("{0}:数量{1}", thread.currentthread.name, number); console.writeline("------------------------------------------"); //thread.sleep(0); } } } }
以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持。
推荐阅读