java - java中的多线程示例

Question

这是来自 tutorials point 的 java 示例程序:

// Create a new thread.
class NewThread implements Runnable {
   Thread t;
   NewThread() {
      // Create a new, second thread
      t = new Thread(this, "Demo Thread");
      System.out.println("Child thread: " + t);
      t.start(); // Start the thread
   }

   // This is the entry point for the second thread.
   public void run() {
      try {
         for(int i = 5; i > 0; i--) {
            System.out.println("Child Thread: " + i);
            // Let the thread sleep for a while.
            Thread.sleep(50);
         }
     } catch (InterruptedException e) {
         System.out.println("Child interrupted.");
     }
     System.out.println("Exiting child thread.");
   }
}

public class ThreadDemo {
   public static void main(String args[]) {
      new NewThread(); // create a new thread
      try {
         for(int i = 5; i > 0; i--) {
           System.out.println("Main Thread: " + i);
           Thread.sleep(100);
         }
      } catch (InterruptedException e) {
         System.out.println("Main thread interrupted.");
      }
      System.out.println("Main thread exiting.");
   }
}

程序的输出如下。

Child thread: Thread[Demo Thread,5,main]
Main Thread: 5
Child Thread: 5
Child Thread: 4
Main Thread: 4
Child Thread: 3
Child Thread: 2
Main Thread: 3
Child Thread: 1
Exiting child thread.
Main Thread: 2
Main Thread: 1
Main thread exiting.

我无法理解为什么主线程在子线程之前执行。在程序中可以看到首先执行了new NewThread()。 NewThread 类实例化一个新线程并在新线程上调用 start() 函数，然后新线程调用 run() 函数。main 函数中的循环仅在创建新线程后才会出现。然而当程序运行时，主线程中的循环先于子线程运行。请帮我理解。

Answer 1

The loop in the main thread is run before the child thread. Please help me understand.

默认情况下，线程没有任何形式的同步，并且可以按任何顺序运行，并且执行可能会交错。您可以获取锁并使用它们来确保顺序，例如，如果主线程在启动子线程之前获取锁，则让子线程等待获取锁，并让主线程在完成其任务后释放锁.