java线程之ThreadGroup

所有线程都隶属于一个线程组。那可以是一个默认线程组，亦可是一个创建线程时明确指定的组。在创建之初，线程被限制到一个组里，而且不能改变到一个不同的组。每个应用都至少有一个线程从属于系统线程组。若创建多个线程而不指定一个组，它们就会自动归属于系统线程组。

线程组也必须从属于其他线程组。必须在构建器里指定新线程组从属于哪个线程组。若在创建一个线程组的时候没有指定它的归属，则同样会自动成为系统线程组的一名属下。因此，一个应用程序中的所有线程组最终都会将系统线程组作为自己的“父”

之所以要提出“线程组”的概念，一般认为，是由于“安全”或者“保密”方面的理由。根据Arnold和Gosling的说法：“线程组中的线程可以修改组内的其他线程，包括那些位于分层结构最深处的。一个线程不能修改位于自己所在组或者下属组之外的任何线程”。

测试代码：

package com.lang;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CountDownLatch;

import junit.framework.TestCase;

public class TestThreadGroup extends TestCase {

	public void testThreadGroup1() {
		final CountDownLatch cdl = new CountDownLatch(15);
		List<ThreadGroup> list = new ArrayList<ThreadGroup>();
		for (int i = 0; i < 5; i++) {
			list.add(new ThreadGroup("ThreadGroup" + i));
		}
		for (int i = 0; i < 15; i++) {
			int index = i % 5;
			ThreadGroup threadGroup = list.get(index);
			Thread thread = new Thread(threadGroup, new Runnable() {
				@Override
				public void run() {
					for (int j = 0; j < 3; j++) {
						System.out.println(Thread.currentThread() + ", " + j);
					}
					cdl.countDown();
				}
			});
			int pri = index + 1;
			thread.start();
			threadGroup.setMaxPriority(pri);
		}

		try {
			cdl.await();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

	public void testThreadGroup2() {
		final CountDownLatch cdl = new CountDownLatch(3);
		ThreadGroup threadGroup = new ThreadGroup("ThreadGroup");
		for (int i = 0; i < 3; i++) {
			Thread thread = new Thread(threadGroup, new Runnable() {
				@Override
				public void run() {
					for (int j = 0; j < 3; j++) {
						System.out.println(Thread.currentThread() + ", " + j);
					}
					cdl.countDown();
				}
			});
			thread.start();
		}
		Thread[] threads = new Thread[threadGroup.activeCount()];
		threadGroup.enumerate(threads);
		for (Thread thread : threads) {
			if (thread != null) {
				System.out.println(thread);
			}
		}

		try {
			cdl.await();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}

}

运行testThreadGroup1：

Thread[Thread-0,5,ThreadGroup0], 0
Thread[Thread-0,5,ThreadGroup0], 1
Thread[Thread-0,5,ThreadGroup0], 2
Thread[Thread-1,5,ThreadGroup1], 0
Thread[Thread-1,5,ThreadGroup1], 1
Thread[Thread-1,5,ThreadGroup1], 2
Thread[Thread-3,5,ThreadGroup3], 0
Thread[Thread-3,5,ThreadGroup3], 1
Thread[Thread-3,5,ThreadGroup3], 2
Thread[Thread-2,5,ThreadGroup2], 0
Thread[Thread-2,5,ThreadGroup2], 1
Thread[Thread-2,5,ThreadGroup2], 2
Thread[Thread-4,5,ThreadGroup4], 0
Thread[Thread-4,5,ThreadGroup4], 1
Thread[Thread-4,5,ThreadGroup4], 2
Thread[Thread-9,5,ThreadGroup4], 0
Thread[Thread-9,5,ThreadGroup4], 1
Thread[Thread-7,3,ThreadGroup2], 0
Thread[Thread-9,5,ThreadGroup4], 2
Thread[Thread-7,3,ThreadGroup2], 1
Thread[Thread-7,3,ThreadGroup2], 2
Thread[Thread-8,4,ThreadGroup3], 0
Thread[Thread-14,5,ThreadGroup4], 0
Thread[Thread-14,5,ThreadGroup4], 1
Thread[Thread-14,5,ThreadGroup4], 2
Thread[Thread-8,4,ThreadGroup3], 1
Thread[Thread-8,4,ThreadGroup3], 2
Thread[Thread-13,4,ThreadGroup3], 0
Thread[Thread-13,4,ThreadGroup3], 1
Thread[Thread-13,4,ThreadGroup3], 2
Thread[Thread-12,3,ThreadGroup2], 0
Thread[Thread-12,3,ThreadGroup2], 1
Thread[Thread-12,3,ThreadGroup2], 2
Thread[Thread-11,2,ThreadGroup1], 0
Thread[Thread-11,2,ThreadGroup1], 1
Thread[Thread-11,2,ThreadGroup1], 2
Thread[Thread-6,2,ThreadGroup1], 0
Thread[Thread-6,2,ThreadGroup1], 1
Thread[Thread-6,2,ThreadGroup1], 2
Thread[Thread-10,1,ThreadGroup0], 0
Thread[Thread-10,1,ThreadGroup0], 1
Thread[Thread-10,1,ThreadGroup0], 2
Thread[Thread-5,1,ThreadGroup0], 0
Thread[Thread-5,1,ThreadGroup0], 1
Thread[Thread-5,1,ThreadGroup0], 2

运行testThreadGroup2：

Thread[Thread-0,5,ThreadGroup], 0
Thread[Thread-0,5,ThreadGroup], 1
Thread[Thread-0,5,ThreadGroup], 2
Thread[Thread-1,5,ThreadGroup]
Thread[Thread-2,5,ThreadGroup]
Thread[Thread-1,5,ThreadGroup], 0
Thread[Thread-1,5,ThreadGroup], 1
Thread[Thread-1,5,ThreadGroup], 2
Thread[Thread-2,5,ThreadGroup], 0
Thread[Thread-2,5,ThreadGroup], 1
Thread[Thread-2,5,ThreadGroup], 2