「java.util.concurrent并发包9」之 CountDownLatch
一 概述
CountDownLatch.java类中定义的构造函数:
public void CountDownLatch(int count) {...}
构造器中的计数值count实际上就是闭锁需要等待的线程数量。这个值只能被设置一次,而且CountDownLatch没有提供任何机制去重新设置这个计数值。
与CountDownLatch的第一次交互是主线程等待其他线程。主线程必须在启动其他线程后立即调用countDownLatch.await()方法。这样主线程的操作就会在这个方法上阻塞,直到其他线程完成各自的任务。
其他N个线程必须引用闭锁对象,因为他们需要通知CountDownLatch对象已经完成了各自的任务。这种通知机制是通过 countDownLatch.countDown()方法来完成的;每调用一次这个方法,count值就减1。所以当N个线程都调用了这个方法,count的值等于0,然后主线程就能通过await()方法,恢复执行自己的任务。
二 使用场景
1. 实现最大的并行性:有时我们想同时启动多个线程,实现最大程度的并行性。例如,我们想测试一个单例类。如果我们创建一个初始计数为1的CountDownLatch,并让所有线程都在这个锁上等待,那么我们可以很轻松地完成测试。我们只需调用 一次countDown()方法就可以让所有的等待线程同时恢复执行。
2. 开始执行前等待n个线程完成各自任务:例如应用程序启动类要确保在处理用户请求前,所有N个外部系统已经启动和运行了。
(延伸:第二个容易想到,第一个还是挺别致的~)
三 代码实例
描述: 在这个例子中,我们有一个应用调用主类,所有外部系统准备(数据,资源,网络准备等)结束后通知闭锁,主类一致在闭锁等待,直到所有外部准备完成。
抽象类定义: 抽象准备接口(抽象类)
1 public abstract class BasePreparer implements Runnable { 2 3 private CountDownLatch countDownLatch; 4 5 private String prepareName; 6 7 private boolean prepareDone; 8 9 public BasePreparer(String prepareName, CountDownLatch countDownLatch) { 10 this.countDownLatch = countDownLatch; 11 this.prepareName = prepareName; 12 this.prepareDone = false; 13 } 14 15 protected abstract void prepare(); 16 17 @Override 18 public void run() { 19 try { 20 prepare(); 21 prepareDone = true; 22 } catch (Exception e) { 23 prepareDone = false; 24 } finally { 25 if (countDownLatch != null) { 26 countDownLatch.countDown(); 27 } 28 } 29 } 30 31 public String getPrepareName() { 32 return prepareName; 33 } 34 35 public boolean isPrepareDone() { 36 return prepareDone; 37 } 38 }
实现类1: 模拟数据准备(后面2,3类似,只是成员变量简单变化)
1 public class DataPreparer extends BasePreparer { 2 3 public DataPreparer(CountDownLatch countDownLatch) { 4 super("data prepare", countDownLatch); 5 } 6 7 @Override 8 protected void prepare() { 9 System.out.println(this.getPrepareName() + "is doing"); 10 try { 11 TimeUnit.SECONDS.sleep(1); 12 } catch (InterruptedException e) { 13 e.printStackTrace(); 14 } 15 System.out.println(this.getPrepareName() + "is done"); 16 } 17 }
实现类2: 模拟网络准备
1 public class NetworkPreparer extends BasePreparer { 2 3 public NetworkPreparer(CountDownLatch countDownLatch) { 4 super("network prepare", countDownLatch); 5 } 6 7 @Override 8 protected void prepare() { 9 System.out.println(this.getPrepareName() + "is doing"); 10 try { 11 TimeUnit.SECONDS.sleep(1); 12 } catch (InterruptedException e) { 13 e.printStackTrace(); 14 } 15 System.out.println(this.getPrepareName() + "is done"); 16 } 17 }
实现类3: 模拟资源准备
1 public class ResourcePreparer extends BasePreparer { 2 3 public ResourcePreparer(CountDownLatch countDownLatch) { 4 super("resource prepare", countDownLatch); 5 } 6 7 @Override 8 protected void prepare() { 9 System.out.println(this.getPrepareName() + "is doing"); 10 try { 11 TimeUnit.SECONDS.sleep(1); 12 } catch (InterruptedException e) { 13 e.printStackTrace(); 14 } 15 System.out.println(this.getPrepareName() + "is done"); 16 } 17 }
主调用类
1 public class CountDownLatchMain { 2 3 private static final int LATCH_NUMBER = 3; 4 5 public static void main(String[] args) throws InterruptedException { 6 7 List<BasePreparer> preparerList = Lists.newArrayListWithExpectedSize(LATCH_NUMBER); 8 CountDownLatch countDownLatch = new CountDownLatch(LATCH_NUMBER); 9 10 preparerList.add(new ResourcePreparer(countDownLatch)); 11 preparerList.add(new DataPreparer(countDownLatch)); 12 preparerList.add(new NetworkPreparer(countDownLatch)); 13 14 ExecutorService executorService = Executors.newFixedThreadPool(LATCH_NUMBER); 15 preparerList.forEach(executorService::execute); 16 countDownLatch.await(); 17 18 List<BasePreparer> readyList = preparerList.stream().filter(BasePreparer::isPrepareDone).collect(Collectors.toList()); 19 System.out.println(readyList.size() == LATCH_NUMBER); 20 21 } 22 }
运行结果
1 resource prepareis doing 2 network prepareis doing 3 data prepareis doing 4 network prepareis done 5 data prepareis done 6 resource prepareis done 7 true