java線程概念案例說明

java線程一直是一個比較容易困擾的地方,很多初學者都不是很清楚,

在此,我用了幾個小實驗,來將其基本概念說明一下,首先把run(),start()區分開來,

看看為什麼直接調用run()和用start()啟動一個線程的差別

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");

   }
 }

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

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()和其他方法的調用沒任何不同,main方法按順序執行了它,並列印出最後一句,

退出程式.下面看看把調用改為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();
  }
 }
}
輸出結果:

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

這表明,start()方法重新建立了一個線程,在main方法執行結束後,由於start()方法建立的

線程沒有運行結束,因此主線程未能退出,直到線程 t 也執行完畢,這裡要注意,預設建立的

線程是使用者線程(非守護線程),再改一下代碼

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();
  }
 }
}
輸出為:

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

為什麼沒有列印出十句呢,因為我們將 t 線程設定為了daemon(守護)線程,

程式中只有守護線程存在的時候,是可以退出的,所以只列印了七句便退出了.

再改一下代碼

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

結果可見,使用者線程可以被System.exit(0)強制kill掉,所以也只列印出七句.