Java多线程的经典案例,3个窗口同时卖100张火车票。要保证没有重复票和错票。
错误案例:
public class TestSellTicketsRunnable {
public static void main(String[] args) {
Tickets tickets = new Tickets();
Thread thread1 = new Thread(tickets, "窗口1");
thread1.start();
Thread thread2 = new Thread(tickets, "窗口2");
thread2.start();
Thread thread3 = new Thread(tickets, "窗口3");
thread3.start();
}
}
class Tickets implements Runnable {
static int countTickets = 100;
@Override
public void run() {
while (true) {
if (countTickets > 0) {
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + "卖票,票号是:" + countTickets);
countTickets--;
} else {
break;
}
}
}
}
运行的结果如下图所以,可以看到会导致有重复票和错误票产生。(如果不写sleep()方法,不一定能出现错误的现象)
原因分析:
线程1执行了if方法之后,CPU执行权限被线程2抢去了,然后线程2开始执行。当线程2执行到if之后,又被线程3抢去了。然后线程3执行完了打印方法,CPU执行权限被线程1抢去了,线程1执行打印方法,这个时候的票数已经出现异常了。等线程1执行完了,轮到线程2的时候,又会出现票数异常。所以,可以看到最后两个打印的结果是错误的结果。
正确写法:
public class TestSellTicketsRunnable {
public static void main(String[] args) {
Tickets tickets = new Tickets();
Thread thread1 = new Thread(tickets, "窗口1");
thread1.start();
Thread thread2 = new Thread(tickets, "窗口2");
thread2.start();
Thread thread3 = new Thread(tickets, "窗口3");
thread3.start();
}
}
class Tickets implements Runnable {
static int countTickets = 100;
@Override
public void run() {
while (true) {
synchronized (this) {
if (countTickets > 0) {
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + "卖票,票号是:" + countTickets);
countTickets--;
} else {
break;
}
}
}
}
}
这里加上了 synchronized (this) 这么一段代码,叫做线程同步,也可以叫做锁。保证多线程在处理同一个对象(或者说同一个资源)的时候,会把synchronized()里面大括号包的内容执行完,再轮到下个线程。
打印结果如下: