通過一道面試題瞭解Condition線程通信

Condition

• Condition介面描述了可能會與鎖有關聯的條件變數。這些變數在用法與使用Object.wait訪問的隱式監視器類似，但提供了更強大的功能。需要特別指出的是，單個Lock可能與多個Condition對象關聯。為了避免相容性問題，Condition方法的名稱與對應的Object版本中的不同；
• 在Condition對象中，與wait、notify和notifyAll方法對應的分別是await、signal和signalAll；
• Condition實例實質上被綁定到一個鎖上。要為特定Lock實例獲得Condition實例，請使用器newCondition()方法。

面試題

  1 package com.ccfdod.juc;
  2 
  3 import java.util.concurrent.locks.Condition;
  4 import java.util.concurrent.locks.Lock;
  5 import java.util.concurrent.locks.ReentrantLock;
  6 
  7 /**
  8 * 題目：編寫一個程式，開啟 3 個線程，這三個線程的 ID 分別為 A、B、C，每個線程將自己的 ID 在屏幕上列印 10 遍，要求輸出的結果必須按順序顯示。
  9 * 如：ABCABCABC……
 10 */
 11 class AlternateDemo {
 12     //標誌當前由哪一個線程輸出，1代表A，2代表B，3代表C
 13     private int number = 1;
 14 
 15     Lock lock = new ReentrantLock();
 16 
 17     //Condition的強大之處在於它可以為多個線程間建立不同的Condition
 18     Condition condition1 = lock.newCondition();
 19     Condition condition2 = lock.newCondition();
 20     Condition condition3 = lock.newCondition();
 21 
 22     // loopNum:當前迴圈輪數
 23     public void loopA(int loopNum) {
 24         //上鎖
 25         lock.lock();
 26         try {
 27             while (number != 1) {
 28                 //等待
 29                 condition1.await();
 30             }
 31 
 32             System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);
 33             number = 2;
 34             //喚醒
 35             condition2.signal();
 36 
 37         } catch (InterruptedException e) {
 38             e.printStackTrace();
 39         } finally {
 40             //確保釋放鎖
 41             lock.unlock();
 42         }
 43     }
 44 
 45     public void loopB(int loopNum) {
 46         lock.lock();
 47         try {
 48             while (number != 2) {
 49                 condition2.await();
 50             }
 51 
 52             System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);
 53             number = 3;
 54             condition3.signal();
 55 
 56         } catch (InterruptedException e) {
 57             e.printStackTrace();
 58         } finally {
 59             lock.unlock();
 60         }
 61     }
 62 
 63     public void loopC(int loopNum) {
 64         lock.lock();
 65         try {
 66             while (number != 3) {
 67                 condition3.await();
 68             }
 69 
 70             System.out.println(Thread.currentThread().getName() + ", currentLoopNum is " + loopNum);
 71             number = 1;
 72             condition1.signal();
 73 
 74         } catch (InterruptedException e) {
 75             e.printStackTrace();
 76         } finally {
 77             lock.unlock();
 78         }
 79     }
 80 }
 81 
 82 public class TestABCAlternate {
 83 
 84     public static void main(String[] args) {
 85         AlternateDemo ad = new AlternateDemo();
 86 
 87         new Thread(new Runnable() {
 88 
 89             @Override
 90             public void run() {
 91                 for (int i = 0; i < 10; i++)
 92                     ad.loopA(i);
 93             }
 94         }, "A").start();
 95 
 96         new Thread(new Runnable() {
 97 
 98             @Override
 99             public void run() {
100                 for (int i = 0; i < 10; i++)
101                     ad.loopB(i);
102             }
103         }, "B").start();
104 
105         new Thread(new Runnable() {
106 
107             @Override
108             public void run() {
109                 for (int i = 0; i < 10; i++)
110                     ad.loopC(i);
111             }
112         }, "C").start();
113     }
114 }