python多線程中join()的理解

在 Python 的多線程編程中，常常碰到 thread.join()這樣的代碼。那麼今天我們用實際代碼來解釋一下 join 函數的做用。

第一，當一個進程啓動以後，會默認產生一個主線程，由於線程是程序執行流的最小單元，當設置多線程時，主線程會建立多個子線程，在python中，默認狀況下（其實就是setDaemon(False)），主線程執行完本身的任務之後，就退出了，此時子線程會繼續執行本身的任務，直到本身的任務結束，例子見下面。

# coding: utf-8
# 測試多線程中join的功能
import threading, time
def doWaiting():
    print 'start waiting1: ' + time.strftime('%H:%M:%S') + "\n"
    time.sleep(3)
    print 'stop waiting1: ' + time.strftime('%H:%M:%S') + "\n"
def doWaiting1():
    print 'start waiting2: ' + time.strftime('%H:%M:%S') + "\n"
    time.sleep(8)
    print 'stop waiting2: ', time.strftime('%H:%M:%S') + "\n"
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') + "\n"

print 'end join: ' + time.strftime('%H:%M:%S') + "\n"

代碼執行結果以下：

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，當咱們使用setDaemon(True)方法，設置子線程爲守護線程時，主線程一旦執行結束，則所有線程所有被終止執行，可能出現的狀況就是，子線程的任務尚未徹底執行結束，就被迫中止，例子見下面。

 1 # coding: utf-8
 2 # 測試多線程中join的功能
 3 import threading, time
 4 def doWaiting():
 5     print 'start waiting1: ' + time.strftime('%H:%M:%S') + "\n"
 6     time.sleep(3)
 7     print 'stop waiting1: ' + time.strftime('%H:%M:%S') + "\n"
 8 def doWaiting1():
 9     print 'start waiting2: ' + time.strftime('%H:%M:%S') + "\n"
10     time.sleep(8)
11     print 'stop waiting2: ', time.strftime('%H:%M:%S') + "\n"
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') + "\n"
22 
23 print 'end join: ' + time.strftime('%H:%M:%S') + "\n"

代碼執行結果以下：

start waiting1: 22:34:46

start waiting2: 22:34:46
start join: 22:34:46


end join: 22:34:46

第三，沒有設置守護線程且沒有設置join函數的timeout參數時，主線程將會一直等待，直到子線程所有結束，主線程才結束，程序退出。代碼以下：

 1 # coding: utf-8
 2 # 測試多線程中join的功能
 3 import threading, time
 4 def doWaiting():
 5     print 'start waiting1: ' + time.strftime('%H:%M:%S') + "\n"
 6     time.sleep(3)
 7     print 'stop waiting1: ' + time.strftime('%H:%M:%S') + "\n"
 8 def doWaiting1():
 9     print 'start waiting2: ' + time.strftime('%H:%M:%S') + "\n"
10     time.sleep(8)
11     print 'stop waiting2: ', time.strftime('%H:%M:%S') + "\n"
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') + "\n"
20 for tt in tsk:
21     tt.join()
22 print 'end join: ' + time.strftime('%H:%M:%S') + "\n"

代碼執行結果以下：

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，當沒有設置守護線程且join函數的參數timeout=2時，主線程將會等待多個子線程timeout的累加和這樣的一段時間，時間一到，主線程結束，可是並無殺死子線程，子線程依然能夠繼續執行，直到子線程所有結束，程序退出。代碼以下：

 1 # coding: utf-8
 2 # 測試多線程中join的功能
 3 import threading, time
 4 def doWaiting():
 5     print 'start waiting1: ' + time.strftime('%H:%M:%S') + "\n"
 6     time.sleep(3)
 7     print 'stop waiting1: ' + time.strftime('%H:%M:%S') + "\n"
 8 def doWaiting1():
 9     print 'start waiting2: ' + time.strftime('%H:%M:%S') + "\n"
10     time.sleep(8)
11     print 'stop waiting2: ', time.strftime('%H:%M:%S') + "\n"
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') + "\n"
20 for tt in tsk:
21     tt.join(2)
22 print 'end join: ' + time.strftime('%H:%M:%S') + "\n"

代碼執行結果以下：

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，當設置守護線程join函數的參數timeout=2時，主線程將會等待多個子線程timeout的累加和這樣的一段時間，時間一到，主線程結束，殺死未執行完的子線程，程序退出。代碼以下：

 1 # coding: utf-8
 2 # 測試多線程中join的功能
 3 import threading, time
 4 def doWaiting():
 5     print 'start waiting1: ' + time.strftime('%H:%M:%S') + "\n"
 6     time.sleep(3)
 7     print 'stop waiting1: ' + time.strftime('%H:%M:%S') + "\n"
 8 def doWaiting1():
 9     print 'start waiting2: ' + time.strftime('%H:%M:%S') + "\n"
10     time.sleep(8)
11     print 'stop waiting2: ', time.strftime('%H:%M:%S') + "\n"
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') + "\n"
22 for tt in tsk:
23     tt.join(2)
24 print 'end join: ' + time.strftime('%H:%M:%S') + "\n"

代碼執行結果以下：

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