概述
- 多线程给我们带来的好处是可以并发的执行多个任务，特别是对于I/O密集型的业务，使用多线程，可以带来成倍的性能增长。
- 可是当我们多个线程需要修改同一个数据，在不做任何同步控制的情况下，产生的结果往往是不可预料的，比如两个线程，一个输出hello，一个输出world，实际运行的结果，往往可能是一个是hello world，一个是world hello。
- python里提供了多个用于控制多线程同步的同步原语，这些原语，包含在python的标准库threading.py当中。我今天简单的介绍一下python里的这些控制多线程同步的原语，包括：Locks、RLocks、Semaphores、Events、Conditions和Barriers，你也可以继承这些类，实现自己的同步控制原语。
Lock(锁)
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Locks是python里最简单的同步原语，只包括两个状态：locked和unlocked，刚创建时状态是unlocked。Locks有两个方法，acquire和release。acquire方法加锁，release方法释放锁，如果acquire枷锁失败，则阻塞，表明其他线程已经加锁。release方法只有当状态是locked调用方法True，如果是unlocked状态，调用release方法会抛出RunTimeError异常。例如代码：
from threading import Lock, Thread lock = Lock() g = 0 def add_one(): """ Just used for demonstration. It’s bad to use the ‘global’ statement in general. """ global g lock.acquire() g += 1 lock.release() def add_two(): global g lock.acquire() g += 2 lock.release() threads = [] for func in [add_one, add_two]: threads.append(Thread(target=func)) threads[-1].start() for thread in threads: """ Waits for threads to complete before moving on with the main script. """ thread.join() print(g) 最终输出的结果是3，通过Lock的使用，虽然在两个线程中修改了同一个全局变量，但两个线程是顺序计算出结果的。
RLock(循环锁)
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上面的Lock对象虽然能达到同步的效果，但是无法得知当前是那个线程获取到了锁。如果锁没被释放，则其他获取这个锁的线程都会被阻塞住。如果不想阻塞，可以使用RLock，例如：
# 使用Lock import threading num = 0 lock = Threading.Lock() lock.acquire() num += 1 lock.acquire() # 这个地方阻塞 num += 2 lock.release() # 使用RLock lock = Threading.RLock() lock.acquire() num += 3 lock.acquire() # 这不会阻塞 num += 4 lock.release() lock.release() # 这个地方注意是释放两次锁
Semaphores
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Semaphores是个最简单的计数器，有两个方法acquire()和release()，如果有多个线程调用acquire()方法，acquire()方法会阻塞住，每当调用次acquire方法，就做一次减1操作，每当release()方法调用此次，就加1，如果最后的计数数值大于调用acquire()方法的线程数目，release()方法会抛出ValueError异常。下面是个生产者消费者的示例。
import random, time from threading import BoundedSemaphore, Thread max_items = 5 container = BoundedSemaphore(max_items) def producer(nloops): for i in range(nloops): time.sleep(random.randrange(2, 5)) print(time.ctime(), end=": ") try: container.release() print("Produced an item.") except ValueError: print("Full, skipping.") def consumer(nloops): for i in range(nloops): time.sleep(random.randrange(2, 5)) print(time.ctime(), end=": ") if container.acquire(False): print("Consumed an item.") else: print("Empty, skipping.") threads = [] nloops = random.randrange(3, 6) print("Starting with %s items." % max_items) threads.append(Thread(target=producer, args=(nloops,))) threads.append(Thread(target=consumer, args=(random.randrange(nloops, nloops+max_items+2),))) for thread in threads: # Starts all the threads. thread.start() for thread in threads: # Waits for threads to complete before moving on with the main script. thread.join() print("All done.")
- threading模块还提供了一个Semaphore对象，它允许你可以任意次的调用release函数，但是最好还是使用BoundedSemaphore对象，这样在release调用次数过多时会报错，有益于查找错误。Semaphores最长用来限制资源的使用，比如最多十个进程。
Events
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event可以充当多进程之间的通信工具，基于一个内部的标志，线程可以调用set()和clear()方法来操作这个标志，其他线程则阻塞在wait()函数，直到标志被设置为True。下面的代码展示了如何利用Events来追踪行为。
import random, time from threading import Event, Thread event = Event() def waiter(event, nloops): for i in range(nloops): print(“%s. Waiting for the flag to be set.” % (i+1)) event.wait() # Blocks until the flag becomes true. print(“Wait complete at:”, time.ctime()) event.clear() # Resets the flag. print() def setter(event, nloops): for i in range(nloops): time.sleep(random.randrange(2, 5)) # Sleeps for some time. event.set() threads = [] nloops = random.randrange(3, 6) threads.append(Thread(target=waiter, args=(event, nloops))) threads[-1].start() threads.append(Thread(target=setter, args=(event, nloops))) threads[-1].start() for thread in threads: thread.join() print(“All done.”)
Conditions
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conditions是比events更加高级一点的同步原语，可以用户多线程间的通信和通知。比如A线程通知B线程资源已经可以被消费。其他的线程必须在调用wait()方法前调用acquire()方法。同样的，每个线程在资源使用完以后，要调用release()方法，这样其他线程就可以继续执行了。下面是使用conditions实现的一个生产者消费者的例子。
import random, time from threading import Condition, Thread condition = Condition() box = [] def producer(box, nitems): for i in range(nitems): time.sleep(random.randrange(2, 5)) # Sleeps for some time. condition.acquire() num = random.randint(1, 10) box.append(num) # Puts an item into box for consumption. condition.notify() # Notifies the consumer about the availability. print("Produced:", num) condition.release() def consumer(box, nitems): for i in range(nitems): condition.acquire() condition.wait() # Blocks until an item is available for consumption. print("%s: Acquired: %s" % (time.ctime(), box.pop())) condition.release() threads = [] nloops = random.randrange(3, 6) for func in [producer, consumer]: threads.append(Thread(target=func, args=(box, nloops))) threads[-1].start() # Starts the thread. for thread in threads: thread.join() print("All done.")
- conditions还有其他很多用户，比如实现一个数据流API，当数据准备好了可以通知其他线程去处理数据。
Barriers
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barriers是个简单的同步原语，可以用户多个线程之间的相互等待。每个线程都调用wait()方法，然后阻塞，直到所有线程调用了wait(),然后所有线程同时开始运行。例如：
from random import randrange from threading import Barrier, Thread from time import ctime, sleep num = 4 b = Barrier(num) names = [“Harsh”, “Lokesh”, “George”, “Iqbal”] def player(): name = names.pop() sleep(randrange(2, 5)) print(“%s reached the barrier at: %s” % (name, ctime())) b.wait() threads = [] print(“Race starts now…”) for i in range(num): threads.append(Thread(target=player)) threads[-1].start() for thread in threads: thread.join() print() print(“Race over!”)
总结
多线程同步，说难也难，说不难也很容易，关键是要看你的业务场景和解决问题的思路，尽量降低多线程之间的依赖，理清楚业务流程，选择合适的方法，则事尽成。
转载自我的博客：捕蛇者说
