启动了四个线程，为什么只执行了一个

运行结果只有携程卖票，其他的线程却不卖，请问是为什么


class MyRunnable implements Runnable{
    private Integer ticketscount=1000;
    @Override
    public void run() {
        synchronized(this){
            while(ticketscount>0){
                System.out.println(Thread.currentThread().getName()+"卖出了一张票，还剩"+--ticketscount+"张票");
            }
        }
        
    }
   
}
public class TicketsRunnable{
    public static void main(String [] args){
        MyRunnable mr=new MyRunnable();
        Thread th1=new Thread(mr,"携程");
        Thread th2=new Thread(mr,"途牛");
        Thread th3=new Thread(mr,"12306");
        Thread th4=new Thread(mr,"手机客户端");
        th1.start();
        th2.start();
        th3.start();
        th4.start();        
    }
}