ThreadGroup

java.lang.ThreadGroup，ThreadGroup有很多优势，最重要的一点就是它可以对线程进行遍历，知道那些线程已经运行完毕，还有那些线程在运行。关于ThreadGroup的使用代码如下：

  

class MyThread extends Thread { 	boolean stopped; 	MyThread(ThreadGroup tg, String name) { 		super(tg, name);		stopped = false; 	} 	public void run() {		System.out.println(Thread.currentThread().getName() + " starting.");		try {			for (int i = 1; i < 1000; i++) {				System.out.print(".");				Thread.sleep(250);				synchronized (this) {					if (stopped)break;				}			}		} catch (Exception exc) {			System.out.println(Thread.currentThread().getName() + " interrupted.");		}		System.out.println(Thread.currentThread().getName() + " exiting.");	}	synchronized void myStop() {		stopped = true;	}}public class Main {	public static void main(String args[]) throws Exception {		ThreadGroup tg = new ThreadGroup("My Group");		MyThread thrd = new MyThread(tg, "MyThread #1");		MyThread thrd2 = new MyThread(tg, "MyThread #2");		MyThread thrd3 = new MyThread(tg, "MyThread #3");		thrd.start();		thrd2.start();		thrd3.start();		Thread.sleep(1000);		System.out.println(tg.activeCount() + " threads in thread group.");		Thread thrds[] = new Thread[tg.activeCount()];		tg.enumerate(thrds);		for (Thread t : thrds)			System.out.println(t.getName());		thrd.myStop();		Thread.sleep(1000);		System.out.println(tg.activeCount() + " threads in tg.");		tg.interrupt();	}}

1.ThreadGroup可以遍历线程，知道那些线程已经运行完毕，那些还在运行 

2.可以通过ThreadGroup.activeCount知道有多少线程从而可以控制插入的线程数