二、线程的创建方式

线程的创建方式最准确的描述有两种

方式一:继承Thread并重写run方法

public class MyThread extends Thread {

    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName());
    }

    public static void main(String[] args) {
        MyThread myThread = new MyThread();
        myThread.setName("线程demo");
        myThread.start();
    }
}

方式二：实现Runnable接口并重写run方法

public class MyRunable implements Runnable,Serializable {
    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName());
        try {
            System.in.read();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        Thread thread = new Thread(new MyRunable());
        thread.setName("xdclass");
        thread.start();
    }
}

注意：只有调用start()方法才会启动线程

源码分析

调用Thread.start()方法最终是调用run()方法，run()方法的源码如下:

// 如果target不为空则是执行target的run方法
/* What will be run. */
private Runnable target;

public synchronized void start() {
    /**
        * This method is not invoked for the main method thread or "system"
        * group threads created/set up by the VM. Any new functionality added
        * to this method in the future may have to also be added to the VM.
        *
        * A zero status value corresponds to state "NEW".
        */
    if (threadStatus != 0)
        throw new IllegalThreadStateException();

    /* Notify the group that this thread is about to be started
        * so that it can be added to the group's list of threads
        * and the group's unstarted count can be decremented. */
    group.add(this);

    boolean started = false;
    try {
        start0();
        started = true;
    } finally {
        try {
            if (!started) {
                group.threadStartFailed(this);
            }
        } catch (Throwable ignore) {
            /* do nothing. If start0 threw a Throwable then
                it will be passed up the call stack */
        }
    }
}

// 调用底层代码(c)，最终调用run方法
private native void start0();

@Override
public void run() {
    if (target != null) {
        target.run();
    }
}

问题：同时继承Thread和实现Runnable接口，会执行哪个run方法

继承Thread创建线程，是重写Thread的run方法，而实现Runnable接口则是实现Runnable接口的run方法
run方法一旦被重写，就不会存在经典的三行代码，也就不会执行Runnable接口的run方法了，故此同时继承Thread和实现Runnable接口，只会执行Thread子类重写的run方法

if (target != null) {
        target.run();
    }