java多线程，关于join()执行顺序问题。

Question

在执行main函数的first.join();时，为何两个线程都运行了

运行截图如下

代码如下：

public class Main {
    void f() {
        FirstThread first = new FirstThread(this);
        SecondThread second = new SecondThread(this);
        first.start();
        second.start();
        try {
            System.out.println("Waiting for first thread to finish...");
            first.join();
            System.out.println("It's a long wait!");
            System.out.println("Waking up second thread..");
            synchronized (this) {
                this.notify();
            }
            System.out.println("Waiting for second thread to finish..");
            second.join();
        } catch (InterruptedException e) {
        }
        System.out.println("I'm ready to finish too.");
    }
    public static void main(String[] args) {
        Main m = new Main();
        m.f();
    }
}

class FirstThread extends Thread {
    Object lock;
    FirstThread(Object o) { lock = o; }
    public void run() {
        try {
            System.out.println("First thread starts running.");
            sleep(10000);
            System.out.println("First thread finishes running.");
        } catch (InterruptedException e) {
        }
    }
}

class SecondThread extends Thread {
    Object lock;
    SecondThread(Object o) { lock = o; }
    public void run() {
        System.out.println("Second thread starts running.");
        System.out.println("Second thread suspend itself.");
        try {
            synchronized (lock) {
                lock.wait();
            }
        } catch (InterruptedException e) {
        }
        System.out.println("Second thread runs again and finishes.");
    }
}

Answer 1

因为你两个线程都start了，当然会运行，跟join没关系。first.join()意思是当前线程阻塞，直到first线程完成。