关于线程基础的问题

[dodomila_]

用java编写一个程序实现多线程，在线程中输出线程的名字到文件test.txt中，隔300毫秒输出一次，共输出20次。
在做这道题的时候想用实现Runnable接口达到多个线程一共输出20次的效果。我是参考这段代码做的：

package test;

public class MutliThreadDemo {
    public static void main(String [] args){
        MutliThread m=new MutliThread();
        Thread t1=new Thread(m,"window 1");
        Thread t2=new Thread(m,"window 2");
        Thread t3=new Thread(m,"window 3");
        t1.start();
        t2.start();
        t3.start();
    }
}
class MutliThread implements Runnable{
    private int ticket=20;//所有线程一共拥有20张票
    private int a;
    public void run(){
        while(ticket>0){
            System.out.println(ticket--+" is saled by "+Thread.currentThread().getName());
            
        }
    }
}

但是给文件里输出的时候就是21条。这是我的代码：

package test;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.PrintWriter;

public class ThreadTest {
	 public static void main(String [] args){
		 	DemoThread dt=new DemoThread();
	        Thread t1=new Thread(dt);
	        Thread t2=new Thread(dt);
	        Thread t3=new Thread(dt);
	        t1.start();
	        t2.start();
	        t3.start();
	    }
}
class DemoThread implements Runnable{
    private int num=20;//所有线程一共输出0次
   
    private String fl = "F:\\Test.txt";
    File f = new File(fl);
    
    public void run(){
    	 if(f.exists() && f.isFile()){
			  System.out.println("使用已经存在的Test.txt文件");
		  }else{
			  try{
				  //创建文件
				  f.createNewFile();
				  System.out.println("创建Test.txt文件");
			  }catch(IOException e){
				  System.out.println("创建Test.txt文件失败,错误信息："+e.getMessage());
				  return;
			  }
		  }
        while(num>0){
        	try {
				FileOutputStream fos = new FileOutputStream(f,true);
				PrintWriter pw = new PrintWriter(fos);
				pw.write(num--+" ---"+Thread.currentThread().getName());//每输出一次num-1
				pw.println();
				pw.close();
			} catch (FileNotFoundException e) {
				System.out.println(e.getMessage());
			}
        }
    }
}

而且我总感觉我写的有问题，求大神讲解一下为什么，正确的应该是什么。

[X元素]

private int num=20; 试着使用volatile

[X元素]

最好的办法是使用 原子操作，

[dodomila_]

引用 2 楼 u011619071 的回复:

最好的办法是使用 原子操作，

我把num定义成volatile的输出的还是21条数据啊

[qq_15915835]

用 AtomicInteger 来计数

[dodomila_]

引用 4 楼 qq_15915835 的回复:

用 AtomicInteger 来计数

没有基础的方法来解决吗？我不懂啊

[qq_15915835]

引用 5 楼 dodomila_ 的回复:

[quote=引用 4 楼 qq_15915835 的回复:] 用 AtomicInteger 来计数

没有基础的方法来解决吗？我不懂啊[/quote] 对非线程安全变量操作加锁，这里就是num

[txazo]

public class MutliThreadDemo {

    public static void main(String[] args) throws IOException {
        MutliThread m = new MutliThread();
        Thread t1 = new Thread(m, "window 1");
        Thread t2 = new Thread(m, "window 2");
        Thread t3 = new Thread(m, "window 3");
        t1.start();
        t2.start();
        t3.start();
        System.in.read();
    }

}

class MutliThread implements Runnable {

    private AtomicInteger a = new AtomicInteger(0);

    public void run() {
        int num = 0;
        while ((num = a.incrementAndGet()) <= 20) {
            System.out.println(num + " is saled by " + Thread.currentThread().getName());

        }
    }

}

给一个正确的例子，可以在网上搜下Java内存模型的文章学习下

[dodomila_]

引用 7 楼 txazo 的回复:

public class MutliThreadDemo {

    public static void main(String[] args) throws IOException {
        MutliThread m = new MutliThread();
        Thread t1 = new Thread(m, "window 1");
        Thread t2 = new Thread(m, "window 2");
        Thread t3 = new Thread(m, "window 3");
        t1.start();
        t2.start();
        t3.start();
        System.in.read();
    }

}

class MutliThread implements Runnable {

    private AtomicInteger a = new AtomicInteger(0);

    public void run() {
        int num = 0;
        while ((num = a.incrementAndGet()) <= 20) {
            System.out.println(num + " is saled by " + Thread.currentThread().getName());

        }
    }

}

给一个正确的例子，可以在网上搜下Java内存模型的文章学习下

十分感谢

[dodomila_]

引用 7 楼 txazo 的回复:

public class MutliThreadDemo {

    public static void main(String[] args) throws IOException {
        MutliThread m = new MutliThread();
        Thread t1 = new Thread(m, "window 1");
        Thread t2 = new Thread(m, "window 2");
        Thread t3 = new Thread(m, "window 3");
        t1.start();
        t2.start();
        t3.start();
        System.in.read();
    }

}

class MutliThread implements Runnable {

    private AtomicInteger a = new AtomicInteger(0);

    public void run() {
        int num = 0;
        while ((num = a.incrementAndGet()) <= 20) {
            System.out.println(num + " is saled by " + Thread.currentThread().getName());

        }
    }

}

给一个正确的例子，可以在网上搜下Java内存模型的文章学习下

那麻烦问一下，我贴的那个参考案例是错误的是么？但是那个运行下来是20没错啊