log4j无法在线程中打印MDC键值对

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.ThreadContext;

public class MyThread extends Thread {
    private static final Logger logger = LogManager.getLogger(MyThread.class);

    private String userId;

    public MyThread(String userId) {
        this.userId = userId;
    }

    @Override
    public void run() {
        ThreadContext.put("userId", userId);
        logger.info("Hello, world!");
        ThreadContext.clearAll();
    }

    public static void main(String[] args) {
        MyThread thread1 = new MyThread("user1");
        MyThread thread2 = new MyThread("user2");

        thread1.start();
        thread2.start();
    }
}