ThreadGroup:多个相同功能的线程可合并到一个线程组中。线程组可用于监控组里面的活动线程数，同时操作多个线程，wait,notify。等等。。

同时可为线程组和各个线程命名。

/** * Created by li on 2016/11/24. */public class DemoThread extends Thread {    private final static Object object = new Object();    public static void main(String[] args) throws InterruptedException {        ThreadGroup threadGroup = new ThreadGroup("myGroup");        Thread thread1 = new Thread(threadGroup, new DemoThread(), "t1");        Thread thread2 = new Thread(threadGroup, new DemoThread(), "t2");        thread1.start();        thread2.start();        System.out.println(threadGroup.activeCount());        System.out.println("主线程干的事");        Thread.sleep(5000);        System.out.println(threadGroup.activeCount());    }    @Override    public void run() {        System.out.println(Thread.currentThread().getThreadGroup().getName() + "++" + Thread.currentThread().getName());        try {            Thread.sleep(3000);        } catch (InterruptedException e) {            e.printStackTrace();        }    }}