Java-JUC（十二）：有3個線程。線程A和線程B並行執行，線程C需要A和B執行完成后才能執行。可以怎么實現？

方案（一）CountDownLatch：

使用CountDownLatch+Semaphore方式實現：

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Semaphore;

public class TestABC {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch=new CountDownLatch(2);
        Semaphore semaphoreC = new Semaphore(1);
        
        Thread threadA = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(newjava.util.Random().nextInt(1000));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName());
                countDownLatch.countDown();
            }
        }, "Thread-A");

        Thread threadB = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(newjava.util.Random().nextInt(1000));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName());
                countDownLatch.countDown();
            }
        }, "Thread-B");

        Thread threadC = new Thread(new Runnable() {
            @Override
            public void run() {                
                try {
                    semaphoreC.acquire();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName());
                semaphoreC.release();
            }
        }, "Thread-C");
        
        // 占用C鎖，直到A/B線程完成后，才釋放C鎖。
        semaphoreC.acquire();
        
        threadA.start();
        threadB.start();
        threadC.start();
        
        countDownLatch.await(); 
        // 釋放C鎖，讓C線程有獲取鎖的可能
        semaphoreC.release();
        
    }
}

上邊使用CountDownLatch+Semaphore方式實現，但是缺點：上邊這種方式會導致線程阻塞情況。下邊這種方案是可以實現不阻塞線程的用法：

import java.util.concurrent.CountDownLatch;

public class TestABC {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch=new CountDownLatch(2);
        
        Thread threadA = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(new java.util.Random().nextInt(1000));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName());
                countDownLatch.countDown();
            }
        }, "Thread-A");

        Thread threadB = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(new java.util.Random().nextInt(1000));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName());
                countDownLatch.countDown();
            }
        }, "Thread-B");

        Thread threadC = new Thread(new Runnable() {
            @Override
            public void run() {    
                // 在C中等待A/B運算結束
                try {
                    countDownLatch.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                    throw new RuntimeException("CountDownLatch等待失敗。。。",e);
                } 
                
                System.out.println(Thread.currentThread().getName());
            }
        }, "Thread-C");
        
        threadA.start();
        threadB.start();
        threadC.start();        
    }
}

方案（二）：CyclicBarrier

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

public class TestABC {
    public static void main(String[] args) throws InterruptedException {
        CyclicBarrier cyclicBarrier=new CyclicBarrier(3);
        
        Thread threadA = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(new java.util.Random().nextInt(1000));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName());

                // 沖破柵欄代表A線程結束
                try {
                    cyclicBarrier.await();
                } catch (InterruptedException | BrokenBarrierException e) {
                    e.printStackTrace();
                    throw new RuntimeException("cylicBarrier.await()拋出異常：",e);
                }
            }
        }, "Thread-A");

        Thread threadB = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(new java.util.Random().nextInt(1000));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName());
                
                // 沖破柵欄代表B線程結束
                try {
                    cyclicBarrier.await();
                } catch (InterruptedException | BrokenBarrierException e) {
                    e.printStackTrace();
                    throw new RuntimeException("cylicBarrier.await()拋出異常：",e);
                }
            }
        }, "Thread-B");

        Thread threadC = new Thread(new Runnable() {
            @Override
            public void run() {
                // 等待前兩個(A/B)線程結束，只有前兩個(A/B)線程結束了才能滿足3個線程都沖破柵欄，
                try {
                    // 等待柵欄被沖破，沖破柵欄的條件是：A/B/C三個線程都到達await()。
                    // 只有柵欄沖破，才能向下執行，否則先到達的線程等待。
                    cyclicBarrier.await();
                } catch (InterruptedException | BrokenBarrierException e) {
                    e.printStackTrace();
                    throw new RuntimeException("cylicBarrier.await()拋出異常：",e);
                }
                // 滿足了三個線程都沖破柵欄才向下執行
                System.out.println(Thread.currentThread().getName());
            }
        }, "Thread-C");
        
        threadA.start();
        threadB.start();
        threadC.start();        
    }
}