之前遇到過這個問題，直接上程式碼記錄下：

public class RunableTest implements Runnable {

    volatile int target="A".codePointAt(0);
    static int minOrder="A".codePointAt(0);
    static int maxOrder="C".codePointAt(0);

    @Override
    public void run() {
        synchronized (this){
            int order=Thread.currentThread().getName().codePointAt(0);
            for (int i=0;i<10;i++){
                while (target!=order){
                    try{
                        wait();
                    }catch (Exception ex){

                    }
                }
                System.out.println(Thread.currentThread().getName());
                target=order+1;
                if(target>maxOrder){
                    target=minOrder;
                }
                notifyAll();
            }

        }

    }
}
 

public class TestMain {

    public static void main(String[] args){


        Runnable runnable=new RunableTest();

        Thread threadA=new Thread(runnable,"A");
        Thread threadB=new Thread(runnable,"B");
        Thread threadC=new Thread(runnable,"C");

        threadA.start();
        threadB.start();
        threadC.start();



    }
}
 

A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C