python多线程之t.setDaemon(True) 和 t.join()
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0.目录
1.参考
2.结论
(1)通过 t.setDaemon(True) 将子线程设置为守护进程（默认False），主线程结束后，守护子线程随之中止。
(2) t.join() 用于阻塞主线程，可以想象成将某个子线程的执行过程插入(join)到主线程的时间线上，主线程的后续代码延后执行。注意和 t.start() 分开写在两个for循环中。
(3)第一个for循环同时启动了所有子线程，随后在第二个for循环中执行t.join() ，主线程实际被阻塞的总时长==其中执行时间最长的一个子线程。
3.验证过程
1.参考
C:\Program Files\Anaconda2\Lib\threading.py
def daemon(self): """A boolean value indicating whether this thread is a daemon thread (True) or not (False). This must be set before start() is called, otherwise RuntimeError is raised. Its initial value is inherited from the creating thread; the main thread is not a daemon thread and therefore all threads created in the main thread default to daemon = False. The entire Python program exits when no alive non-daemon threads are left. """
def join(self, timeout=None): """Wait until the thread terminates. This blocks the calling thread until the thread whose join() method is called terminates -- either normally or through an unhandled exception or until the optional timeout occurs. When the timeout argument is present and not None, it should be a floating point number specifying a timeout for the operation in seconds (or fractions thereof). As join() always returns None, you must call isAlive() after join() to decide whether a timeout happened -- if the thread is still alive, the join() call timed out. When the timeout argument is not present or None, the operation will block until the thread terminates. A thread can be join()ed many times. join() raises a RuntimeError if an attempt is made to join the current thread as that would cause a deadlock. It is also an error to join() a thread before it has been started and attempts to do so raises the same exception. """
2.结论
(1)通过 t.setDaemon(True) 将子线程设置为守护进程（默认False），主线程结束后，守护子线程随之中止。
(2) t.join() 用于阻塞主线程，可以想象成将某个子线程的执行过程插入(join)到主线程的时间线上，主线程的后续代码延后执行。注意和 t.start() 分开写在两个for循环中。
(3)第一个for循环同时启动了所有子线程，随后在第二个for循环中执行t.join() ，主线程实际被阻塞的总时长==其中执行时间最长的一个子线程。
3.验证过程
(1)没有将子线程设置为守护进程，主线程结束后，各子线程继续运行。
#!usr/bin/env python #coding:utf-8 import time import random import logging import thread import threading from Queue import Queue lock = threading.Lock() #‘function-call ownership‘ rlock = threading.RLock() #thread ownership def get_logger(): logger = logging.getLogger("threading_example") logger.setLevel(logging.DEBUG) # fh = logging.FileHandler("d:/threading.log") fh = logging.StreamHandler() fmt = ‘%(asctime)s - %(threadName)-10s - %(levelname)s - %(message)s‘ formatter = logging.Formatter(fmt) fh.setFormatter(formatter) logger.addHandler(fh) return logger logger = get_logger() ################################# class MyThread(threading.Thread): def __init__(self, number): threading.Thread.__init__(self) self.number = number def run(self): for i in range(2): logger.debug(i) time.sleep(self.number) def main(): logger.debug(‘MainThread Start‘) threads = [MyThread(3-i) for i in range(3)] for t in threads: t.start() logger.debug(‘MainThread End‘) if __name__ == ‘__main__‘: main()
输出：
2017-08-15 17:04:12,512 - MainThread - DEBUG - MainThread Start 2017-08-15 17:04:12,512 - Thread-1 - DEBUG - 0 2017-08-15 17:04:12,513 - Thread-2 - DEBUG - 0 2017-08-15 17:04:12,513 - Thread-3 - DEBUG - 0 2017-08-15 17:04:12,513 - MainThread - DEBUG - MainThread End 2017-08-15 17:04:13,513 - Thread-3 - DEBUG - 1 2017-08-15 17:04:14,513 - Thread-2 - DEBUG - 1 2017-08-15 17:04:15,513 - Thread-1 - DEBUG - 1
(2) t.setDaemon(True)， 主线程结束后，守护子线程随之中止。
def main(): logger.debug(‘MainThread Start‘) threads = [MyThread(3-i) for i in range(3)] for t in threads: t.setDaemon(True) #将子线程设置为守护进程 t.start() logger.debug(‘MainThread End‘)
