Four people are selling tickets at the same time. The number of all tickets is 40. Each ticket only can be sold one time. import java.util.concurrent.locks.ReentrantLock; public class ThreadTest implements Runnable { int tickets = 40; ReentrantLock lock = new ReentrantLock(); public void run() { while (true) { lock.lock(); try { if (tickets > 0) { System.out.println(Thread.currentThread().getName() + "is selling NO."+ tickets + " ticket."); --tickets; } lock.unlock(); } catch (Exception e) { lock.unlock(); } } } }

public class Demo1 { public static void main(String[] args) { ThreadTest tt = new ThreadTest(); Thread t1 = new Thread(tt); Thread t2 = new Thread(tt); Thread t3 = new Thread(tt); Thread t4 = new Thread(tt); t1.start(); t2.start(); t3.start(); t4.start(); } }

result: Thread-0is Thread-0is Thread-0is Thread-0is Thread-0is Thread-0is Thread-0is Thread-0is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is Thread-3is

selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling selling

NO.40 ticket. NO.39 ticket. NO.38 ticket. NO.37 ticket. NO.36 ticket. NO.35 ticket. NO.34 ticket. NO.33 ticket. NO.32 ticket. NO.31 ticket. NO.30 ticket. NO.29 ticket. NO.28 ticket. NO.27 ticket. NO.26 ticket. NO.25 ticket. NO.24 ticket. NO.23 ticket. NO.22 ticket. NO.21 ticket. NO.20 ticket. NO.19 ticket. NO.18 ticket. NO.17 ticket. NO.16 ticket. NO.15 ticket. NO.14 ticket. NO.13 ticket. NO.12 ticket. NO.11 ticket. NO.10 ticket. NO.9 ticket. NO.8 ticket. NO.7 ticket. NO.6 ticket. NO.5 ticket. NO.4 ticket. NO.3 ticket. NO.2 ticket. NO.1 ticket

In main process, I create object tt and use four threads to run it. When threads run

to the while loop they need a lock to continue, but only one of them can get the lock at the same time. All the threads can print and minus 1 one by one. Suppose thread 0 get the lock. It can execute the below code but other threads have to keep trying to get lock until thread 0 release the lock. The thread 0 would print something and minus one from the number of tickets, and then release the lock and try to get lock again. Usually a thread tends to get the lock continuously like following. Thread 0 ✕ ✕ ✕ ✕ ! ! 。。 。 If without the lock and unlock, for example CPU switch to thread 0 when ticket left one, which is more than zero. So it prints selling No.1 ticket. However, before thread 0 execute ticket -­‐-­‐ the CPU switch to thread 1 and ticket = 1 is bigger than 0, thread 1 prints selling No.1 ticket again. Therefore, the same ticket may be printed many times. At that time, because both thread 0 and thread 1 have past the judgment (ticket >0), they both would minus one from the number of tickets. The number of ticket would become -­‐1. That is why there would be some negative numbers.

We should use the lock carefully to avoid dead lock, so we always use try-­‐catch or try-­‐finally to throw exception, which protect the process go ahead. Thread 1 ! ! ! ! ✕ ✕ 。。 。 Thread 2 ! ! ! ! ! ! 。。 。 Thread 3 ! ! ! ! ! ! 。。 。