通过一道面试题了解Condition线程通信

Condition

• Condition接口描述了可能会与锁有关联的条件变量。这些变量在用法与使用Object.wait访问的隐式监视器类似，但提供了更强大的功能。需要特别指出的是，单个Lock可能与多个Condition对象关联。为了避免兼容性问题，Condition方法的名称与对应的Object版本中的不同；
• 在Condition对象中，与wait、notify和notifyAll方法对应的分别是await、signal和signalAll；
• Condition实例实质上被绑定到一个锁上。要为特定Lock实例获得Condition实例，请使用器newCondition()方法。

面试题

 package com.ccfdod.juc;

 import java.util.concurrent.locks.Condition;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;

 /**
 * 题目：编写一个程序，开启 3 个线程，这三个线程的 ID 分别为 A、B、C，每个线程将自己的 ID 在屏幕上打印 10 遍，要求输出的结果必须按顺序显示。
 * 如：ABCABCABC……
 */
 class AlternateDemo {
     //标志当前由哪一个线程输出，1代表A，2代表B，3代表C
     private int number = 1;

     Lock lock = new ReentrantLock();

     //Condition的强大之处在于它可以为多个线程间建立不同的Condition
     Condition condition1 = lock.newCondition();
     Condition condition2 = lock.newCondition();
     Condition condition3 = lock.newCondition();

     // loopNum:当前循环轮数
     public void loopA(int loopNum) {
         //上锁
         lock.lock();
         try {
             while (number != 1) {
                 //等待
                 condition1.await();
             }

             System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);
             number = 2;
             //唤醒
             condition2.signal();

         } catch (InterruptedException e) {
             e.printStackTrace();
         } finally {
             //确保释放锁
             lock.unlock();
         }
     }

     public void loopB(int loopNum) {
         lock.lock();
         try {
             while (number != 2) {
                 condition2.await();
             }

             System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);
             number = 3;
             condition3.signal();

         } catch (InterruptedException e) {
             e.printStackTrace();
         } finally {
             lock.unlock();
         }
     }

     public void loopC(int loopNum) {
         lock.lock();
         try {
             while (number != 3) {
                 condition3.await();
             }

             System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);
             number = 1;
             condition1.signal();

         } catch (InterruptedException e) {
             e.printStackTrace();
         } finally {
             lock.unlock();
         }
     }
 }

 public class TestABCAlternate {

     public static void main(String[] args) {
         AlternateDemo ad = new AlternateDemo();

         new Thread(new Runnable() {

             @Override
             public void run() {
                 for (int i = 0; i < 10; i++)
                     ad.loopA(i);
             }
         }, "A").start();

         new Thread(new Runnable() {

             @Override
             public void run() {
                 for (int i = 0; i < 10; i++)
                     ad.loopB(i);
             }
         }, "B").start();

         new Thread(new Runnable() {

             @Override
             public void run() {
                 for (int i = 0; i < 10; i++)
                     ad.loopC(i);
             }
         }, "C").start();
     }
 }