多线程Java如何为多个线程同步运行函数

Question

我必须使用名为changeCase的Runnable类的三个实例，并且对于所有三个changeCase实例，只能使用Server类的一个实例。进程执行需要连续运行，这就是为什么我在被重写的run方法中使用了while (true)。每个线程的process方法执行是重叠的。我该如何解决这个问题

我的代码：

package uppercase;

import java.util.Scanner;

class Server {

    void process(String threadName) {

        System.out.println("You are inside the Server " +threadName);

        System.out.println("Enter your sentence");
        Scanner sc1 = new Scanner(System.in);
        String input = sc1.nextLine();

        System.out.println("Press 1 to convert to Uppercase or Press 2 to convert to Lowercase");
        Scanner sc2 = new Scanner(System.in);
        int num = sc2.nextInt();

        switch (num) {
            case 1:
                changetoUpperCase(input);
                break;
            case 2:
                changetoLowerCase(input);
                break;
            default:
                System.out.println("Invalid number given\n\n");
                break;
        }
    }

    void changetoUpperCase(String msg) {

        printMessage(msg.toUpperCase());
    }

    void changetoLowerCase(String msg) {

        printMessage(msg.toLowerCase());
    }

    void printMessage(String msg) {

        System.out.println("[ " +msg+ " ]\n\n");

        try {
            Thread.sleep(1000);
        }catch (Exception e) {
           System.out.println(e.getMessage());
        }
    }
}

class changeCase implements Runnable {

    String threadName;
    Server target;
    Thread t;

    public changeCase(Server c, String s) {

        this.threadName = s;
        this.target = c;
    }

    @Override
    synchronized public void run() {

        while(true) {

            target.process(threadName);
        }

        //target.process(threadName);
    }

}

public class UpperCase {

    public static void main(String[] args) {

        Server s1 = new Server();

        Thread t1 = new Thread(new changeCase(s1, "Thread1"));
        Thread t2 = new Thread(new changeCase(s1, "Thread2"));
        Thread t3 = new Thread(new changeCase(s1, "Thread3"));

        t1.start();
        t2.start();
        t3.start();
    }

}

我需要的输出：

run:
You are inside the Server Thread1
Enter your sentence
hello world
Press 1 to convert to Uppercase or Press 2 to convert to Lowercase
1
[ HELLO WORLD ]


You are inside the Server Thread3
Enter your sentence
HELLO WORLD
Press 1 to convert to Uppercase or Press 2 to convert to Lowercase
2
[ hello world ]


You are inside the Server Thread2
Enter your sentence
hello world
Press 1 to convert to Uppercase or Press 2 to convert to Lowercase
4
Invalid number given


You are inside the Server Thread3
Enter your sentence

我得到的输出：

run:
You are inside the Server Thread1
Enter your sentence
You are inside the Server Thread2
Enter your sentence
You are inside the Server Thread3
Enter your sentence