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();}}
}