知道线程何时结束是很有用的.例如,在前面的示例中,为了演示使主线程的存活时间长于其他线程的好处,就需要做到这一点.在这些程序中,这是通过让主线程的睡眠时间多于他创建的子线程来实现的.当然,这不能说是一个令人满意的或是可通用的解决方法.

幸运的是,Thread提供了两种方法,可以确定线程是否结束.第一种方法是可以在线程中调用isAlive(),其基本形式如下所示:

final boolean isAlive();

如果调用isAlive()方法的线程仍在运行,该方法就返回true,否则返回false.为使用isAlive(),更新代码:

public class test2
{
    // @param args
    public static void main(String args[])
    {
     MyThread mt1 = new MyThread("child#1");
     MyThread mt2 = new MyThread("child#2");
     MyThread mt3 = new MyThread("child#3");
     
     mt1.start();
     mt2.start();
     mt3.start();
     
     do {
         System.out.print(".");
         try {
             Thread.sleep(100);
         }catch(InterruptedException exc) {
             System.out.println("main thread  interrupted");
         }
     } while(mt1.isAlive() || mt2.isAlive() ||  mt3.isAlive());
     System.out.println("main thread ending");
    }
}
class MyThread extends Thread implements  Runnable
{
    String thrdName;
    
    MyThread(String name)
    {
        thrdName = name;
    }
    
    public void run()
    {
        System.out.println(thrdName+" start");
        try {
            for(int count=0;count<10;count++)
            {
                Thread.sleep(400);
                System.out.println(thrdName+"  count is "+count);
            }
        }catch(InterruptedException exc) {
            System.out.println(thrdName+"  interrupted");
        }
    }
}

该版本的输出与前面很相似,但是main()在其他线程结束后立即终止.另一个区别是他使用isAlive()来等待子线程结束.另一种等待线程结束的方法是调用join().如下所示:

final void join() throws InterruptedException

该方法将等待,直到他调用的线程终止.他的名字表示调用线程会一直等待,直到指定线程加入他,join()的另一种允许指定等待指定线程终止的最大时间.

下面是一个使用join()来确保主线程的最后结束的程序:

    public static void main(String args[])
    {
     MyThread mt1 = new MyThread("child#1");
     MyThread mt2 = new MyThread("child#2");
     MyThread mt3 = new MyThread("child#3");
     
     mt1.start();
     mt2.start();
     mt3.start();
        try {
            mt1.join(); //等待直到线程终结
            mt2.join();
            mt3.join();
            System.out.println("mt 123 joined");
        }catch(InterruptedException exc) {
            System.out.println("main thread  interrupted");
        }
     System.out.println("main thread ending");
    }

输出如下所示:

child#1 start
child#2 start
child#3 start
child#1 count is 0
child#3 count is 0
child#2 count is 0
child#3 count is 1
child#1 count is 1
child#2 count is 1
child#3 count is 2
child#1 count is 2
child#2 count is 2
child#1 count is 3
child#2 count is 3
child#3 count is 3
child#3 count is 4
child#1 count is 4
child#2 count is 4
child#1 count is 5
child#2 count is 5
child#3 count is 5
child#1 count is 6
child#3 count is 6
child#2 count is 6
child#1 count is 7
child#3 count is 7
child#2 count is 7
child#2 count is 8
child#1 count is 8
child#3 count is 8
child#1 count is 9
child#2 count is 9
child#3 count is 9
mt 123 joined
main thread ending

如你所见,当对join()的调用返回后,线程已经停止执行.