输出：
2017-08-15 17:06:20,822 - MainThread - DEBUG - MainThread Start 2017-08-15 17:06:20,822 - Thread-1 - DEBUG - 0 2017-08-15 17:06:20,823 - Thread-2 - DEBUG - 0 2017-08-15 17:06:20,823 - MainThread - DEBUG - MainThread End 2017-08-15 17:06:20,823 - Thread-3 - DEBUG - 0
(3)错误用法： d t1.join()阻塞了主线程的for循环，t1结束后才执行t2.start()...实际就变成了单线程顺序执行。
def main(): logger.debug(‘MainThread Start‘) threads = [MyThread(3-i) for i in range(3)] for t in threads: t.start() t.join() logger.debug(‘{} start() join() activeCount: {}‘.format(t.getName(), threading.activeCount())) logger.debug(‘MainThread End‘)
输出：
2017-08-15 17:17:38,219 - MainThread - DEBUG - MainThread Start 2017-08-15 17:17:38,230 - Thread-1 - DEBUG - 0 2017-08-15 17:17:41,230 - Thread-1 - DEBUG - 1 2017-08-15 17:17:44,232 - MainThread - DEBUG - Thread-1 start() join() activeCount: 1 2017-08-15 17:17:44,232 - Thread-2 - DEBUG - 0 2017-08-15 17:17:46,232 - Thread-2 - DEBUG - 1 2017-08-15 17:17:48,233 - MainThread - DEBUG - Thread-2 start() join() activeCount: 1 2017-08-15 17:17:48,233 - Thread-3 - DEBUG - 0 2017-08-15 17:17:49,234 - Thread-3 - DEBUG - 1 2017-08-15 17:17:50,234 - MainThread - DEBUG - Thread-3 start() join() activeCount: 1 2017-08-15 17:17:50,234 - MainThread - DEBUG - MainThread End
(4)异常用法：只对for循环的最后一个子线程执行了t.join() ，如果该子线程的执行时长不是所有子线程中最久的，可能达不到预期效果。
def main(): logger.debug(‘MainThread Start‘)
# threads = [MyThread(3-i) for i in range(3)] threads = [MyThread((3-i)*2) for i in range(3)] #修改了等待时间 for t in threads: t.start() logger.debug(‘MainThread ing‘) # for t in threads: t.join() logger.debug(‘{} is_alive: {} join() activeCount: {}‘.format(t.getName(), t.is_alive(), threading.activeCount())) logger.debug(‘MainThread End‘)
输出：
2017-08-15 18:18:10,924 - MainThread - DEBUG - MainThread Start 2017-08-15 18:18:10,927 - Thread-1 - DEBUG - 0 2017-08-15 18:18:10,930 - Thread-2 - DEBUG - 0 2017-08-15 18:18:10,931 - Thread-3 - DEBUG - 0 2017-08-15 18:18:10,931 - MainThread - DEBUG - MainThread ing 2017-08-15 18:18:12,931 - Thread-3 - DEBUG - 1 2017-08-15 18:18:14,931 - Thread-2 - DEBUG - 1 2017-08-15 18:18:14,931 - MainThread - DEBUG - Thread-3 is_alive: False join() activeCount: 3 2017-08-15 18:18:14,931 - MainThread - DEBUG - MainThread End 2017-08-15 18:18:16,928 - Thread-1 - DEBUG - 1
(5)正常用法：第二个for循环保证每一个子线程都执行了t.join()， 虽然t1运行结束后才执行t2.join(), 但是第一个for循环已经启动了所有子线程，所以主线程实际被阻塞的总时长==其中执行时间最长的一个子线程。
def main(): logger.debug(‘MainThread Start‘) # threads = [MyThread(3-i) for i in range(3)] threads = [MyThread((i+1)*2) for i in range(3)] #修改了等待时间 for t in threads: t.start() logger.debug(‘MainThread ing‘) for t in threads: logger.debug(‘{} is_alive: {} join() activeCount: {}‘.format(t.getName(), t.is_alive(), threading.activeCount())) t.join() logger.debug(‘MainThread End‘)
输出：
2017-08-15 17:30:00,499 - MainThread - DEBUG - MainThread Start 2017-08-15 17:30:00,499 - Thread-1 - DEBUG - 0 2017-08-15 17:30:00,500 - Thread-2 - DEBUG - 0 2017-08-15 17:30:00,500 - Thread-3 - DEBUG - 0 2017-08-15 17:30:00,500 - MainThread - DEBUG - MainThread ing 2017-08-15 17:30:00,500 - MainThread - DEBUG - Thread-1 is_alive: True join() activeCount: 4 2017-08-15 17:30:02,500 - Thread-1 - DEBUG - 1 2017-08-15 17:30:04,500 - Thread-2 - DEBUG - 1 2017-08-15 17:30:04,500 - MainThread - DEBUG - Thread-2 is_alive: True join() activeCount: 3 2017-08-15 17:30:06,500 - Thread-3 - DEBUG - 1 2017-08-15 17:30:08,503 - MainThread - DEBUG - Thread-3 is_alive: True join() activeCount: 2 2017-08-15 17:30:12,500 - MainThread - DEBUG - MainThread End
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