線程的創建和安全問題(傳智播客)

一.線程的創建
1.繼承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.實現Runnable接口
當某一個類由父類時，無法繼承Thread類，可通過實現Runnable接口來實現線程創建

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.實現細節

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實現創建線程的區別

• Runnable可避免子類繼承Thread類中除run和start以外的方法
• Runnable可避免java單繼承的缺陷
• Runnable有共享數據

二.線程安全
1.使用Runnable實現賣票(當線程任務執行很快，要想模擬出2個程序同時執行的效果，使用循壞讓程序一直執行)

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() + "賣出了票號爲"+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();   }
}

分析出現0，-1，-2以及4個9的原因。
2.線程安全產生的原因

• 多個線程有共享數據
• 操作共享數據的代碼有多份