如何创建一个等待布尔变量变为true的线程？ [英] How to create a thread that waits for a boolean variable to become true?

问题描述

我有一个函数需要在布尔变量为真时调用。我尝试在线程中使用while循环，但它不起作用。这是我尝试过的：

I have a function that needs to be called once a boolean variable is true. I tried using a while loop in a thread but it doesn't work. Here is what I've tried:

public class MyRunnable implements Runnable {

public void run() {
    while (true) {
         if (conditions == true) { 
             System.out.println("second");
             break;
         }
    }
}

public static void main(String args[]) {
    boolean condition = false;
    (new Thread(new MyRunnable())).start();
    System.out.println("first\n");
    // set conndition to true
    condition = true;

    }

}

结果应该是是：

first
second

推荐答案

请勿忙碌等待以获取此类条件。使用阻塞习语。对于你的简单情况，你会得到一个新的CountDownLatch（1）。首先，这是你的代码，但修复后编译并运行你期望的方式：

Do not busy-wait for such conditions. Use a blocking idiom. For your simple case you would get away with a new CountDownLatch(1). First, here's your code, but fixed to compile and run the way you expect:

public class MyRunnable implements Runnable {
  volatile boolean condition = false;

  public void run() {
    while (true) {
      if (condition) {
        System.out.println("second");
        break;
      }
    }
  }
  public static void main(String args[]) {
    final MyRunnable r = new MyRunnable();
    new Thread(r).start();
    System.out.println("first\n");
    r.condition = true;
  }
}

作为比较，一个<$ c $的程序c> CountDownLatch ：

public class MyRunnable implements Runnable {
  final CountDownLatch latch = new CountDownLatch(1);

  public void run() {
    try { latch.await(); } catch (InterruptedException e) {}
    System.out.println("second");
  }

  public static void main(String args[]) {
    final MyRunnable r = new MyRunnable();
    new Thread(r).start();
    System.out.println("first\n");
    r.latch.countDown();
  }
}

要真正注意到差异，请添加 Thread.sleep（20000）在 println（first）之后，听到声音的差异您的计算机粉丝努力消耗第一个程序浪费的能量。

To truly notice the difference, add a Thread.sleep(20000) after println("first") and hear the difference in the sound of your computer's fan working hard to dissipate the energy the first program is wasting.

这篇关于如何创建一个等待布尔变量变为true的线程？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！