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
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