Thread类的activeCount()方法用于返回当前线程的线程组中活动线程的数量。返回的值只是一个估计值，因为当此方法遍历内部数据结构时，线程数可能会动态更改。
语法
public static int activeCount()
返回
它返回当前线程的线程组中活动线程的数量。
示例
public class JavaActiveCountExp extends Thread
{
JavaActiveCountExp(String threadname, ThreadGroup tg)
{
super(tg, threadname);
start();
}
public void run()
{
System.out.println("running thread name is: "+Thread.currentThread().getName());
}
public static void main(String arg[])
{
// creating the thread group
ThreadGroup g1 = new ThreadGroup("parent thread group");
// creating a thread
JavaActiveCountExp t1 = new JavaActiveCountExp("Thread-1", g1);
// creating another thread
JavaActiveCountExp t2 = new JavaActiveCountExp("Thread-2", g1);
// checking the number of active thread
System.out.println("number of active thread: "+ g1.activeCount());
}
}
执行上面示例代码，得到以下结果：
number of active thread: 2
running thread name is: Thread-1
running thread name is: Thread-2
