有人能帮助我理解什么是Java CountDownLatch以及什么时候使用它吗?
我不太清楚这个项目是怎么运作的。据我所知，这三个线程同时开始，每个线程将在3000ms后调用CountDownLatch。所以倒数会一个一个递减。锁存变为零后，程序打印“完成”。也许我理解的方式是错误的。
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
class Processor implements Runnable {
private CountDownLatch latch;
public Processor(CountDownLatch latch) {
this.latch = latch;
}
public void run() {
System.out.println("Started.");
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
latch.countDown();
}
}
// -----------------------------------------------------
public class App {
public static void main(String[] args) {
CountDownLatch latch = new CountDownLatch(3); // coundown from 3 to 0
ExecutorService executor = Executors.newFixedThreadPool(3); // 3 Threads in pool
for(int i=0; i < 3; i++) {
executor.submit(new Processor(latch)); // ref to latch. each time call new Processes latch will count down by 1
}
try {
latch.await(); // wait until latch counted down to 0
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Completed.");
}
}
As mentioned in JavaDoc (https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CountDownLatch.html), CountDownLatch is a synchronization aid, introduced in Java 5. Here the synchronization does not mean restricting access to a critical section. But rather sequencing actions of different threads.
The type of synchronization achieved through CountDownLatch is similar to that of Join.
Assume that there is a thread "M" which needs to wait for other worker threads "T1", "T2", "T3" to complete its tasks
Prior to Java 1.5, the way this can be done is, M running the following code
T1.join();
T2.join();
T3.join();
上面的代码确保线程M在T1、T2、T3完成它的工作之后恢复它的工作。T1, T2, T3可以按任意顺序完成它们的工作。
同样可以通过CountDownLatch实现，其中T1、T2、T3和线程M共享相同的CountDownLatch对象。
"M"请求:countDownLatch.await();
其中“T1”，“T2”，“T3”是countDownLatch.countdown();
连接方法的一个缺点是M必须知道T1、T2、T3。如果后来添加了一个新的工作线程T4，那么M也必须知道它。使用CountDownLatch可以避免这种情况。
实现后的动作顺序为[T1,T2,T3](T1,T2,T3的顺序可以任意)-> [M]
来自oracle关于CountDownLatch的文档:
一种同步辅助工具，允许一个或多个线程等待在其他线程中执行的一组操作完成。
CountDownLatch使用给定的计数进行初始化。由于调用了countDown()方法，await方法阻塞直到当前计数为零，在此之后所有等待的线程都被释放，并且await的任何后续调用立即返回。这是一个一次性现象——计数不能重置。
CountDownLatch是一种通用的同步工具，可用于许多目的。
初始化为1的CountDownLatch用作简单的开/关锁存器或门:所有调用await的线程都在门处等待，直到由调用countDown()的线程打开。
初始化为N的CountDownLatch可用于使一个线程等待到N个线程完成某个操作，或者某个操作已完成N次。
public void await()
throws InterruptedException
导致当前线程等待，直到锁存器计数到零，除非线程被中断。
如果当前计数为零，则此方法立即返回。
public void countDown()
减少锁存器的计数，如果计数为零，则释放所有等待线程。
如果当前计数大于零，则递减。如果新的计数为零，则所有等待的线程都将重新启用以进行线程调度。
解释你的例子。
You have set count as 3 for latch variable
CountDownLatch latch = new CountDownLatch(3);
You have passed this shared latch to Worker thread : Processor
Three Runnable instances of Processor have been submitted to ExecutorService executor
Main thread ( App ) is waiting for count to become zero with below statement
latch.await();
Processor thread sleeps for 3 seconds and then it decrements count value with latch.countDown()
First Process instance will change latch count as 2 after it's completion due to latch.countDown().
Second Process instance will change latch count as 1 after it's completion due to latch.countDown().
Third Process instance will change latch count as 0 after it's completion due to latch.countDown().
Zero count on latch causes main thread App to come out from await
App program prints this output now : Completed
是的，你理解对了。
CountDownLatch的工作原理是闩锁，主线程将等待门被打开。一个线程等待n个线程，这是在创建CountDownLatch时指定的。
任何调用CountDownLatch.await()的线程(通常是应用程序的主线程)都将等待，直到count达到零或被另一个线程中断。所有其他线程都需要在完成或准备就绪后调用CountDownLatch.countDown()进行倒计时。
只要count达到0，等待线程就会继续。CountDownLatch的一个缺点/优点是它不可重用:一旦count达到0，就不能再使用CountDownLatch了。
编辑:
当一个线程(如主线程)需要等待一个或多个线程完成后才能继续处理时，使用CountDownLatch。
在Java中使用CountDownLatch的一个经典示例是一个服务器端核心Java应用程序，该应用程序使用服务体系结构，其中多个服务由多个线程提供，在所有服务成功启动之前应用程序不能开始处理。
注:
OP的问题有一个非常简单的例子，所以我没有包括一个。
As mentioned in JavaDoc (https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/CountDownLatch.html), CountDownLatch is a synchronization aid, introduced in Java 5. Here the synchronization does not mean restricting access to a critical section. But rather sequencing actions of different threads.
The type of synchronization achieved through CountDownLatch is similar to that of Join.
Assume that there is a thread "M" which needs to wait for other worker threads "T1", "T2", "T3" to complete its tasks
Prior to Java 1.5, the way this can be done is, M running the following code
T1.join();
T2.join();
T3.join();
上面的代码确保线程M在T1、T2、T3完成它的工作之后恢复它的工作。T1, T2, T3可以按任意顺序完成它们的工作。
同样可以通过CountDownLatch实现，其中T1、T2、T3和线程M共享相同的CountDownLatch对象。
"M"请求:countDownLatch.await();
其中“T1”，“T2”，“T3”是countDownLatch.countdown();
连接方法的一个缺点是M必须知道T1、T2、T3。如果后来添加了一个新的工作线程T4，那么M也必须知道它。使用CountDownLatch可以避免这种情况。
实现后的动作顺序为[T1,T2,T3](T1,T2,T3的顺序可以任意)-> [M]
