1.先看看jdk文档
void run()
If this thread was constructed using a separate Runnable run object, then that Runnable object's run method is called; otherwise, this method does nothing and returns.
void start()
Causes this thread to begin execution; the Java Virtual Machine calls the run method of this thread.
线程调用run()方法是直接执行其run方法（正常情况）。
线程调用start()方法后进入准备状态，等待CPU的调度，当切换到该线程时，线程进入运行状态，执行run()方法。
既然都是执行线程的run方法，为什么不直接调用run方法？
2.再看代码
public class Main {
public static void main(String[] args) {
Mythread t = new Mythread();
Thread thread1 = new Thread(t);
Thread thread2 = new Thread(t);
System.out.println("begin = " + System.currentTimeMillis());
thread1.setName("我是线程1");
thread2.setName("我是线程2");
thread1.run();
thread2.start();
System.out.println("end = " + System.currentTimeMillis());
}
}
class Mythread extends Thread{
public void run() {
try {
System.out.println("threadName: "+this.currentThread().getName()+" begin");
Thread.sleep(1000);
System.out.println("threadName: "+this.currentThread().getName()+" end");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
运行结果：
可以发现直接调用run方法时是被主线程当做普通的方法调用，并不是真正的多线程。