多线程 -- 同步锁

• 多个线程同时操作一个变量

import time
import threading

def addNum():
    #在每个线程中都获取这个全局变量
    global num

    temp = num
    time.sleep(0.1)
    #对此公共变量进行-1操作
    num = temp-1

#设定一个共享变量
num = 100
thread_list = []
for i in range(100):
    t = threading.Thread(target=addNum)
    t.start()
    thread_list.append(t)

#等待所有线程执行完毕
for t in thread_list:
    t.join()

print('final num:', num )

输出：
99

 

• 同步锁，acquire和release之间的代码，在同一时间只会被一个线程执行

import time
import threading

def addNum():
    global num

    lock.acquire()
    temp = num
    time.sleep(0.1)
    num = temp-1
    lock.release()

num = 100
thread_list = []
lock = threading.Lock()

for i in range(100):
    t = threading.Thread(target=addNum)
    t.start()
    thread_list.append(t)

for t in thread_list:
    t.join()

print('final num:', num )

输出：
0

• 死锁

import threading,time

class MyThread(threading.Thread):
    def doA(self):
        lockA.acquire()
        print(self.name, "gotlockA", time.ctime())
        time.sleep(3)
        lockB.acquire()
        print(self.name, "gotlockB", time.ctime())
        lockB.release()
        lockA.release()

    def doB(self):
        lockB.acquire()
        print(self.name, "gotlockB", time.ctime())
        time.sleep(2)
        lockA.acquire()
        print(self.name, "gotlockA", time.ctime())
        lockA.release()
        lockB.release()

    def run(self):
        self.doA()
        self.doB()
if __name__ == "__main__":

    lockA = threading.Lock()
    lockB = threading.Lock()
    threads = []
    for i in range(5):
        threads.append(MyThread())
    for t in threads:
        t.start()
    for t in threads:
        t.join()

• 解决死锁 -- RLock

import threading

class Account:
    def __init__(self, _id, balance):
        self.id = _id
        self.balance = balance
        self.lock = threading.RLock()

    def withdraw(self, amount):
        with self.lock:
            self.balance -= amount

    def deposit(self, amount):
        with self.lock:
            self.balance += amount

    #lock.acquire中嵌套lock.acquire的场景
    def drawcash(self, amount):

        with self.lock:
            interest = 0.05
            count = amount+amount*interest
            self.withdraw(count)

def transfer(_from, to, amount):

     _from.withdraw(amount)

     to.deposit(amount)

alex = Account('alex', 1000)
yuan = Account('yuan', 1000)

t1 = threading.Thread(target=transfer, args=(alex, yuan, 100))
t1.start()

t2 = threading.Thread(target=transfer, args=(yuan, alex, 200))
t2.start()

t1.join()
t2.join()

print('>>>', alex.balance)
print('>>>', yuan.balance)