Java線程編程中isAlive()和join()的使用詳解

更新時間：2015年09月29日 15:34:13 投稿：goldensun

這篇文章主要介紹了Java線程編程中isAlive()和join()的使用詳解,是Java入門學習中的基礎(chǔ)知識,需要的朋友可以參考下

一個線程如何知道另一線程已經(jīng)結(jié)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(jié)束。第一，可以在線程中調(diào)用isAlive()。這種方法由Thread定義，它的通常形式如下：

  final boolean isAlive( )

如果所調(diào)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(jié)束的更常用的方法是調(diào)用join()，描述如下：

  final void join( ) throws InterruptedException

該方法等待所調(diào)用線程結(jié)束。該名字來自于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(jié)束定義一個最大時間。下面是前面例子的改進版本。運用join()以確保主線程最后結(jié)束。同樣，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.");
  }
}

程序輸出如下所示：

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.

如你所見，調(diào)用join()后返回，線程終止執(zhí)行。

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