CountDownLatch 是什么？
A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
提供一种同步辅助功能，它允许一个或多个线程等待，直到其它线程完成一组操作之后。
CountDownLatch 如何使用？
其实在 java.util.concurrent.CountDownLatch 类注释中已经给了一些典型的用例。
下面有一个需求：主线程中启动一些子线程，接下来主线程会等待所有子线程工作完毕之后继续运行。
测试使用的代码
import java.util.Random;
import java.util.concurrent.CountDownLatch;
import java.util.stream.IntStream;
/**
* @author H
*/
public class CountDownLatchTest {
private static final Random RANDOM = new Random(System.currentTimeMillis());
public static void main(String[] args) throws InterruptedException {
final CountDownLatch countDownLatch = new CountDownLatch(5);
System.out.println("[main] Prepare handle multi-thread tasks.");
IntStream.rangeClosed(1, 5)
.forEach(i ->
new Thread(() -> {
try {
System.out.println("[task-start] " + Thread.currentThread().getName() + " is working.");
Thread.sleep(RANDOM.nextInt(1000));
System.out.println("[task-finished] " + Thread.currentThread().getName() + " is finished.");
} catch (InterruptedException e) {
e.printStackTrace();
}
countDownLatch.countDown();
}, String.valueOf(i)).start()
);
countDownLatch.await();
System.out.println("[main] Finished.");
}
}
// [main] Prepare handle multi-thread tasks.
// [task-start] 2 is working.
// [task-start] 1 is working.
// [task-start] 3 is working.
// [task-start] 4 is working.
// [task-start] 5 is working.
// [task-finished] 1 is finished.
// [task-finished] 5 is finished.
// [task-finished] 2 is finished.
// [task-finished] 3 is finished.
// [task-finished] 4 is finished.
// [main] Finished.
CountDownLatch 的原理是什么？
在 JDK 源码中，CountDownLatch 是通过计数器实现的。 线程会在await() 方法调用处阻塞；每当调用一次 countDown() 方法，那么将会触发一次释放操作，计数器也就会减一，直到由于调用 countDown() 方法而导致计数器达到零为止，之后所有等待的线程都将会被释放。
需要注意，在获取 CountDownLatch 实例的时候需要在构造方法中传入线程的数量。这是一个固定的值并且不能为负数，CountDownLatch 没有提供重置此数量的方法，如果有需要重置操作可以去看看 java.util.concurrent.CyclicBarrier
CountDownLatch 的构造方法
/**
* Constructs a {@code CountDownLatch} initialized with the given count.
*
* @param count the number of times {@link #countDown} must be invoked
* before threads can pass through {@link #await}
* @throws IllegalArgumentException if {@code count} is negative
*/
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
如何自己实现一个 CountDownLatch ？
下面给出一个简易版的 CountDownLatch，每次调用 down() 方法，计数器自加操作，await() 方法调用出一直阻塞，直到计数器等于线程总数才会获得释放。
public class CustomCountDownLatch {
private final int total;
private int counter = 0;
public CustomCountDownLatch(int total) {
this.total = total;
}
public void down() {
synchronized (this) {
counter++;
notifyAll();
}
}
public void await() throws InterruptedException {
synchronized (this) {
while (counter != total) {
wait();
}
}
}
}
测试用例
import java.util.Random;
import java.util.stream.IntStream;
/**
* @author H
*/
public class CustomCountDownLatchTest {
private static final Random RANDOM = new Random(System.currentTimeMillis());
public static void main(String[] args) throws InterruptedException {
final CustomCountDownLatch customCountDownLatch = new CustomCountDownLatch(5);
System.out.println("[main] Prepare handle multi-thread tasks.");
IntStream.rangeClosed(1, 5)
.forEach(i ->
new Thread(() -> {
try {
System.out.println("[task-start] " + Thread.currentThread().getName() + " is working.");
Thread.sleep(RANDOM.nextInt(1000));
System.out.println("[task-finished] " + Thread.currentThread().getName() + " is finished.");
} catch (InterruptedException e) {
e.printStackTrace();
}
customCountDownLatch.down();
}, String.valueOf(i)).start()
);
customCountDownLatch.await();
System.out.println("[main] Finished.");
}
}
// [main] Prepare handle multi-thread tasks.
// [task-start] 2 is working.
// [task-start] 3 is working.
// [task-start] 4 is working.
// [task-start] 5 is working.
// [task-start] 1 is working.
// [task-finished] 4 is finished.
// [task-finished] 2 is finished.
// [task-finished] 3 is finished.
// [task-finished] 1 is finished.
// [task-finished] 5 is finished.
// [main] Finished.
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