正如每個Java文檔所描述的那樣，CountDownLatch是一個同步工具類，它允許一個或多個線程一直等待，直到其他線程的操作執行完后再執行。

CountDownLatch是在java1.5被引入的，跟它一起被引入的并發工具類還有CyclicBarrier、Semaphore、ConcurrentHashMap和BlockingQueue，它們都存在于java.util.concurrent包下。CountDownLatch這個類能夠使一個線程等待其他線程完成各自的工作后再執行。例如，應用程序的主線程希望在負責啟動框架服務的線程已經啟動所有的框架服務之后再執行。

CountDownLatch是通過一個計數器來實現的，計數器的初始值為線程的數量。每當一個線程完成了自己的任務后，計數器的值就會減1。當計數器值到達0時，它表示所有的線程已經完成了任務，然后在閉鎖上等待的線程就可以恢復執行任務。

CountDownLatch的方法

//可中斷等待 public void await() throws InterruptedException; //等待直到超時或者中斷 public boolean await(long timeout, TimeUnit unit)throws InterruptedException; //計數器減1 public void countDown();

CountDownLatch的應用

控制線程同時執行

package main.java.study;

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.CountDownLatch;

public class CountDownLatchTest {

?? ?public class MapOper implements Runnable {
?? ??? ?
?? ???? CountDownLatch latch ;

?? ???? public MapOper(CountDownLatch latch) {
?? ???????? this.latch = latch;
?? ???? }

?? ???? public void run() {
?? ???????? try {
?? ??????? ??? ?
?? ??????? ??? ? SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
?? ???????????? System.out.println(Thread.currentThread().getName() + "start:" + df.format(new Date()));
?? ??????????? ?
?? ???????????? latch.await();
?? ???????????? System.out.println(Thread.currentThread().getName() + "work:" + df.format(new Date()));
?? ??????????? ?
?? ???????? } catch (InterruptedException e) {
?? ???????????? // TODO Auto-generated catch block
?? ???????????? e.printStackTrace();
?? ???????? }
?? ???????? System.out.println(Thread.currentThread().getName()+" Sync Started!");
?? ???? }
?? ?}

?? ?
?? ?public static void main(String[] args) throws InterruptedException {
?? ??? ?// TODO Auto-generated method stub
?? ??? ?CountDownLatchTest test = new CountDownLatchTest();
?? ??? ?
?? ??? ?CountDownLatch latch = new CountDownLatch(1);
?? ??? ?
?? ??? ?Thread t1 = new Thread(test.new MapOper(latch));
?? ??? ?Thread t2 = new Thread(test.new MapOper(latch));
?? ??? ?Thread t3 = new Thread(test.new MapOper(latch));
?? ??? ?Thread t4 = new Thread(test.new MapOper(latch));
?? ??? ?
?? ??? ?t1.setName("Thread1");
?? ??? ?t2.setName("Thread2");
?? ??? ?t3.setName("Thread3");
?? ??? ?t4.setName("Thread4");
?? ??? ?
?? ??? ?t1.start();
?? ??? ?Thread.sleep(1500);
?? ??? ?t2.start();
?? ??? ?Thread.sleep(1500);
?? ??? ?t3.start();
?? ??? ?Thread.sleep(1500);
?? ??? ?t4.start();
?? ??? ?
?? ??? ?
?? ??? ?System.out.println("thread already start, sleep for a while...");
?? ??? ?
?? ??? ?Thread.sleep(1000);
?? ??? ?latch.countDown();

?? ?}
?? ?
?? ?

}

執行結果：線程在不同時刻啟動，但是等待后在同一時刻工作。

Thread1start:2019-05-25 13:24:01
Thread2start:2019-05-25 13:24:02
Thread3start:2019-05-25 13:24:04
thread already start, sleep for a while...
Thread4start:2019-05-25 13:24:05
Thread1work:2019-05-25 13:24:06
Thread1 Sync Started!
Thread2work:2019-05-25 13:24:06
Thread2 Sync Started!
Thread3work:2019-05-25 13:24:06
Thread3 Sync Started!
Thread4work:2019-05-25 13:24:06
Thread4 Sync Started!

CountDownLatch的原理

調用countDownLatch.await()方法的線程，它會處于掛起狀態，直到所有的線程都執行完countDownLatch.countDown方法，最終將計數器減為0，才會被喚醒繼續執行。

CountDownLatch的源碼

private static final class Sync extends AbstractQueuedSynchronizer {private static final long serialVersionUID = 4982264981922014374L;Sync(int count) {setState(count);}int getCount() {return getState();}protected int tryAcquireShared(int acquires) {//只有state為0時，才返回1，其他返回-1，小于0則 表示acquire失敗。return (getState() == 0) ? 1 : -1;}protected boolean tryReleaseShared(int releases) {// Decrement count; signal when transition to zerofor (;;) {int c = getState();if (c == 0)return false;int nextc = c-1;//每次state減1if (compareAndSetState(c, nextc))return nextc == 0;}}} public void await() throws InterruptedException {sync.acquireSharedInterruptibly(1);}//countDown，release(1)public void countDown() {sync.releaseShared(1);}

AbstractQueuedSynchronizer相關源碼參考：https://blog.csdn.net/demon7552003/article/details/90105335

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