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。