一、CountDownLatch是什么？
CountDownLatch是通过一个计数器来实现的，当我们在new 一个CountDownLatch对象的时候需要带入该计数器值，该值就表示了线程的数量。每当一个线程完成自己的任务后，计数器的值就会减1。当计数器的值变为0时，就表示所有的线程均已经完成了任务，然后就可以恢复等待的线程继续执行。
在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。
CountDownlatch与CyclicBarrier的区别：
1.CountDownLatch的作用是允许1或N个线程等待其他线程完成执行； 而CyclicBarrier则是允许N个线程相互等待
2.CountDownLatch的计数器无法被重置；CyclicBarrier的计数器可以被重置后使用，因此它被称为是循环的barrier。
二、代码示例
1.飞机示例
private static CountDownLatchT1 countDownLatch = new CountDownLatchT1(5);
/**
* Plan线程，等待乘客到达起飞
*/
static class PlanThread extends Thread{
@Override
public void run() {
log.info("飞机在等待，总共有" + countDownLatch.getCount() + "个乘客乘坐飞机...");
try {
// 飞机等待
//countDownLatch.await();
countDownLatch.await(10, TimeUnit.SECONDS);
} catch (InterruptedException e) {
e.printStackTrace();
}
log.info("所有乘客都已经到齐了，起飞吧...");
}
}
/**
* 乘客上飞机
*/
static class PeopleThread extends Thread{
@Override
public void run() {
log.info(Thread.currentThread().getName() + "乘客，到达飞机....");
// 乘客到达飞机 count - 1
countDownLatch.countDown();
}
}
public static void main(String[] args) {
// 飞机线程启动
new PlanThread().start();
for(long i = 0,j = countDownLatch.getCount() ; i < j ; i++){
new PeopleThread().start();
}
}
2.源码分析
package com.yl.springboottest.consurrency.countdownlatch;
import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
/**
* 描述:J.U.C之并发工具类：CountDownLatch
*
* @author: yanglin
* @Date: 2020-11-27-10:19
* @Version: 1.0
*/
@Slf4j
public class CountDownLatchT1 {
/**
* Synchronization control For
* Uses AQS state to represent count.
* Sync继承AQS
*
* 同步控制使用AQS状态表示计数。
*/
private static final class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 4982264981922014374L;
Sync(int count) {
setState(count);
}
// 获取同步状态
int getCount() {
return getState();
}
// 获取同步状态
@Override
protected int tryAcquireShared(int acquires) {
// getState()获取同步状态，其值等于计数器的值，从这里我们可以看到如果计数器值不等于0，
// 则会调用doAcquireSharedInterruptibly(int arg)，该方法为一个自旋方法会尝试
// 一直去获取同步状态
return (getState() == 0) ? 1 : -1;
}
// 释放同步状态
// tryReleaseShared(int arg)方法被CountDownLatch的内部类Sync重写：
@Override
protected boolean tryReleaseShared(int releases) {
// Decrement count; signal when transition to zero 减量计数；转换为零时发出信号
for (;;) {
// 获取锁状态
int c = getState();
// c == 0 直接返回，释放锁成功
if (c == 0)
return false;
// 计算新“锁计数器”
int nextc = c-1;
// 更新锁状态（计数器）
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
}
private final Sync sync;
/**
* 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
*
* 构造一个用给定计数初始化的 CountDownLatch
*/
public CountDownLatchT1(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
*
* <p>If the current count is zero then this method returns immediately.
*
* <p>If the current count is greater than zero then the current
* thread becomes disabled for thread scheduling purposes and lies
* dormant until one of two things happen:
* <ul>
* <li>The count reaches zero due to invocations of the
* {@link #countDown} method; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
* </ul>
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* @throws InterruptedException if the current thread is interrupted
* while waiting
*
* 使当前线程在锁存器倒计数至零之前一直等待，除非线程被中断
*/
public void await() throws InterruptedException {
// await其内部使用AQS的acquireSharedInterruptibly(int arg)：
sync.acquireSharedInterruptibly(1);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted},
* or the specified waiting time elapses.
*
* 使当前线程等待，直到闩锁计数到零，除非线程是{@linkplainthread #interrupt interrupted}，
* 或指定的等待时间已过。
*
* <p>If the current count is zero then this method returns immediately
* with the value {@code true}.
*
* <p>If the current count is greater than zero then the current
* thread becomes disabled for thread scheduling purposes and lies
* dormant until one of three things happen:
* <ul>
* <li>The count reaches zero due to invocations of the
* {@link #countDown} method; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>The specified waiting time elapses.
* </ul>
*
* <p>If the count reaches zero then the method returns with the
* value {@code true}.
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p>If the specified waiting time elapses then the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all.
*
* 如果指定的等待时间已经过去，则值{@code false} 返回。如果时间小于或等于零，方法根本不会等。
*
* @param timeout the maximum time to wait 等待的最长时间
* @param unit the time unit of the {@code timeout} argument
* @return {@code true} if the count reached zero and {@code false}
* if the waiting time elapsed before the count reached zero
* @throws InterruptedException if the current thread is interrupted
* while waiting
*/
public boolean await(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
/**
* Decrements the count of the latch, releasing all waiting threads if
* the count reaches zero.
*
* <p>If the current count is greater than zero then it is decremented.
* If the new count is zero then all waiting threads are re-enabled for
* thread scheduling purposes.
*
* <p>If the current count equals zero then nothing happens.
*
* 递减锁存器的计数，如果计数到达零，则释放所有等待的线程。
*/
public void countDown() {
// 内部调用AQS的releaseShared(int arg)方法来释放共享锁同步状态
sync.releaseShared(1);
}
/**
* Returns the current count.
*
* <p>This method is typically used for debugging and testing purposes.
*
* @return the current count
*
* 返回当前计数。
*/
public long getCount() {
return sync.getCount();
}
/**
* Returns a string identifying this latch, as well as its state.
* The state, in brackets, includes the String {@code "Count ="}
* followed by the current count.
*
* @return a string identifying this latch, as well as its state
*/
@Override
public String toString() {
return super.toString() + "[Count = " + sync.getCount() + "]";
}
}
总结
CountDownLatch内部通过共享锁实现。在创建CountDownLatch实例时，需要传递一个int型的参数：count，该参数为计数器的初始值，也可以理解为该共享锁可以获取的总次数。当某个线程调用await() 方法，程序首先判断count的值是否为0，如果不会0的话则会一直等待直到为0为止。当其他线程调用 countDown()方法时，则执行释放共享锁状态，使count值 - 1。当在创建CountDownLatch时初始化的count参数，必须要有count线程调用countDown方法才会使计数器count等于0，锁才会释放，前面等待的线程才会继续运行。注意CountDownLatch不能回滚重置。