多线程售票问题以及线程同步机制

售票问题

package windowSale;

/**
 * @author zss
 */
public class WindowSale1 {
    public static int  countTicket=10;

    public static void main(String[] args) {
        Window1 window1 = new Window1();
        Window1 window2 = new Window1();
        Window1 window3 = new Window1();
        window1.start();
        window2.start();
        window3.start();
    }
}
class Window1 extends Thread{
    boolean judge=true;
    @Override
    public  void  run(){
        while (judge){
            sell();
        }
    }
    public  void sell(){
        if (WindowSale1.countTicket<=0){
            judge=false;
            System.out.println("售票结束");

            }
        else {
            try {
                Thread.sleep(50);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            WindowSale1.countTicket = WindowSale1.countTicket - 1;
            System.out.println(Thread.currentThread().getName() + "售卖一张票" + "还剩" + WindowSale1.countTicket);
        }

           // System.out.println();

    }
}

三个窗口售卖，会发现我们的票数剩余0张还可以继续售卖。这是因为在我们的票数还有两张时，我们的1号窗口进来还没有进行减去操作，2号与3号都进来，导致2张票被买了三次。

线程同步机制

在多线程编程，一些敏感数据不允许被多个线程同时访问，此时就使用同步访问技术，保证数据在任意同一时刻，最多由一个线程访问，保证数据的完整性。

同步的具体方法

• synchronized(对象){//得到对象的锁，才能操作同步带代码

​ //需要被同步的代码}，操作的必须是同一个对象

package windowSale;

/**
 * @author zss
 */
public class WindowSale1 {
    public static void main(String[] args) {
        SellTicket sellTicket=new SellTicket();
        Thread thread1=new Thread(sellTicket);
        Thread thread2=new Thread(sellTicket);
        Thread thread3=new Thread(sellTicket);
        thread1.start();
        thread2.start();
        thread3.start();
    }
}
class SellTicket extends Thread{
    int ticketNum=100;
    boolean judge=true;
    Object obj=new Object();
    @Override
    public  void  run() {
        while (judge){
            sell();
        }
    }
    public  void sell(){

        synchronized (this.obj){
        if (ticketNum<=0){
            System.out.println("售票结束");
            judge=false;
        }
        else {
            ticketNum--;
            System.out.println(Thread.currentThread().getName()+"售卖一张票，号数为"+(ticketNum+1)+"还剩"+ticketNum);
        }
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    }
}

• synchronized还可以放在方法声明中，表示整个方法为同步方法

public synchronized void m（String name）{

}

package windowSale;

/**
 * @author zss
 */
public class WindowSale1 {
    public static void main(String[] args) {
        SellTicket sellTicket=new SellTicket();
        Thread thread1=new Thread(sellTicket);
        Thread thread2=new Thread(sellTicket);
        Thread thread3=new Thread(sellTicket);
        thread1.start();
        thread2.start();
        thread3.start();
    }
}
class SellTicket extends Thread{
    int ticketNum=100;
    boolean judge=true;
    @Override
    public  void  run() {
        while (judge){
            sell();
        }
    }
    public  void sell(){
        if (ticketNum<=0){
            System.out.println("售票结束");
            judge=false;
        }
        else {
            ticketNum--;
            System.out.println(Thread.currentThread().getName()+"售卖一张票，号数为"+(ticketNum+1)+"还剩"+ticketNum);
        }
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

此时发现没有超票售出，但是售票的顺序却不正确，而且其实内部已经发生了超卖现象，但是我们只不过用代码块掩盖 了

package windowSale;

/**
 * @author zss
 */
public class WindowSale1 {
    public static void main(String[] args) {
        SellTicket sellTicket=new SellTicket();
        Thread thread1=new Thread(sellTicket);
        Thread thread2=new Thread(sellTicket);
        Thread thread3=new Thread(sellTicket);
        thread1.start();
        thread2.start();
        thread3.start();
    }
}
class SellTicket extends Thread{
    int ticketNum=100;
    boolean judge=true;
    @Override
    public  void  run() {
        while (judge){
            sell();
        }
    }
    public synchronized void sell(){
        if (ticketNum<=0){
            System.out.println("售票结束");
            judge=false;
        }
        else {
            ticketNum--;
            System.out.println(Thread.currentThread().getName()+"售卖一张票，号数为"+(ticketNum+1)+"还剩"+ticketNum);
        }
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}