CountDownLatch基于AQS阻塞工具用法详解

package com.conrrentcy.juc;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
public class CountDownLatchExample {
    private static final Logger log = LoggerFactory.getLogger(CountDownLatchExample.class);
     //线程数量
    private static final int THREAD_NUM = 10;
    // CountdownLatch阻塞模拟
    public static void main(String[] args) throws InterruptedException {
        // 创建线程池 用于执行线程
        ExecutorService executorService = Executors.newCachedThreadPool();
        //创建countDownLatch
        final CountDownLatch countDownLatch = new CountDownLatch(THREAD_NUM);
        long startTime = System.currentTimeMillis();
        //循环创建线程
        for (int i = 0; i < THREAD_NUM; i++) {
            final int a = i;
            executorService.execute(() -> {
                try {
                    test(a);
                } catch (Exception e) {
                    log.error("Exception", e);
                } finally {
                    countDownLatch.countDown();
                }
            });
        }
        countDownLatch.await();
        long endTime = System.currentTimeMillis();
        log.info("执行完毕,{}-{}",startTime,endTime);
        executorService.shutdown();
    }
    private static void test(int num) throws InterruptedException {
        Thread.sleep(100);
        log.info("{}-{}", num,System.currentTimeMillis());
        Thread.sleep(100);
    }
}

//当前对象中私有阻塞工具
private final Sync sync;
// 模板方法模式重写AQS工具
private static final class Sync extends AbstractQueuedSynchronizer {
   private static final long serialVersionUID = 4982264981922014374L;
   // 共享阻塞AQS
   Sync(int count) {
       setState(count);
   }
   // 获取当前还剩多少资源可以使用
   int getCount() {
       return getState();
   }
   protected int tryAcquireShared(int acquires) {
       return (getState() == 0) ? 1 : -1;
   }
   protected boolean tryReleaseShared(int releases) {
       for (;;) {
           int c = getState();
           if (c == 0)
               return false;
           int nextc = c-1;
           if (compareAndSetState(c, nextc))
               return nextc == 0;
       }
   }
}
//构造方法创建一个锁对象
public CountDownLatch(int count) {
   if (count < 0) throw new IllegalArgumentException("count < 0");
   this.sync = new Sync(count);
}

// 该方法时释放一个共享锁。当所有锁都被释放完成后主线程就能继续执行了。
public void countDown() {
    sync.releaseShared(1);
}

//拦截主线程的方法。主线程在这里等待条件达成后继续执行。
public void await() throws InterruptedException {
    //在这里阻塞线程的执行
    sync.acquireSharedInterruptibly(1);
}
public final void acquireSharedInterruptibly(int arg)
       throws InterruptedException {
   if (Thread.interrupted())
       throw new InterruptedException();
    //这里判断是否还有可以共享的资源
   // 如果有则返回-1 否则返回 1，重写AQS的方法参见（1.AQS框架以及构造方法）
   if (tryAcquireShared(arg) < 0)
       // 有资源则运行阻塞自旋等待所有线程执行完毕
       doAcquireSharedInterruptibly(arg);
   // 无资源可用就让线程继续执行
}
// 带延迟的减少数据拦截方法
// 返回的结果是没有跑完全部线程就继续执行下一步了。
public boolean await(long timeout, TimeUnit unit)
    throws InterruptedException {
    return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
    throws InterruptedException {
    //线程如果被中断则抛出异常
    if (Thread.interrupted())
        throw new InterruptedException();
    // 表示如果线程被执行完了直接返回成功，如果没有执行完则看等待时间来决定是否要继续执行。
    return tryAcquireShared(arg) >= 0 ||
        doAcquireSharedNanos(arg, nanosTimeout);
}

    private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }