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.");
}
}
这个来自Java Doc的例子帮助我清楚地理解了这些概念:
class Driver { // ...
void main() throws InterruptedException {
CountDownLatch startSignal = new CountDownLatch(1);
CountDownLatch doneSignal = new CountDownLatch(N);
for (int i = 0; i < N; ++i) // create and start threads
new Thread(new Worker(startSignal, doneSignal)).start();
doSomethingElse(); // don't let run yet
startSignal.countDown(); // let all threads proceed
doSomethingElse();
doneSignal.await(); // wait for all to finish
}
}
class Worker implements Runnable {
private final CountDownLatch startSignal;
private final CountDownLatch doneSignal;
Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
this.startSignal = startSignal;
this.doneSignal = doneSignal;
}
public void run() {
try {
startSignal.await();
doWork();
doneSignal.countDown();
} catch (InterruptedException ex) {} // return;
}
void doWork() { ... }
}
视觉解释:
显然，CountDownLatch允许一个线程(这里是Driver)等待一堆正在运行的线程(这里是Worker)完成它们的执行。
CountDownLatch在Java中是一种同步器，它允许一个线程在开始处理之前等待一个或多个线程。
CountDownLatch工作在闩锁原理，线程将等待直到门打开。在创建CountDownLatch时，一个线程等待指定的n个线程。
例如:final CountDownLatch = new CountDownLatch(3);
这里我们将计数器设置为3。
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为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工作后调用CountDownLatch.countDown()来进行倒计时。一旦count达到0，等待的线程就开始运行。
这里的计数是通过CountDownLatch.countDown()方法递减的。
调用await()方法的线程将一直等待，直到初始计数为零。
为了使计数为零，其他线程需要调用countDown()方法。
一旦计数为零，调用await()方法的线程将恢复(开始执行)。
CountDownLatch的缺点是它不可重用:一旦计数为零，它就不再可用。
如果你在调用latch.countDown()后添加一些调试，这可能会帮助你更好地理解它的行为。
latch.countDown();
System.out.println("DONE "+this.latch); // Add this debug
输出将显示Count正在递减。这个“计数”实际上是你启动的可运行任务(处理器对象)的数量，其中countDown()没有被调用，因此在主线程调用latch.await()时被阻塞。
DONE java.util.concurrent.CountDownLatch@70e69696[Count = 2]
DONE java.util.concurrent.CountDownLatch@70e69696[Count = 1]
DONE java.util.concurrent.CountDownLatch@70e69696[Count = 0]
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]
NikolaB解释得很好，但是举例有助于理解，所以这里有一个简单的例子…
import java.util.concurrent.*;
public class CountDownLatchExample {
public static class ProcessThread implements Runnable {
CountDownLatch latch;
long workDuration;
String name;
public ProcessThread(String name, CountDownLatch latch, long duration){
this.name= name;
this.latch = latch;
this.workDuration = duration;
}
public void run() {
try {
System.out.println(name +" Processing Something for "+ workDuration/1000 + " Seconds");
Thread.sleep(workDuration);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(name+ "completed its works");
//when task finished.. count down the latch count...
// basically this is same as calling lock object notify(), and object here is latch
latch.countDown();
}
}
public static void main(String[] args) {
// Parent thread creating a latch object
CountDownLatch latch = new CountDownLatch(3);
new Thread(new ProcessThread("Worker1",latch, 2000)).start(); // time in millis.. 2 secs
new Thread(new ProcessThread("Worker2",latch, 6000)).start();//6 secs
new Thread(new ProcessThread("Worker3",latch, 4000)).start();//4 secs
System.out.println("waiting for Children processes to complete....");
try {
//current thread will get notified if all chidren's are done
// and thread will resume from wait() mode.
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("All Process Completed....");
System.out.println("Parent Thread Resuming work....");
}
}
