线程的创建和安全问题(传智播客)

一.线程的创建
1.继承Thread

public class MyThread extends Thread{
    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName()+"......run called......");
    }

    public static void main(String[] args) {
        MyThread t0 = new MyThread();
        MyThread t1 = new MyThread();
        t0.start();
        t1.start();
        System.out.println(Thread.currentThread().getName()+"......main called......");
    }
}

2.实现Runnable接口
当某一个类由父类时，无法继承Thread类，可通过实现Runnable接口来实现线程创建

public class MyRunnable implements Runnable{
    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName()+"......run called......");
    }

    public static void main(String[] args) {
        MyRunnable r = new MyRunnable();
        Thread t0 = new Thread(r);
        Thread t1 = new Thread(r);
        t0.start();
        t1.start();
        System.out.println(Thread.currentThread().getName()+"......main called......");
    }
}

3.实现细节

public class MyThread {
    private Runnable r;

    public MyThread(Runnable r) {
        this.r = r;
    }
    
    public void run(){
        
    }
    
    public void start(){
        if(r!=null)
            this.run();
    }
}

4.Thread和Runnable实现创建线程的区别

• Runnable可避免子类继承Thread类中除run和start以外的方法
• Runnable可避免java单继承的缺陷
• Runnable有共享数据

二.线程安全
1.使用Runnable实现卖票(当线程任务执行很快，要想模拟出2个程序同时执行的效果，使用循坏让程序一直执行)

public class MyRunnable implements Runnable{
    private int ticketNo = 100;

    @Override
    public void run() {
        while(true) {
            if(ticketNo>0) {
                try {
                    sleep(10);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + "卖出了票号为"+ticketNo+"的票");
                ticketNo--;
            }
        }
    }

    public static void main(String[] args) {
        MyRunnable r = new MyRunnable();
        Thread t0 = new Thread(r);
        Thread t1 = new Thread(r);
        Thread t2 = new Thread(r);
        Thread t3 = new Thread(r);
        t0.start();
        t1.start();
        t2.start();
        t3.start();   }
}

分析出现0，-1，-2以及4个9的原因。
2.线程安全产生的原因

• 多个线程有共享数据
• 操作共享数据的代码有多份