isActive()方法：判断当前的线程是否处于活动状态。

活动状态是指线程已经启动且尚未终止，线程处于正在运行或准备开始运行的状态，就认为线程是存活的。

class Alive implements Runnable {@Overridepublic void run(){for(int i=0;i<4;i++){System.out.println(Thread.currentThread().getName()+" "+i);try{Thread.sleep(500);}catch(InterruptedException e){e.printStackTrace();}}}
}public class TestAliveThread {public static void main(String[] args){Thread thread = new Thread(new Alive());thread.setName("Alive");thread.start();System.out.println(thread.getName()+" "+thread.isAlive());try{Thread.sleep(4000);}catch(InterruptedException e){e.printStackTrace();}System.out.println(thread.getName()+" "+thread.isAlive());}
}