【java并发】CountDownLatch

本文主要是介绍【java并发】CountDownLatch，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

【应用场景】

有这样一种业务需求：需要几个线程完成之后，主线程才能继续。举个例子：车辆出厂，一辆车出厂，需要生产各个配件，并且安装，都完成后还需要质检，最后才可以出厂。类似这样的需求，我们可以考虑使用CountDownLatch来控制并发。

【CountDownLatch】

java.util.concurrent包有这样一个类，类似计数器，其内部维护着一个加锁的count计数，CountDownLatch通过构造函数传入一个初始计数值，调用cutDown()方法使count减1；调用await()方法，则线程就会一直阻塞，直到其他线程通过cutDown方法，将计数减到0，才可以继续执行。

public class CountDownLatch {    private static final class Sync extends AbstractQueuedSynchronizer {private static final long serialVersionUID = 4982264981922014374L;Sync(int count) {setState(count);}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;}}}private final Sync sync;public CountDownLatch(int count) {if (count < 0) throw new IllegalArgumentException("count < 0");this.sync = new Sync(count);}  public void await() throws InterruptedException {sync.acquireSharedInterruptibly(1);}  public boolean await(long timeout, TimeUnit unit)throws InterruptedException {return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));}public void countDown() {sync.releaseShared(1);}public long getCount() {return sync.getCount();}public String toString() {return super.toString() + "[Count = " + sync.getCount() + "]";}
}

【应用实现】

package zheng;import java.util.concurrent.CountDownLatch;public class test {public enum EnumTest {发动机, 底盘, 车身, 其他;// 线程名称}public static void main(String[] args) {final CountDownLatch countDownLatch = new CountDownLatch(3);new Thread(new Runnable() {@Overridepublic void run() {System.out.println("主线程:正在等待车辆质检");try {countDownLatch.await();System.out.println("发动机、底盘、车身、其他,已配置好, 车辆出厂啦");} catch (InterruptedException e) {e.printStackTrace();}}}).start();for (EnumTest threadName : EnumTest.values()) {final String tN = String.valueOf(threadName);new Thread(new Runnable() {@Overridepublic void run() {System.out.println(tN + " 正在配置");try {Thread.sleep(100);} catch (Exception e) {e.printStackTrace();}System.out.println(tN + " 安装完成");countDownLatch.countDown();}}).start();}}}

运行结果：