join和 Daemon守护线程
一、前言
一个程序至少有一个主线程，主线程启动子线程后，它们之间并没有隶属关系。主线程和子线程执行是并行的，相互独立。主线程执行完毕后默认不等子线程执行结束就接着往下走了，如果有其他程序就会运行另外的程序，如果没有就等待子线程执行完成后结束程序。
import threading
import time
import random
class MyThread(threading.Thread):
def __init__(self, n):
super(MyThread, self).__init__()
self.n = n
def run(self):
print('task %s is operating' % self.n)
t_time = random.randint(1, 8)
time.sleep(t_time)
# getNname获取线程名称
print(self.getName(), 'finished', 'I sleep %d seconds' % t_time)
class Person(object):
def __init__(self, name):
self.name = name
def get_info(self):
# time.sleep(10)
print('my name is %s.' % self.name)
if __name__ == '__main__':
start_time = time.time()
for i in range(5):
t = MyThread(i)
t.start()
# t.join()
print('main thread finished.')
print('*****执行另一个程序了******')
p = Person('bigberg')
p.get_info()
print('*************************')
print('cost: %s' % (time.time() - start_time))
结果：
# 线程开始执行 task 0 is operating task 1 is operating task 2 is operating task 3 is operating task 4 is operating # 主线程执行完毕 main thread finished. # 可以执行另一个程序了 *****执行另一个程序了****** my name is bigberg. ************************* # 这里花费的时间是主线程的运行时间，显然没有计算子线程的时间 cost: 0.0019881725311279297 # 子线程在主线程结束后依然在运行 Thread-3 finished I sleep 2 seconds Thread-5 finished I sleep 2 seconds Thread-2 finished I sleep 3 seconds Thread-4 finished I sleep 4 seconds Thread-1 finished I sleep 5 seconds # 所有子线程完毕后程序结束 Process finished with exit code 0
二、join 等待子线程完成
如果在线程实例后加上join默认主线程是阻塞的，主线程会等待该子线程运行完成后在结束。
# -*- coding: UTF-8 -*-
import threading
import time
import random
class MyThread(threading.Thread):
def __init__(self, n):
super(MyThread, self).__init__()
self.n = n
def run(self):
print('task %s is operating' % self.n)
t_time = random.randint(1, 8)
time.sleep(t_time)
print(self.getName(), 'finished', 'I sleep %d seconds' % t_time)
if __name__ == '__main__':
start_time = time.time()
for i in range(5):
t = MyThread(i)
t.start()
t.join() # 添加join，阻塞主线程
print('main thread finished.')
print('cost: %s' % (time.time() - start_time))
# 注
# 如果对每个线程都加join,那么并发就没有了，实际上线程都是串行的
# 前一个线程执行完了，才会执行下一个线程
# 主线程最后运行完毕
结果：
task 0 is operating Thread-1 finished I sleep 2 seconds task 1 is operating Thread-2 finished I sleep 6 seconds task 2 is operating Thread-3 finished I sleep 4 seconds task 3 is operating Thread-4 finished I sleep 8 seconds task 4 is operating Thread-5 finished I sleep 5 seconds # 这里主线程已经是最后执行完毕的了 main thread finished. # 消耗的时间也是每个线程的运行时间之和 cost: 25.005265712738037
2.1 计算并发运行时间
如果不想计算出总的运行时间，而是所有线程的并发运行时间呢？就像上例中的那样，最长运行时间是8秒，那么所有线程都能在8秒内全部运行完毕。
把t.join()单独移到for循环外面是不行的，因为这样并发运行总会在最后一个线程出阻塞。如下：
# -*- coding: UTF-8 -*-
import threading
import time
import random
class MyThread(threading.Thread):
def __init__(self, n):
super(MyThread, self).__init__()
self.n = n
def run(self):
print('task %s is operating' % self.n)
t_time = random.randint(1, 8)
time.sleep(t_time)
print(self.getName(), 'finished', 'I sleep %d seconds' % t_time)
if __name__ == '__main__':
start_time = time.time()
for i in range(5):
t = MyThread(i)
t.start()
t.join() # 添加join，阻塞主线程
print('main thread finished.')
print('cost: %s' % (time.time() - start_time))
结果：
task 0 is operating task 1 is operating task 2 is operating task 3 is operating task 4 is operating Thread-1 finished I sleep 2 seconds Thread-3 finished I sleep 2 seconds Thread-5 finished I sleep 3 seconds # 其实是在线程5，也就是最后一个线程出阻塞的 main thread finished. cost: 3.001293659210205 Thread-2 finished I sleep 4 seconds Thread-4 finished I sleep 5 seconds
正确的方法，定义一个空列表，获取所以的线程实例，for 循环阻塞所有的线程实例
# -*- coding: UTF-8 -*-
import threading
import time
import random
class MyThread(threading.Thread):
def __init__(self, n):
super(MyThread, self).__init__()
self.n = n
def run(self):
print('task %s is operating' % self.n)
t_time = random.randint(1, 8)
time.sleep(t_time)
print(self.getName(), 'finished', 'I sleep %d seconds' % t_time)
if __name__ == '__main__':
t_list = []
start_time = time.time()
for i in range(5):
t = MyThread(i)
t.start()
t_list.append(t)
for t in t_list:
t.join()
print('main thread finished.')
print('cost: %s' % (time.time() - start_time))
结果，事实上也符合我们刚才的推论，运行时间最长的线程所消耗的时间，就是总的并发时间
task 0 is operating task 1 is operating task 2 is operating task 3 is operating task 4 is operating Thread-3 finished I sleep 3 seconds Thread-5 finished I sleep 3 seconds Thread-2 finished I sleep 7 seconds Thread-1 finished I sleep 7 seconds Thread-4 finished I sleep 8 seconds main thread finished. cost: 8.001787185668945 # 并发时间在8秒左右
总结：主线程创建一个子线程后，如果子线程调用join()方法，主线程会在调用的地方等待，直到该子线程运行完成才会接着往下执行。
三、守护线程setDaemon
setDaemon()方法：在主线程中创建子线程，该子线程调用setDaemon方法后成为主线程的守护线程。这种情况下如果主线程执行结束，那么不管子线程是否完成，一并和主线程退出。这里基本和join()方法相反。此外，还有个要特别注意的：必须在start() 方法调用之前设置，如果不设置为守护线程，程序会被无限挂起。
# -*- coding: UTF-8 -*-
import threading
import time
import random
class MyThread(threading.Thread):
def __init__(self, n):
super(MyThread, self).__init__()
self.n = n
def run(self):
print('task %s is operating' % self.n)
t_time = random.randint(1, 8)
time.sleep(t_time)
print(self.getName(), 'finished', 'I sleep %d seconds' % t_time)
if __name__ == '__main__':
start_time = time.time()
for i in range(5):
t = MyThread(i)
t.setDaemon(True)
t.start()
print('main thread finished.', threading.current_thread(), threading.active_count())
print('cost: %s' % (time.time() - start_time))
注：threading.current_thread()查看当前运行的线程
threading.active_count() 查看活跃线程数
线程数 = 主线程 + 子线程数
结果：
task 0 is operating task 1 is operating task 2 is operating task 3 is operating task 4 is operating main thread finished. <_MainThread(MainThread, started 8656)> 6 cost: 0.0009999275207519531 Process finished with exit code 0 # 很显然把子线程设置为主线程的守护线程后，主线程一旦结束，程序就执行退出运行，不会再等待子线程运行。
注：如果程序中有其他非守护线程时，还是会等待非守护线程运行完毕，程序才会结束。
