JAVA多线程添加同步代码块后还是出现数据安全问题

Question

public class MyRunnable implements Runnable {
	private int tickets = 100;
	private Object obj = new Object();
	
	@Override
	public void run() {
		while (true) {
//添加同步代码块后，锁同意对象
			synchronized (obj) {
				if (tickets > 0) {
					try {
						Thread.sleep(100);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
					System.out.println(Thread.currentThread().getName() + "售卖第" + (101 - tickets) + "张票");
					tickets--;
				}else if (tickets==0) {
					break;
				}
			}
		}
	}
}


public class MyThread {
	public static void main(String args[]) {
		MyRunnable mr = new MyRunnable();
		// Thread th1 = new Thread(mr);
		// Thread th2 = new Thread(mr);
		// Thread th3 = new Thread(mr);

		Thread th1 = new Thread(mr, "售票口一");
		Thread th2 = new Thread(mr, "售票口二");
		Thread th3 = new Thread(mr, "售票口三");

		th1.start();
		th2.start();
		th3.start();
	}
}

Answer 1

synchronized (obj) {

if (tickets > 0) {

try {

System.out.println(Thread.currentThread().getName() + "售卖第" + (101 - tickets) + "张票");

tickets--;

Thread.sleep(100);

} catch (InterruptedException e) {

e.printStackTrace();

}

}

}