多线程编程中使用wait方法导致IllegalMonitorStateException异常的原因是什么？

// 线程打印完毕后，设置下一个要打印的线程标识，并唤醒其他线程
if (current_thread.equals("a")) {
    current_thread = "b";
} else if (current_thread.equals("b")) {
    current_thread = "c";
} else if (current_thread.equals("c")) {
    current_thread = "a";
}

package 并发编程.work2;

public class Test {
    private static volatile String current_thread = "A";
    private static final Object lock = new Object();//新增锁对象

    public static void main(String[] args) {
        Thread t1 = new Thread(new PrintThreadName("A"), "A");
        Thread t2 = new Thread(new PrintThreadName("B"), "B");
        Thread t3 = new Thread(new PrintThreadName("C"), "C");
        t1.start();
        t2.start();
        t3.start();
    }

    static class PrintThreadName implements Runnable {
        private String threadName;

        public PrintThreadName(String threadName) {
            this.threadName = threadName;
        }

        @Override
        public void run() {
            for (int i = 0; i < 5; i++) {
                synchronized (lock) { // 使用独立锁对象
                    while (!current_thread.equals(threadName)) {
                        try {
                            lock.wait(); // 使用独立锁对象
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.print(threadName);
                    if (threadName.equals("A")) {
                        current_thread = "B";
                    } else if (threadName.equals("B")) {
                        current_thread = "C";
                    } else {
                        current_thread = "A";
                    }
                    lock.notifyAll(); // 使用独立锁对象
                }
            }
        }
    }
}