代码一：

public static void main(String[] args) throws Exception {
    Thread thread = new Thread(() -> {
      try {
        TimeUnit.SECONDS.sleep(10);
        System.out.println("thread-0 thread exit.");
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    }, "thread-0");
    thread.start();
    thread.join();
    System.out.println("main thread exit.");
  }

控制台(10s后打印并退出)：

thread-0 thread exit.
main thread exit.

Process finished with exit code 0

代码二：

public static void main(String[] args) throws Exception {
    Thread.currentThread().join();
    System.out.println("main thread exit.");
  }

控制台(一直等待中)：

问题：第一段代码是main线程阻塞在thread-0线程上，当thread-0执行完毕，main线程也同时退出，那第二段代码中main线程阻塞在哪个线程上呢，为什么没有退出？