The essence thread.join

Java code   

1. // waiter  
2. synchronized (object) {  
3.     while (condition is not satisfied) {  
4.         Object .wait ()  
5.     }  
6.     dosomething();  
7. }  
8. //informer  
9. synchronized (object) {  
10.     Changing conditions  
11.     Object .notifyAll ();  
12. }  

    When it comes to join now, we all know that the trial scene join method, that is, when we call in the thread B.join A thread (), only the thread A will enter BLOCK (actually WAITING OR TIMED_WAITING) state , when the execution is complete thread B after, a thread will continue to 
look at the join to realize source, you will find inside also said wait - notification model 

Java code   

1. public final synchronized void join(long millis)  
2.     throws InterruptedException {  
3.         long base = System.currentTimeMillis();  
4.         long now = 0;  
5.   
6.         if (millis < 0) {  
7.             throw new IllegalArgumentException("timeout value is negative");  
8.         }  
9.   
10.         if (millis == 0) {  
11.             while (isAlive()) {  
12.                 wait(0);  
13.             }  
14.         } else {  
15.             while (isAlive()) {  
16.                 long delay = millis - now;  
17.                 if (delay <= 0) {  
18.                     break;  
19.                 }  
20.                 wait(delay);  
21.                 now = System.currentTimeMillis() - base;  
22.             }  
23.         }  
24.     }  

    A thread that is first through synchronized, to acquire a lock thread B, then thread B while to determine whether survival, the survival of the wait blocks until the end of the exit B thread execution, calls notifyAll () method when the thread exits. 
The reason to use this method while, in order to wake up after being re-confirm whether the conditions are satisfied. 
    So A thread will wait until the end of the thread execution will continue B