java三线程循环有序打印ABC

迅雷笔试题：

编写一个程序，开启3个线程，这3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，要求输出结果必须按ABC的顺序显示；如：ABCABC….依次递推。

解决思路：每个线程运行时先检查他依赖的线程是否已完成工作，线程B依赖线程A的完成，线程C依赖线程B和线程A的完成，线程A依赖线程C的完成。如果当前线程依赖的线程没有执行完，则阻塞当前线程直到条件满足再执行。

Condition.await()会使当前线程暂时阻塞，并释放ReentrantLock锁.

Condition.signalAll()会通知激活Condition.await()而阻塞的线程，这时被激活的线程就会继续检查（通过while循环））是否满足条件，如果满足而且再次获得ReentrantLock锁就能继续运行，否则继续等待。

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

 public class ThreadDemo implements Runnable {

     private ReentrantLock lock = new ReentrantLock();
     private Condition condition = lock.newCondition();
     //标记线程A的状态，true为刚执行完。为什么要两个变量？因为只使用一个变量，当线程A执行完后，a为true，B线程就会不停执行
     //所以线程B执行要执行必须满足a==true&&a2==true
     private volatile Boolean a = false, a2 = false;
     //标记线程B的状态，true为刚执行完。
     private volatile Boolean b = false;
     //标记线程C的状态，true为刚执行完。
     private volatile Boolean c = true;

     @Override
     public void run() {
         String name = Thread.currentThread().getName();
         lock.lock();
         //进入临界区
         try {
             for (int i = 0; i < 10; i++) {
                 if (name.equals("B")) {
                     //只有a和a2同时为true时才打印B，否则阻塞当前线程
                     while (!a || !a2) {
                         condition.await();//条件不满足，暂时阻塞线程，暂时释放lock
                     }
                     b = true;
                     a2 = false;
                 } else if (name.equals("C")) {
                     while (!a || !b) {
                         condition.await();
                     }
                     c = true;
                     b = false;
                 } else if (name.equals("A")) {
                     while (!c) {
                         condition.await();
                     }
                     a = true;
                     a2 = true;
                     b = false;
                     c = false;
                 }
                 System.out.print(name);
                 condition.signalAll();//通知正在等待的线程，此时有可能已经满足条件
             }
         } catch (InterruptedException e) {
             e.printStackTrace();
         } finally {
             lock.unlock();// 记得要释放锁
         }
     }

     public static void main(String[] args) throws InterruptedException {
         ThreadDemo task = new ThreadDemo();
         Thread thread1 = new Thread(task);
         Thread thread2 = new Thread(task);
         Thread thread3 = new Thread(task);
         thread1.setName("A");
         thread2.setName("B");
         thread3.setName("C");
         thread1.start();
         thread2.start();
         thread3.start();
     }

 }

下面是更简单的实现方法：

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

 public class ThreadDemo implements Runnable {

     private ReentrantLock lock = new ReentrantLock();
     private Condition condition = lock.newCondition();
     private int state = 0;

     @Override
     public void run() {
         String name = Thread.currentThread().getName();
         lock.lock();
         // 进入临界区
         try {
             for (int i = 0; i < 10; i++) {
                 if (name.equals("B")) {
                     // 只有a和a2同时为true时才打印B，否则阻塞当前线程
                     while (state % 3 != 1) {
                         condition.await();// 条件不满足，暂时阻塞线程，暂时释放lock
                     }
                 } else if (name.equals("C")) {
                     while (state % 3 != 2) {
                         condition.await();
                     }
                 } else if (name.equals("A")) {
                     while (state % 3 != 0) {
                         condition.await();
                     }
                 }
                 state++;
                 System.out.print(name);
                 condition.signalAll();// 通知正在等待的线程，此时有可能已经满足条件
             }
         } catch (InterruptedException e) {
             e.printStackTrace();
         } finally {
             lock.unlock();// 记得要释放锁
         }
     }

     public static void main(String[] args) throws InterruptedException {
         ThreadDemo task = new ThreadDemo();
         Thread thread1 = new Thread(task);
         Thread thread2 = new Thread(task);
         Thread thread3 = new Thread(task);
         thread1.setName("A");
         thread2.setName("B");
         thread3.setName("C");
         thread1.start();
         thread2.start();
         thread3.start();
     }

 }