一个线程如何知道另一线程已经结束？Thread类提供了回答此问题的方法。

有两种方法可以判定一个线程是否结束。第一，可以在线程中调用isAlive()。这种方法由Thread定义，它的通常形式如下：

final boolean isAlive( )

如果所调用线程仍在运行，isAlive()方法返回true，如果不是则返回false。但isAlive（）很少用到，等待线程结束的更常用的方法是调用join()，描述如下：

final void join( ) throws InterruptedException

该方法等待所调用线程结束。该名字来自于要求线程等待直到指定线程参与的概念。join()的附加形式允许给等待指定线程结束定义一个最大时间。下面是前面例子的改进版本。运用join()以确保主线程最后结束。同样，它也演示了isAlive()方法。

// Using join() to wait for threads to finish. class NewThread implements Runnable { String name; // name of thread Thread t; NewThread(String threadname) { name = threadname; t = new Thread(this, name); System.out.println("New thread: " + t); t.start(); // Start the thread } // This is the entry point for thread. public void run() { try { for(int i = 5; i > 0; i--) { System.out.println(name + ": " + i); Thread.sleep(1000); } } catch (InterruptedException e) { System.out.println(name + " interrupted."); } System.out.println(name + " exiting."); } } class DemoJoin { public static void main(String args[]) { NewThread ob1 = new NewThread("One"); NewThread ob2 = new NewThread("Two"); NewThread ob3 = new NewThread("Three"); System.out.println("Thread One is alive: "+ ob1.t.isAlive()); System.out.println("Thread Two is alive: "+ ob2.t.isAlive()); System.out.println("Thread Three is alive: "+ ob3.t.isAlive()); // wait for threads to finish try { System.out.println("Waiting for threads to finish."); ob1.t.join(); ob2.t.join(); ob3.t.join(); } catch (InterruptedException e) { System.out.println("Main thread Interrupted"); } System.out.println("Thread One is alive: "+ ob1.t.isAlive()); System.out.println("Thread Two is alive: "+ ob2.t.isAlive()); System.out.println("Thread Three is alive: "+ ob3.t.isAlive()); System.out.println("Main thread exiting."); } }

程序输出如下所示：

New thread: Thread[One,5,main] New thread: Thread[Two,5,main] New thread: Thread[Three,5,main] Thread One is alive: true Thread Two is alive: true Thread Three is alive: true Waiting for threads to finish. One: 5 Two: 5 Three: 5 One: 4 Two: 4 Three: 4 One: 3 Two: 3 Three: 3 One: 2 Two: 2 Three: 2 One: 1 Two: 1 Three: 1 Two exiting. Three exiting. One exiting. Thread One is alive: false Thread Two is alive: false Thread Three is alive: false Main thread exiting.

如你所见，调用join()后返回，线程终止执行。