在 Python 的多线程编程中，经常碰到 thread.join()这样的代码。那么今天咱们用实际代码来解释一下 join 函数的作用。
第一，当一个进程启动之后，会默认产生一个主线程，因为线程是程序执行流的最小单元，当设置多线程时，主线程会创建多个子线程，在python中，默认情况下（其实就是setDaemon(False)），主线程执行完自己的任务以后，就退出了，此时子线程会继续执行自己的任务，直到自己的任务结束，例子见下面。
# coding: utf-8
# 测试多线程中join的功能
import threading, time
def doWaiting():
print 'start waiting1: ' + time.strftime('%H:%M:%S') + "
"
time.sleep(3)
print 'stop waiting1: ' + time.strftime('%H:%M:%S') + "
"
def doWaiting1():
print 'start waiting2: ' + time.strftime('%H:%M:%S') + "
"
time.sleep(8)
print 'stop waiting2: ', time.strftime('%H:%M:%S') + "
"
tsk = []
thread1 = threading.Thread(target = doWaiting)
thread1.start()
tsk.append(thread1)
thread2 = threading.Thread(target = doWaiting1)
thread2.start()
tsk.append(thread2)
print 'start join: ' + time.strftime('%H:%M:%S') + "
"
print 'end join: ' + time.strftime('%H:%M:%S') + "
"
代码执行结果如下：
start waiting1: 22:24:20 start join: 22:24:20 start waiting2: 22:24:20 end join: 22:24:20 stop waiting1: 22:24:23 stop waiting2: 22:24:28
第二，当我们使用setDaemon(True)方法，设置子线程为守护线程时，主线程一旦执行结束，则全部线程全部被终止执行，可能出现的情况就是，子线程的任务还没有完全执行结束，就被迫停止，例子见下面。
1 # coding: utf-8
2 # 测试多线程中join的功能
3 import threading, time
4 def doWaiting():
5 print 'start waiting1: ' + time.strftime('%H:%M:%S') + "
"
6 time.sleep(3)
7 print 'stop waiting1: ' + time.strftime('%H:%M:%S') + "
"
8 def doWaiting1():
9 print 'start waiting2: ' + time.strftime('%H:%M:%S') + "
"
10 time.sleep(8)
11 print 'stop waiting2: ', time.strftime('%H:%M:%S') + "
"
12 tsk = []
13 thread1 = threading.Thread(target = doWaiting)
14 thread1.setDaemon(True)
15 thread1.start()
16 tsk.append(thread1)
17 thread2 = threading.Thread(target = doWaiting1)
18 thread2.setDaemon(True)
19 thread2.start()
20 tsk.append(thread2)
21 print 'start join: ' + time.strftime('%H:%M:%S') + "
"
22
23 print 'end join: ' + time.strftime('%H:%M:%S') + "
"
代码执行结果如下：
start waiting1: 22:34:46 start waiting2: 22:34:46 start join: 22:34:46 end join: 22:34:46
第三，没有设置守护线程且没有设置join函数的timeout参数时，主线程将会一直等待，直到子线程全部结束，主线程才结束，程序退出。代码如下：
1 # coding: utf-8
2 # 测试多线程中join的功能
3 import threading, time
4 def doWaiting():
5 print 'start waiting1: ' + time.strftime('%H:%M:%S') + "
"
6 time.sleep(3)
7 print 'stop waiting1: ' + time.strftime('%H:%M:%S') + "
"
8 def doWaiting1():
9 print 'start waiting2: ' + time.strftime('%H:%M:%S') + "
"
10 time.sleep(8)
11 print 'stop waiting2: ', time.strftime('%H:%M:%S') + "
"
12 tsk = []
13 thread1 = threading.Thread(target = doWaiting)
14 thread1.start()
15 tsk.append(thread1)
16 thread2 = threading.Thread(target = doWaiting1)
17 thread2.start()
18 tsk.append(thread2)
19 print 'start join: ' + time.strftime('%H:%M:%S') + "
"
20 for tt in tsk:
21 tt.join()
22 print 'end join: ' + time.strftime('%H:%M:%S') + "
"
代码执行结果如下：
start waiting1: 22:41:50 start join: 22:41:50 start waiting2: 22:41:50 stop waiting1: 22:41:53 stop waiting2: 22:41:58 end join: 22:41:58
第四，当没有设置守护线程且join函数的参数timeout=2时，主线程将会等待多个子线程timeout的累加和这样的一段时间，时间一到，主线程结束，但是并没有杀死子线程，子线程依然可以继续执行，直到子线程全部结束，程序退出。代码如下：
1 # coding: utf-8
2 # 测试多线程中join的功能
3 import threading, time
4 def doWaiting():
5 print 'start waiting1: ' + time.strftime('%H:%M:%S') + "
"
6 time.sleep(3)
7 print 'stop waiting1: ' + time.strftime('%H:%M:%S') + "
"
8 def doWaiting1():
9 print 'start waiting2: ' + time.strftime('%H:%M:%S') + "
"
10 time.sleep(8)
11 print 'stop waiting2: ', time.strftime('%H:%M:%S') + "
"
12 tsk = []
13 thread1 = threading.Thread(target = doWaiting)
14 thread1.start()
15 tsk.append(thread1)
16 thread2 = threading.Thread(target = doWaiting1)
17 thread2.start()
18 tsk.append(thread2)
19 print 'start join: ' + time.strftime('%H:%M:%S') + "
"
20 for tt in tsk:
21 tt.join(2)
22 print 'end join: ' + time.strftime('%H:%M:%S') + "
"
代码执行结果如下：
start waiting1: 23:02:34 start waiting2: 23:02:34 start join: 23:02:34 stop waiting1: 23:02:37 end join: 23:02:38 stop waiting2: 23:02:42
第五，当设置守护线程join函数的参数timeout=2时，主线程将会等待多个子线程timeout的累加和这样的一段时间，时间一到，主线程结束，杀死未执行完的子线程，程序退出。代码如下：
1 # coding: utf-8
2 # 测试多线程中join的功能
3 import threading, time
4 def doWaiting():
5 print 'start waiting1: ' + time.strftime('%H:%M:%S') + "
"
6 time.sleep(3)
7 print 'stop waiting1: ' + time.strftime('%H:%M:%S') + "
"
8 def doWaiting1():
9 print 'start waiting2: ' + time.strftime('%H:%M:%S') + "
"
10 time.sleep(8)
11 print 'stop waiting2: ', time.strftime('%H:%M:%S') + "
"
12 tsk = []
13 thread1 = threading.Thread(target = doWaiting)
14 thread1.setDaemon(True)
15 thread1.start()
16 tsk.append(thread1)
17 thread2 = threading.Thread(target = doWaiting1)
18 thread2.setDaemon(True)
19 thread2.start()
20 tsk.append(thread2)
21 print 'start join: ' + time.strftime('%H:%M:%S') + "
"
22 for tt in tsk:
23 tt.join(2)
24 print 'end join: ' + time.strftime('%H:%M:%S') + "
"
代码执行结果如下：
start waiting1: 23:23:57 start waiting2: 23:23:57 start join: 23:23:57 stop waiting1: 23:24:00 end join: 23:24:01