理解java中的锁

Question

我正在了解java中的锁，但我很难理解这个概念。这是一个带锁的 Java 代码示例

public class LockQ {

    class Shared {
       static int count = 0;
    }

    class LockExample implements Runnable {
       Lock lock;

       public LockExample(Lock lock) {
          this.lock = lock;
       }

       @Override
       public void run() {
           lock.lock();

           for (int i = 0; i < 5; i++) {
                System.out.println("Working on count = " + Thread.currentThread().getName());
                Shared.count++;
            }

            lock.unlock();
        }
    }

    public static void main(String[] args) {
        LockQ obj = new LockQ();
        obj.demo();
    }

    public void demo() {
        Lock lock = new ReentrantLock();

        LockExample l1 = new LockExample(lock);
        LockExample l2 = new LockExample(lock);
        LockExample l3 = new LockExample(lock);

        Thread t1 = new Thread(l1);
        Thread t2 = new Thread(l2);
        Thread t3 = new Thread(l3);

        t1.start();
        t2.start();
        t3.start();

        /*
        try {
            t2.join();
            t3.join();
            t1.join();
        } catch (InterruptedException e) {
             throw new RuntimeException(e);
        }
        */

        System.out.println("Final count value " + Shared.count);
    }
}

这是一个示例输出

Working on count = Thread-0 
Working on count = Thread-0 
Working on count = Thread-0 
Working on count = Thread-0 
Working on count = Thread-0 
Working on count = Thread-1 
Working on count = Thread-1
Working on count = Thread-1 
Working on count = Thread-1 
Working on count = Thread-1 
Working on count = Thread-2 
Working on count = Thread-2 
Working on count = Thread-2 
Working on count = Thread-2
Working on count = Thread-2 
Final count value 0

我的困惑是

为什么最后的计数值是0？它在循环内的所有打印语句之后被打印，这意味着所有三个线程都已完成执行。如果连接语句未注释，代码将按预期工作，我明白了

最终计数值15

最后

Answer 1

如果您不

.join()

线程，则不能保证它们在后续工作发生之前完成（

.join()

等待线程退出后再继续）！

所以发生的情况是你的主程序运行，然后在等待线程退出时挂起，但由于它发生得非常快，你可能看不到效果