Java線程編程中isAlive()和join()的使用詳解

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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，可以線上程中調用isAlive()。這種方法由Thread定義，它的通常形式如下：

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1	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：

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1	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，它也示範了isAlive()方法。

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// 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.");  }}

程式輸出如下所示：

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New thread: Thread[One,5,main]New thread: Thread[Two,5,main]New thread: Thread[Three,5,main]Thread One is alive: trueThread Two is alive: trueThread Three is alive: trueWaiting for threads to finish.One: 5Two: 5Three: 5One: 4Two: 4Three: 4One: 3Two: 3Three: 3One: 2Two: 2Three: 2One: 1Two: 1Three: 1Two exiting.Three exiting.One exiting.Thread One is alive: falseThread Two is alive: falseThread Three is alive: falseMain thread exiting.

如你所見，調用join()後返回，線程終止執行。

Java線程編程中isAlive()和join()的使用詳解