java - 当 main 方法中声明 2 个对象时，代码会同时编译吗？

Question

我在这里研究多线程操作。但我目前对 main 方法的执行顺序感到怀疑。请向我解释一下这些，问题标记在下面。

这是我正在研究的一个简单程序

public class HelloWorld implements Runnable  {
    private Thread t;
    private String threadname;

    HelloWorld(String name){
        threadname= name;
        System.out.println("Create " +threadname);
    }

    public void run() {
        System.out.println("Running " +threadname);
        try {
            for(int i=4; i>0;i--) {
                System.out.println("Thread" +threadname +", "+i);
                Thread.sleep(50);
            }
        }catch(InterruptedException e) {
            System.out.println("Thread" +threadname +"interrupted ");
        }
        System.out.println("Thread" +threadname +"exiting ");
    }

    public void start() {
        System.out.println("Starting " +threadname);
        if(t==null)
        {
            t=new Thread(this, threadname);
            t.start();
        }
    }

    public static void main(String[] args) {
        HelloWorld obj1= new HelloWorld ("Thread-1");
        obj1.start();

        HelloWorld obj2= new HelloWorld ("Thread-2");
        obj2.start();
    }
}

实际结果

Create Thread-1
Starting Thread-1
Create Thread-2
Starting Thread-2
Running Thread-1
ThreadThread-1, 4
Running Thread-2
ThreadThread-2, 4
ThreadThread-2, 3
ThreadThread-1, 3
ThreadThread-1, 2
ThreadThread-2, 2
ThreadThread-1, 1
ThreadThread-2, 1
ThreadThread-2exiting 
ThreadThread-1exiting 

我的问题:

Create Thread-1
Starting Thread-1
Create Thread-2(why here will switch from 1 to 2)
Starting Thread-2
Running Thread-1
ThreadThread-1, 4
Running Thread-2(At here, I understand the thread is switch when Thread.sleep(50) is being executed;)
ThreadThread-2, 4
ThreadThread-2, 3
ThreadThread-1, 3
ThreadThread-1, 2
ThreadThread-2, 2
ThreadThread-1, 1
ThreadThread-2, 1
ThreadThread-2exiting 
ThreadThread-1exiting 

问题:为什么这里会从1切换到2？main 方法中的 2 个对象是否同时运行？

Answer 1

This is because you are using multi threading and multi threading provides parallel execution.

在 main 方法中，您启动了两个子线程 obj1.start() 和 obj2.start()。直到 obj1.start() 为止，只有一个子线程(主线程是父线程)。但是当您启动第二个子线程 obj2.start() 时，现在有两个子线程正在并行执行，这就是发生切换的原因。两个线程将具有单独的执行路径并且将并行运行。

另外，由于CPU使用了线程调度算法(Round Robin等)，所以会发生切换。