Java线程编程中isAlive()和join()的使用详解

2020-01-07 14:23goldensun JAVA教程

这篇文章主要介绍了Java线程编程中isAlive()和join()的使用详解,是Java入门学习中的基础知识,需要的朋友可以参考下

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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									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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									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.