重复测试条件直到它变为真的过程称为轮询Polling?。
轮询通常在循环的帮助下实现，以检查特定条件是否为真。如果是，则采取某些行动。这浪费了许多CPU周期并使实现效率低下。
例如，在一个经典的排队问题中，一个线程正在生成数据而另一个正在使用它。
为了避免轮询，Java使用三种方法，即wait（），notify（）和notifyAll（）。
所有这些方法都属于对象类，因此所有类都有它们。它们必须仅在同步块中使用。
请注意，这个程序可能只在离线IDE中运行，因为它包含在几个点上输入。
// Java program to demonstrate inter-thread communication
// (wait(), join() and notify()) in Java
import java.util.Scanner;
public class threadexample
{
????public static void main(String[] args)
???????????????????????????throws InterruptedException
????{
????????final PC pc = new PC();
????????// Create a thread object that calls pc.produce()
????????Thread t1 = new Thread(new Runnable()
????????{
????????????@Override
????????????public void run()
????????????{
????????????????try
????????????????{
????????????????????pc.produce();
????????????????}
????????????????catch(InterruptedException e)
????????????????{
????????????????????e.printStackTrace();
????????????????}
????????????}
????????});
????????// Create another thread object that calls
????????// pc.consume()
????????Thread t2 = new Thread(new Runnable()
????????{
????????????@Override
????????????public void run()
????????????{
????????????????try
????????????????{
????????????????????pc.consume();
????????????????}
????????????????catch(InterruptedException e)
????????????????{
????????????????????e.printStackTrace();
????????????????}
????????????}
????????});
????????// Start both threads
????????t1.start();
????????t2.start();
????????// t1 finishes before t2
????????t1.join();
????????t2.join();
????}
????// PC (Produce Consumer) class with produce() and
????// consume() methods.
????public static class PC
????{
????????// Prints a string and waits for consume()
????????public void produce()throws InterruptedException
????????{
????????????// synchronized block ensures only one thread
????????????// running at a time.
????????????synchronized(this)
????????????{
????????????????System.out.println("producer thread running");
????????????????// releases the lock on shared resource
????????????????wait();
????????????????// and waits till some other method invokes notify().
????????????????System.out.println("Resumed");
????????????}
????????}
????????// Sleeps for some time and waits for a key press. After key
????????// is pressed, it notifies produce().
????????public void consume()throws InterruptedException
????????{
????????????// this makes the produce thread to run first.
????????????Thread.sleep(1000);
????????????Scanner s = new Scanner(System.in);
????????????// synchronized block ensures only one thread
????????????// running at a time.
????????????synchronized(this)
????????????{
????????????????System.out.println("Waiting for return key.");
????????????????s.nextLine();
????????????????System.out.println("Return key pressed");
????????????????// notifies the produce thread that it
????????????????// can wake up.
????????????????notify();
????????????????// Sleep
????????????????Thread.sleep(2000);
????????????}
????????}
????}
}
输出：
producer thread running Waiting for return key. Return key pressed Resumed
虽然看起来很怪异，但如果你经历两次它真的是小菜一碟。
让我们了解我们的产品和消费方法是如何工作的。
如果您仍然对我们在使用消费线程中使用notify的原因感到困惑，请尝试删除它并再次运行程序。正如您现在必须注意到的，该程序永远不会终止。
原因很简单 - 当您在生产线程上调用wait时，它继续等待并且从未终止。由于程序运行直到其所有线程都已终止，因此它会一直运行。
这个问题还有第二种方法。您可以使用wait（）的第二个变体。
void wait（long timeout）
这将使调用线程仅在指定的时间内休眠。
??Java 多线程原文：https://www.cnblogs.com/breakyizhan/p/13287111.html