wait，notify/notifyAll都要放在synchronized锁里面
如果没放在synchronized锁里面，就要报这样的错误

public class Test5 {

    public static void main(String[] args) throws InterruptedException {

        Thread t1 = new Thread(()->{
            syn();
        },"t1");

        Thread t2 = new Thread(()->{
            syn();
        },"t2");

        t1.start();
        t2.start();

        Thread.sleep(12000);
        synchronized (Test5.class){
            Test5.class.notifyAll();
        }

    }

    public static synchronized void syn(){
        try {
            for (int i = 0; i < 10; i++) {
                if(i == 5){
                    Test5.class.wait();
                }
                Thread.sleep(1000);
                System.out.println(Thread.currentThread().getName());
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}