Thread类与Runnable接口

Runnable

先看看源码中对Runnable的声明

@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();
}

可以看出，Runnable是一个函数式接口，并且里面只有一个抽象方法run()，所以继承Runnable接口必须自己手动实现run()方法。

Thread

先看看API中对Thread类的声明

public class Thread
extends Object
implements Runnable

可以看出，Thread类已经帮我们实现了Runnable接口，并且做了实现。这就是为什么继承Thread类不用重写run()方法的原因，因为Thread类已经帮我们对run方法做了实现，源码中的实现如下：

/**
 * If this thread was constructed using a separate
 * <code>Runnable</code> run object, then that
 * <code>Runnable</code> object's <code>run</code> method is called;
 * otherwise, this method does nothing and returns.
 * <p>
 * Subclasses of <code>Thread</code> should override this method.
 *
 * @see     #start()
 * @see     #stop()
 * @see     #Thread(ThreadGroup, Runnable, String)
 */
@Override
public void run() {
    if (target != null) {
        target.run();
    }
}