彻底理解Runnable和Thread的区别

昨天去面试，面试官问了一个问题：Runnable和Thread有什么区别，因为针对这个问题以前有背过，并且网上大多数都是这些结论，所以脱口而出：

1.Thread有单继承的问题；
2.Runnable便于实现资源共享，而Thread不能；

然而面试官好像并没有什么表示。之后结束后我就找了一下度娘，得到了这篇文章：

https://blog.csdn.net/zhaojianting/article/details/97664370

突然发现“Runnable便于实现资源共享”这一优点好像并不合理，以下是我的代码实现。

使用Thread实现多线程

public class MyThread extends Thread {

    private int ticket = 5;

    @Override
    public void run() {
        while (true) {
            System.out.println("Thread ticket = " + ticket--);
            if (ticket < 0) {
                break;
            }
        }
    }

}

public class Test {
    public static void main(String[] args) {

            //Test Thread

        new MyThread().start();
        new MyThread().start();

    }
}

执行结果如下：
Thread ticket = 5
Thread ticket = 4
Thread ticket = 3
Thread ticket = 2
Thread ticket = 1
Thread ticket = 0
Thread ticket = 5
Thread ticket = 4
Thread ticket = 3
Thread ticket = 2
Thread ticket = 1
Thread ticket = 0

并不能实现资源共享，跟以前背的面试题答案一样；但真的是这样吗？我们仔细看一下，代码中我们创建了两个MyThread对象，每个对象都有自己的ticket成员变量，当然会多卖1倍。现在我们重新测试一下，请看测试代码：

public class Test {
    public static void main(String[] args) {

            //Test Thread

        MyThread t1 = new MyThread();
        new Thread(t1).start();
        new Thread(t1).start();
    }
}

测试结果如下：
Thread ticket = 5
Thread ticket = 4
Thread ticket = 3
Thread ticket = 2
Thread ticket = 1
Thread ticket = 0
Thread ticket = -1

可以看到这次我们只创建了一个MyThread对象，并没出现卖两倍票的情况，Thread也可以实现资源共享。

因为多线程访问同一变量会有并发问题（需要加锁），所以Thread正确的写法如下：

public class MyThread extends Thread {

    private int ticket = 5;

    @Override
    public void run() {
        if (ticket > 0) {
            synchronized (this) {
                if (ticket > 0) {
                    while (true) {
                        System.out.println("Thread:" + Thread.currentThread().getName() + "--Thread ticket = " + ticket--);
                        if (ticket < 0) {
                            break;
                        }
                    }
                }
            }
        }
    }

}

public class Test {
    public static void main(String[] args) {

            //Test Thread

        MyThread t1 = new MyThread();
        new Thread(t1).start();
        new Thread(t1).start();
        new Thread(t1).start();
        new Thread(t1).start();
        new Thread(t1).start();
        new Thread(t1).start();
    }
}

执行结果如下：
Thread:Thread-1--Thread ticket = 5
Thread:Thread-1--Thread ticket = 4
Thread:Thread-1--Thread ticket = 3
Thread:Thread-1--Thread ticket = 2
Thread:Thread-1--Thread ticket = 1
Thread:Thread-1--Thread ticket = 0

现在看Thread和Runnable的源码：

public
class Thread implements Runnable {
    /* Make sure registerNatives is the first thing <clinit> does. */
    private static native void registerNatives();
    static {
        registerNatives();
    }

    private volatile String name;
    private int            priority;
    private Thread         threadQ;
    private long           eetop;

@FunctionalInterface
public interface Runnable {
    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see     java.lang.Thread#run()
     */
    public abstract void run();
}

结论：其实Thread也就是实现了Runnable接口，提供了更多的方法而已。所以说Thread与Runnable并没有什么区别。如果硬要说有什么区别的话，那就是类与接口的区别，继承与实现的区别。另外最重要的是，学习的时候不要忘了思考哦！