并发工具CountDownLatch源码分析
CountDownLatch的作用类似于Thread.join()方法,但比join()更加灵活。
它可以等待多个线程(取决于实例化时声明的数量)都达到预期状态或者完成工作以后,通知其他正在等待的线程继续执行。
简单的说,Thread.join()是等待具体的一个线程执行完毕,CountDownLatch等待多个线程。
比如:如果需要统计4个文件中的内容行数,可以用4个线程分别执行,然后用一个线程等待统计结果,最后执行数据汇总。这样场景就适合使用CountDownLatch。
1、CountDownLatch中的内部类
private static final class Sync extends AbstractQueuedSynchronizer { private static final long serialVersionUID = 4982264981922014374L; Sync(int count) { setState(count); // 更新AQS中的state } int getCount() { return getState(); } protected int tryAcquireShared(int acquires) { return (getState() == 0) ? 1 : -1; } protected boolean tryReleaseShared(int releases) { // Decrement count; signal when transition to zero for (;;) { int c = getState(); if (c == 0) return false; int nextc = c-1; if (compareAndSetState(c, nextc)) return nextc == 0; } } }
其实CountDownLatch的机制和ReentrantLock有点像,都是利用AQS(AbstractQueuedSynchronizer)来实现的。
CountDownLatch的内部类Sync继承AQS,重写了tryAcquireShared()方法和tryReleaseShared()方法。这里的重点是CountDownLatch的构造函数需要传入一个int值count,就是等待的线程数。这个count被Sync用来直接更新为AQS中的state。
2、await()等待方法
//CountDownLatch public void await() throws InterruptedException { sync.acquireSharedInterruptibly(1); //强制获取共享变量 } //AQS public final void acquireSharedInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); if (tryAcquireShared(arg) < 0) // 1:调用AQS中的tryAcquireShare()方法时,Sync重写了tryAcquireShared()方法,获取state,判断state是否为0; doAcquireSharedInterruptibly(arg); // 2:如果state不为0,则返回-1,调用该方法,将线程加入队列,挂起线程。 } //Sync @Override protected int tryAcquireShared(int acquires) { return (getState() == 0) ? 1 : -1; } //AQS 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); } }
3、countDown()等待方法
public void countDown() { sync.releaseShared(1); //释放共享变量 } //AQS public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false; } //Sync @Override protected boolean tryReleaseShared(int releases) { // Decrement count; signal when transition to zero for (;;) { int c = getState(); if (c == 0) //线程数量为0,则表示无法再减,表示所有线程都执行完毕,就唤醒等待队列中的线程; return false; int nextc = c-1; //利用CAS算法将state减1; if (compareAndSetState(c, nextc)) return nextc == 0; } }
//插入countDown的原理图
举个例子吧:
public class CountDownLatchTest { private static CountDownLatch countDownLatch = new CountDownLatch(3); private static ThreadPoolExecutor threadPool = new ThreadPoolExecutor(5, 5, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(10)); public static void main(String[] args) { //等待线程 for (int i = 0; i < 2; i++) { String threadName = "等待线程 " + i; threadPool.execute(new Runnable() { @Override public void run() { try { System.out.println(threadName + " 正在等待..."); //等待 countDownLatch.await(); System.out.println(threadName + " 结束等待..."); } catch (InterruptedException e) { e.printStackTrace(); } } }); } //工作线程 for (int i = 2; i < 5; i++) { String threadName = "工作线程 " + i; threadPool.execute(new Runnable() { @Override public void run() { try { System.out.println(threadName + " 进入..."); //沉睡1秒 TimeUnit.MILLISECONDS.sleep(1000); System.out.println(threadName + " 完成..."); //通知 countDownLatch.countDown(); } catch (InterruptedException e) { e.printStackTrace(); } } }); } threadPool.shutdown(); } }
执行结果:
等待线程 1 正在等待... 等待线程 0 正在等待... 工作线程 2 进入... 工作线程 3 进入... 工作线程 4 进入... 工作线程 3 完成... 工作线程 2 完成... 工作线程 4 完成... 等待线程 0 结束等待... 等待线程 1 结束等待...
从结果也能看到,等待线程先执行,调用countDownLatch.await()方法开始等待。
每个工作线程工作完成以后,都调用countDownLatch.countDown()方法,告知自己的任务完成。countDownLatch初始参数为3,所以3个工作线程都告知自己结束以后,等待线程才开始工作。
参考:
https://www.cnblogs.com/sunshine-ground-poems/p/10384453.html
Over......