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：

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：

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。

// 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：

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。

转载于:https://www.cnblogs.com/cyl048/p/8488349.html