线程之join()

蔚落 2022-04-11 13:00 212阅读 0赞

首先看一段简单的代码：

![复制代码][copycode.gif]

1 from time import ctime,sleep
     2 def play_video(video):
     3     for i in range(2):
     4         print "i am playing video: %s at %s"%(video,ctime())
     5         sleep(4)
     6 
     7 
     8 def play_music(music):
     9     for i in range(2):
    10         print "i am playing music: %s at %s"%(music,ctime())
    11         sleep(2)
    12 
    13 
    14 if __name__=="__main__":
    15     
    16     play_video("speed_and_crazy")
    17     play_music("chengdu")
    18 
    19     print "all are over at %s"%ctime()
    20                 
    21

![复制代码][copycode.gif]

执行结果：

![复制代码][copycode.gif]

C:\Python27>python mui_thread.py
    i am playing video: speed_and_crazy at Mon Jun 26 23:01:59 2017
    i am playing video: speed_and_crazy at Mon Jun 26 23:02:03 2017
    i am playing music: chengdu at Mon Jun 26 23:02:07 2017
    i am playing music: chengdu at Mon Jun 26 23:02:09 2017
    all are over at Mon Jun 26 23:02:11 2017

![复制代码][copycode.gif]

随着人们对多任务的要求，同时为了充分利用cpu资源，多线程编程不可避免，那么我们如何利用python去实现play\_video和play\_music

两个任务同时运行呢？

按 Ctrl+C 复制代码

测试结果：

C:\Python27>python mui_thread.py
    i am playing video: speed_and_crazy at Mon Jun 26 23:18:52 2017
    all are over at Mon Jun 26 23:18:52 2017
    i am playing music: chengdu at Mon Jun 26 23:18:52 2017  #从打印的时间可知，play_video、play_music和父进程几乎同时运行

从结果看，与我们最初的目标相差甚远，怎么没有按照顺序执行，为什么每个函数都只有一条日记输出？

那是因为子线程（play\_video、play\_music）和主线程print "all are over at %s \\n"%ctime()都是同一时间启动，但由于主线程已经运行结束，所以导致子线程也同时终止，在这种条件下，我们如何保证子进程都能够执行完毕呢？

增加thread.join()并 放在循环外

![复制代码][copycode.gif]

from time import ctime,sleep
    import threading
    def play_video(video):
        for i in range(2):
            print "i am playing video: %s at %s \n"%(video,ctime())
            sleep(1)
    
    
    def play_music(music):
        for i in range(2):
            print "i am playing music: %s at %s \n"%(music,ctime())
            sleep(5)
    
    threads=[]
    
    thread1=threading.Thread(target=play_video,args=("speed_and_crazy",))
    
    threads.append(thread1)
    
    thread2=threading.Thread(target=play_music,args=("chengdu",))
    
    threads.append(thread2)
    
    
    
    if __name__=="__main__":
    
        for thread in threads:
            thread.setDaemon(True)
            thread.start()
        thread.join()  #加在循环外，
        print "all are over at %s \n"%ctime()

![复制代码][copycode.gif]

运行结果：

![复制代码][copycode.gif]

1 C:\Python27>python mui_thread.py
     2 i am playing video: speed_and_crazy at Mon Jun 26 23:32:21 2017
     3 i am playing music: chengdu at Mon Jun 26 23:32:21 2017
     4 
     5 
     6 i am playing video: speed_and_crazy at Mon Jun 26 23:32:22 2017
     7 
     8 i am playing music: chengdu at Mon Jun 26 23:32:26 2017
     9 
    10 all are over at Mon Jun 26 23:32:31 2017

![复制代码][copycode.gif]

thread.join()的作用是主线程必须等待子线程都执行完了才能结束，play_video、play_music几乎同时执行
    但是如果改变play_video、play_music里面的sleep的时长，即是下面的代码：

![复制代码][copycode.gif]

from time import ctime,sleep
    import threading
    def play_video(video):
        for i in range(2):
            print "i am playing video: %s at %s \n"%(video,ctime())
            sleep(5)
    
    
    def play_music(music):
        for i in range(2):
            print "i am playing music: %s at %s \n"%(music,ctime())
            sleep(1)
    
    threads=[]
    
    thread1=threading.Thread(target=play_video,args=("speed_and_crazy",))
    
    threads.append(thread1)
    
    thread2=threading.Thread(target=play_music,args=("chengdu",))
    
    threads.append(thread2)
    
    
    
    if __name__=="__main__":
    
        for thread in threads:
            thread.setDaemon(True)
            thread.start()
        thread.join()
        print "all are over at %s \n"%ctime()

![复制代码][copycode.gif]

此时运行结果：

![复制代码][copycode.gif]

C:\Python27>python mui_thread.py
    i am playing video: speed_and_crazy at Mon Jun 26 23:44:13 2017
    i am playing music: chengdu at Mon Jun 26 23:44:13 2017
    
    
    i am playing music: chengdu at Mon Jun 26 23:44:14 2017
    
    all are over at Mon Jun 26 23:44:15 2017

![复制代码][copycode.gif]

我们看到play\_video还有一条log没有打印出来，原因是thread.join()在循环外，此时的thread为play\_music，父进程只会等待play\_music进程执行完就结束，而不会等待play\_video（sleep时间较长）执行完才结束，所以才会有上面的结果

[copycode.gif]: /images/20220411/5cd1aba2e01e4798920c6d9782ff933b.png