Java线程编程中isAlive()和join()的使用详解

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：

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1	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：

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1	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。

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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.");   } }

程序输出如下所示：

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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: 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。