一個線程如何知道另一線程已經結束？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。該名字來自于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()以確保主線程最后結束。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());

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()后返回，線程終止執行。

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