class MyThread(threading.Thread):
def run(self):
global num
time.sleep(1)
if mutex.acquire():
num=num+1
print self.name+' set num to '+str(num)+'\n'
mutex.release()
num=0
mutex=threading.Lock()
def test():
for i in range(5):
t=MyThread()
t.start()
if __name__=='__main__':
test()
"""
"""更简单的死锁情况是一个线程“迭代”请求同一个资源,直接就会造成死锁:
import threading
import time
class MyThread(threading.Thread):
def run(self):
global num
time.sleep(1)
if mutex.acquire(1):
num = num+1
msg = self.name+' set num to '+str(num)
print msg
mutex.acquire()
mutex.release()
mutex.release()
num = 0
mutex = threading.Lock()
def test():
for i in range(5):
t = MyThread()
t.start()
if __name__ == '__main__':
test()
为了支持在同一线程中多次请求同一资源,python提供了“可重入锁”:threading.RLock。RLock内部维护着一个Lock和一个counter变量,counter记录了acquire的次数,从而使得资源可以被多次require。直到一个线程所有的acquire都被release,其他的线程才能获得资源。上面的例子如果使用RLock代替Lock,则不会发生死锁:
import threading
import time
class MyThread(threading.Thread):
def run(self):
global num
time.sleep(1)
if mutex.acquire(1):
num = num+1
msg = self.name+' set num to '+str(num)
print msg
mutex.acquire()
mutex.release()
mutex.release()
num = 0
mutex = threading.RLock()
def test():
for i in range(5):
t = MyThread()
t.start()
if __name__ == '__main__':
test()
"""
演示条件变量同步的经典问题是生产者与消费者问题:假设有一群生产者(Producer)和一群消费者(Consumer)通过一个市场来交互产品。生产者的”策略“是如果市场上剩余的产品少于1000个,那么就生产100个产品放到市场上;而消费者的”策略“是如果市场上剩余产品的数量多余100个,那么就消费3个产品。用Condition解决生产者与消费者问题的代码如下:
import threading
import time
class Producer(threading.Thread):
def run(self):
global count
while True:
if con.acquire():
if count>1000:
con.wait()
else:
count=count+100
print self.name+' produce 100,count='+str(count)
con.release()
time.sleep(1)
class Customer(threading.Thread):
def run(self):
global count
while True:
if con.acquire():
if count>100:
count=count-100
print self.name+ 'consume 100, count='+str(count)
else:
con.wait()
con.release()
time.sleep(1)
count=500
con=threading.Condition()
def test():
for i in range(5):
p=Producer()
p.start()
c=Customer()
c.start()
print i
if __name__=='__main__':
test()
python中默认全局变量在函数中可以读,但是不能写但是
对con只读,所以不用global引入"""