在 Python 的多線程編程中，經常碰到 thread.join()這樣的代碼。那么今天咱們用實際代碼來解釋一下 join 函數的作用。
第一，當一個進程啟動之后，會默認產生一個主線程，因為線程是程序執行流的最小單元，當設置多線程時，主線程會創建多個子線程，在python中，默認情況下（其實就是setDaemon(False)），主線程執行完自己的任務以后，就退出了，此時子線程會繼續執行自己的任務，直到自己的任務結束，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，當我們使用setDaemon(True)方法，設置子線程為守護線程時，主線程一旦執行結束，則全部線程全部被終止執行，可能出現的情況就是，子線程的任務還沒有完全執行結束，就被迫停止，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"
代碼執行結果如下：
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，直到子線程全部結束，主線程才結束，程序退出。代碼如下：
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，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，但是並沒有殺死子線程，子線程依然可以繼續執行，直到子線程全部結束，程序退出。代碼如下：
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，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。代碼如下：
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