关于多线程的问题，为什么count <= 0 的时候也能打印出来呢？

Question

package thread;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

class Print implements Runnable {
private final char printChar;

private static char currentChar = 'A';

private static int count = 10;

private static Lock lock = new ReentrantLock();

private static final Condition condition_A = lock.newCondition();

private static final Condition condition_B = lock.newCondition();

private static final Condition condition_C = lock.newCondition();

Print(char printChar) {
    this.printChar = printChar;
}

public void run() {
    try {
        lock.lock();
        while (count > 0) {
            if (printChar != currentChar) {
                switch (printChar) {
                    case 'A':
                        try {
                            condition_A.await();
                        }
                        catch (InterruptedException e) {
                        }
                        break;
                    case 'B':
                        try {
                            condition_B.await();
                        }
                        catch (InterruptedException e) {
                        }
                        break;
                    case 'C':
                        try {
                            condition_C.await();
                        }
                        catch (InterruptedException e) {
                        }
                        break;
                }
            }
            System.out.print(printChar + "(" + count + ") ~ ");

// if (count > 1) {
switch (printChar) {
case 'A':
currentChar = 'B';
condition_B.signal();
break;
case 'B':
currentChar = 'C';
condition_C.signal();
break;
case 'C':
currentChar = 'A';
condition_A.signal();
break;
// }
}
count--;
}

    }
    finally {
        lock.unlock();
    }

}

}

public class PrintThreadTest2 {

public static void main(String[] args) {
    Print a = new Print('A');
    Print b = new Print('B');
    Print c = new Print('C');
    new Thread(a).start();
    new Thread(b).start();
    new Thread(c).start();
}

}

输出：A(10) ~ B(9) ~ C(8) ~ A(7) ~ B(6) ~ C(5) ~ A(4) ~ B(3) ~ C(2) ~ A(1) ~ B(0) ~ C(-1) ~ 括号中为count

为什么count <= 0 的时候也能打印出来呢？

Answer 1

你可以在while循环的第一句添加一行：

 while (count > 0) {
                System.out.println(Thread.currentThread().getName()+"cout:"+count);

就能看到，每次进入循环的时候count都是>0的，最后一次进入while的时候count=1，但是由于count是全局共享变量，此时某两个个线程执行了count--操作，而当前线程再打印count的时候就成了-1了。