Java知多少（60）isAlive()和join()的使用

如前所述，通常你希望主线程最后结束。在前面的例子中，这点是通过在main()中调用sleep()来实现的，经过足够长时间的延迟以确保所有子线程都先于主线程结束。然而，这不是一个令人满意的解决方法，它也带来一个大问题：一个线程如何知道另一线程已经结束？幸运的是，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。

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Java知多少（上）

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