看看为什么直接调用run()和用start()启动一个线程的差别
1.
package com.dragon;

import java.lang.Thread;

public class ThreadTest extends Thread {
 public void run() {
  for(int i=0;i<10;i++){
      
  System.out.println("this is a Thread test");
   }
 }   

this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
main thread is over

表明,run()和其他方法的调用没任何不同,main方法按顺序执行了它,并打印出最后一句,

退出程序.下面看看把调用改为start()

2.

package com.dragon;

import java.lang.Thread;

public class ThreadTest extends Thread {
 public void run() {
  for(int i=0;i<10;i++){
      
  System.out.println("this is a Thread test");

   }
 }

 public static void main(String[] args) {
  try {
   Thread t = new ThreadTest();
   t.start();
   
   System.out.println("main thread is over");
  } catch (Exception ex) {
   ex.printStackTrace();
  }
 }
}
输出结果:

main thread is over
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test

这表明,start()方法重新创建了一个线程,在main方法执行结束后,由于start()方法创建的

线程没有运行结束,因此主线程未能退出,直到线程 t 也执行完毕,这里要注意,默认创建的

线程是用户线程(非守护线程),再改一下代码

3.

package com.dragon;

import java.lang.Thread;

public class ThreadTest extends Thread {
 public void run() {
  for(int i=0;i<10;i++){
      
  System.out.println("this is a Thread test");

   }
 }

 public static void main(String[] args) {
  try {
   Thread t = new ThreadTest();
   t.setDaemon(true);
   t.start();
   
   System.out.println("main thread is over");
  } catch (Exception ex) {
   ex.printStackTrace();
  }
 }
}
输出为:

main thread is over
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test

为什么没有打印出十句呢,因为我们将 t 线程设置为了daemon(守护)线程,

程序中只有守护线程存在的时候,是可以退出的,所以只打印了七句便退出了.

再改一下代码

4.

package com.dragon;

import java.lang.Thread;

public class ThreadTest extends Thread {
 public void run() {
  for(int i=0;i<100;i++){
      
  System.out.println("this is a Thread test");

   }
 }

 public static void main(String[] args) {
  try {
   Thread t = new ThreadTest();
   t.start();
   System.out.println("main thread is over");
   System.exit(0);
  } catch (Exception ex) {
   ex.printStackTrace();
  }
 }
}
main thread is over
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test
this is a Thread test

结果可见,用户线程可以被System.exit(0)强制kill掉,所以也只打印出七句。