Python 多线程资源共享锁

本文将简单介绍多线程编程中的线程间资源共享和常用的锁机制。

在多线程编程中，常常会涉及到线程间的资源共享, 常用资源共享常用方式：

• 全局变量(global)
• queue(from queue import Queue)

常用的资源共享锁机制：

• Lock
• RLock
• Semphore
• Condition

(一) 线程间资源共享

1. 使用全局变量可以实现线程间的资源共享，关键字global

代码演示：

from threading import Thread, Lock
lock = Lock()
total = 0

'''如果不使用lock那么，最后得到的数字不一定为0；同时loack不支持连续多次acquire，如果这样做了的后果是死锁！'''
def add():
    global total
    global lock
    for i in range(1000000):
        lock.acquire()
        total += 1
        lock.release()
    
def sub():
    global total
    global lock
    for i in range(1000000):
        lock.acquire()
        total -= 1
        lock.release()
    
thread1 = Thread(target=add)
thread2 = Thread(target=sub)


# 将Thread1和2设置为守护线程，主线程完成时，子线程也一起结束
# thread1.setDaemon(True)
# thread1.setDaemon(True)

# 启动线程
thread1.start()
thread2.start()

# 阻塞，等待线程1和2完成，如果不使用join，那么主线程完成后，子线程也会自动关闭。
thread1.join()
thread2.join()

total

2. 使用queue共享资源，queue是线程安全的。

from threading import Thread, Lock
from queue import Queue

def add(q):
    if q.not_full:
        q.put(1)
    
def sub(q):
    if q.not_empty:
        recv = q.get()
        print(recv)
        q.task_done()
        
if __name__ =='__main__':
	# 设置q最多接收3个任务,Queue是线程安全的，所以不需要Lock
    qu = Queue(3)
    thread1 = Thread(target=add, args=(qu,))
    thread2 = Thread(target=sub, args=(qu,))
    thread1.start()
    thread2.start()
    # q队列堵塞，等待所有任务都被处理完。
    qu.join()

(二) 锁(Lock/RLock/Condition/Semphore)

1. Lock

• Lock 不能连续acquire锁，不然会死锁，Lock 资源竞争可能会导致死锁。
• Lock 会降低性能。

from threading import Thread, Lock
lock = Lock()
total = 0

'''如果不使用lock那么，最后得到的数字不一定为0；同时lock不支持连续多次acquire，如果这样做了的后果是死锁！'''
def add():
    global total
    global lock
    for i in range(1000000):
        lock.acquire()
        total += 1
        lock.release()
    
def sub():
    global total
    global lock
    for i in range(1000000):
        lock.acquire()
        total -= 1
        lock.release()
    
thread1 = Thread(target=add)
thread2 = Thread(target=sub)

# 将Thread1和2设置为守护线程，主线程完成时，子线程也一起结束
# thread1.setDaemon(True)
# thread1.setDaemon(True)

# 启动线程
thread1.start()
thread2.start()

# 阻塞，等待线程1和2完成，如果不使用join，那么主线程完成后，子线程也会自动关闭。
thread1.join()
thread2.join()

total

2. RLock

• RLock 可以连续acquire锁，但是需要相应数量的release释放锁
• 因可以连续获取锁，所以实现了函数内部调用带锁的函数

from threading import Thread, Lock, RLock

lock = RLock()
total = 0

def add():
    global lock
    global total
    # RLock实现连续获取锁，但是需要相应数量的release来释放资源
    for i in range(1000000):
        lock.acquire()
        lock.acquire()
        total += 1
        lock.release()
        lock.release()
def sub():
    global lock
    global total
    for i in range(1000000):
        lock.acquire()
        total -= 1
        lock.release()


thread1 = Thread(target=add)
thread2 = Thread(target=sub)
thread1.start()
thread2.start()
thread1.join()
thread2.join()
total

3. Condition 条件变量

• Condition条件变量服从上下文管理协议：使用with语句获取封闭块持续时间的关联锁。
• wait（）方法释放锁，然后阻塞，直到另一个线程通过调用notify（）或notify_all（）唤醒它。一旦被唤醒，wait（）重新获得锁并返回。也可以指定超时。
• 先启动wait接收信号的函数，处于阻塞等待状态，再启动notify的函数发出信号

from threading import Thread, Condition
'''聊天
    Peaple1 : How are you?
    Peaple2 : I`m fine, thank you!
    
    Peaple1 : What`s your job?
    Peaple2 : My job is teacher.
    
'''

def Peaple1(condition):
    with condition:
        print('Peaple1 : ', 'How are you?')
        condition.notify()
        condition.wait()
        
        print('Peaple1 : ', 'What`s your job?')
        condition.notify()
        condition.wait()

def Peaple2(condition):
    with condition:
        condition.wait()
        print('Peaple2 : ', 'I`m fine, thank you!')
        condition.notify()
        
        condition.wait()
        print('Peaple2 : ', 'My job is teacher.')
        condition.notify()


if __name__ == '__main__':
    cond = Condition()
    thread1 = Thread(target=Peaple1, args=(cond,))
    thread2 = Thread(target=Peaple2, args=(cond,))
    
    # 此处thread2要比thread1提前启动，因为notify必须要有wait接收；如果先启动thread1,没有wait接收notify信号，那么将会死锁。
    thread2.start()
    thread1.start()

#     thread1.join()
#     thread2.join()

4. Semphore

• 该类实现信号量对象。信号量管理一个原子计数器，表示release（）调用的数量减去acquire（）调用的数量加上一个初始值。如果需要，acquire（）方法会阻塞，直到它可以返回而不使计数器为负。如果没有给出，则值默认为1。

#Semaphore 是用于控制进入数量的锁
#文件， 读、写， 写一般只是用于一个线程写，读可以允许有多个

import threading
import time

class HtmlSpider(threading.Thread):
    def __init__(self, url, sem):
        super().__init__()
        self.url = url
        self.sem = sem

    def run(self):
        time.sleep(2)
        print("Download {html} success\n".format(html=self.url))
        self.sem.release()

class UrlProducer(threading.Thread):
    def __init__(self, sem):
        super().__init__()
        self.sem = sem

    def run(self):
        for i in range(20):
            self.sem.acquire()
            html_thread = HtmlSpider("https://www.baidu.com/{}".format(i), self.sem)
            html_thread.start()

if __name__ == "__main__":
    # 控制锁的数量, 每次同时会有3个线程获得锁，然后输出
    sem = threading.Semaphore(3)
    url_producer = UrlProducer(sem)
    url_producer.start()

notice:
Using locks, conditions, and semaphores in the with statement
All of the objects provided by this module that have acquire() and release() methods can be used as context managers for a with statement. The acquire() method will be called when the block is entered, and release() will be called when the block is exited. Hence, the following snippet:

with some_lock:
    # do something...
is equivalent to:

some_lock.acquire()
try:
    # do something...
finally:
    some_lock.release()

Currently, Lock, RLock, Condition, Semaphore, and BoundedSemaphore objects may be used as with statement context managers.

(三)简单介绍多进程编程

1. 多进程编程中进程间不能实现全局变量共享，也不能使用queue.Queue
2. 多进程编程通信需要使用Queue，Pipe
3. 如果使用进程池进程编程需要使用Manger的实例的queue来实现通信