睡觉并检查，直到情况成立

本文介绍了睡觉并检查，直到情况成立的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

Java中是否有一个库可以执行以下操作？一个线程应该重复 sleep x毫秒，直到条件变为真或达到最大时间。

Is there a library in Java that does the following? A thread should repeatedly sleep for x milliseconds until a condition becomes true or the max time is reached.

这种情况主要发生在测试等待某些条件变为真时。条件受另一个线程的影响。

This situation mostly occurs when a test waits for some condition to become true. The condition is influenced by another thread.

为了让它更清晰，我希望测试到在失败之前只等待X ms。它不能永远等待条件成为现实。我正在添加一个人为的例子。

Just to make it clearer, I want the test to wait for only X ms before it fails. It cannot wait forever for a condition to become true. I am adding a contrived example.

class StateHolder{ boolean active = false; StateHolder(){ new Thread(new Runnable(){ public void run(){ active = true; } }, "State-Changer").start() } boolean isActive(){ return active; } } class StateHolderTest{ @Test public void shouldTurnActive(){ StateHolder holder = new StateHolder(); assertTrue(holder.isActive); // i want this call not to fail } }

推荐答案

编辑

大多数答案都集中在低级API上，包括等待和通知或条件（更多或更多或不一样的方式）：当你不习惯它时，很难做到正确。证明：其中2个答案没有正确使用等待。 java.util.concurrent 为您提供了一个高级API，其中隐藏了所有这些错综复杂的内容。

Most answers focus on the low level API with waits and notifies or Conditions (which work more or less the same way): it is difficult to get right when you are not used to it. Proof: 2 of these answers don't use wait correctly. java.util.concurrent offers you a high level API where all those intricacies have been hidden.

恕我直言，当并发包中有一个内置类实现相同的功能时，没有必要使用等待/通知模式。

IMHO, there is no point using a wait/notify pattern when there is a built-in class in the concurrent package that achieves the same.

A CountDownLatch 就是这样：

• 当条件成立时，在等待的线程中调用 latch.countdown（）;
• ，使用： boolean ok = latch.await （1，TimeUnit.SECONDS）;

• When the condition becomes true, call latch.countdown();
• in your waiting thread, use : boolean ok = latch.await(1, TimeUnit.SECONDS);

Contrived example：

Contrived example:

final CountDownLatch done = new CountDownLatch(1); new Thread(new Runnable() { @Override public void run() { longProcessing(); done.countDown(); } }).start(); //in your waiting thread: boolean processingCompleteWithin1Second = done.await(1, TimeUnit.SECONDS);

注意：CountDownLatches是线程安全的。

Note: CountDownLatches are thread safe.