如前所述，通常你希望主線程最後結束。在前面的例子中，這點是通過在main()中調用sleep()來實現的，經過足夠長時間的延遲以確保所有子線程都先於主線程結束。然而，這不是一個令人滿意的解決方法，它也帶來一個大問題：一個線程如何知道另一線程已經結束？幸運的是，Thread類提供了回答此問題的方法。

有兩種方法可以判定一個線程是否結束。第一，可以在線程中調用isAlive()。這種方法由Thread定義，它的通常形式如下：
final boolean isAlive( )

如果所調用線程仍在運行，isAlive()方法返回true，如果不是則返回false。但isAlive（）很少用到，等待線程結束的更常用的方法是調用join()，描述如下：
final void join( ) throws InterruptedException

該方法等待所調用線程結束。該名字來自於要求線程等待直到指定線程參與的概念。join()的附加形式允許給等待指定線程結束定義一個最大時間。下面是前面例子的改進版本。運用join()以確保主線程最後結束。同樣，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 www.weicaiyule.cc ());
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.");
}
}

程序輸出如下所示：
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.

如你所見，調用join()後返回，線程終止執行。

七. 多線程編程6.isAlive()和join()的使用