线程运行如何启动？

我正在寻找一个关于线程的小例子。为了创建线程，我们可以通过两种方式来实现：Runnable接口或通过扩展Thread。我使用第一种方式

package test;

public class test implements Runnable{
    public static void main(String args[])
    {
        test t=new test();
        t.run();Thread th=Thread.currentThread();
        th.start();
    }

    @Override
    public void run() {
        // TODO Auto-generated method stub
        System.out.println("hi");
    }
}

我的疑问是当我们打电话时th.start(); then run()被称为。我想知道如何。我内心想start()可能正在打电话run()所以我查看了 Thread 类的文档

以下是start()Thread类中的声明

public synchronized void start() {
    /**
     * This method is not invoked for the main method thread or "system"
     * group threads created/set up by the VM. Any new functionality added
     * to this method in the future may have to also be added to the VM.
     *
     * A zero status value corresponds to state "NEW".
     */
    if (threadStatus != 0)
        throw new IllegalThreadStateException();

    /* Notify the group that this thread is about to be started
     * so that it can be added to the group's list of threads
     * and the group's unstarted count can be decremented. */
    group.add(this);

    boolean started = false;
    try {
        start0();
        started = true;
    } finally {
        try {
            if (!started) {
                group.threadStartFailed(this);
            }
        } catch (Throwable ignore) {
            /* do nothing. If start0 threw a Throwable then
              it will be passed up the call stack */
        }
    }
}

正如你在里面看到的start(),run()没有被调用，但是当我们调用时th.start()然后自动覆盖run()被称为。有人可以解释一下吗

其机制是run在新线程上调用方法是语言外的：它不能用Java代码表示。这是关键的一行start method:

    start0();

start0是一个本机方法，其调用将：

• 导致创建一个新的本机执行线程；
• 导致run要在该线程上调用的方法。