深浅模式
CountDownLatch一般用作多线程倒计时计数器，强制它们等待其他一组（CountDownLatch的初始化决定）任务执行完成，CountDownLatch初始化后计数器值递减到0的时候，不能再复原的。
源码
java
package java.util.concurrent;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
public class CountDownLatch {
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) {
return (getState() == 0) ? 1 : -1;
}
protected boolean tryReleaseShared(int releases) {
// Decrement count; signal when transition to zero
for (;;) {
int c = getState();
if (c == 0)
return false;
int nextc = c - 1;
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
}
private final Sync sync;
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
public boolean await(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
public void countDown() {
sync.releaseShared(1);
}
public long getCount() {
return sync.getCount();
}
public String toString() {
return super.toString() + "[Count = " + sync.getCount() + "]";
}
}java
public class CountDownLatchModule {
//线程数
private static int N = 10;
// 单位：min
private static int countDownLatchTimeout = 5;
public static void main(String[] args) {
//创建CountDownLatch并设置计数值，该count值可以根据线程数的需要设置
CountDownLatch countDownLatch = new CountDownLatch(N);
//创建线程池
ExecutorService cachedThreadPool = Executors.newCachedThreadPool();
for (int i = 0; i < N; i++) {
cachedThreadPool.execute(() ->{
try {
System.out.println(Thread.currentThread().getName() + " do something!");
} catch (Exception e) {
System.out.println("Exception: do something exception");
} finally {
//该线程执行完毕-1
countDownLatch.countDown();
}
});
}
System.out.println("main thread do something-1");
try {
countDownLatch.await(countDownLatchTimeout, TimeUnit.MINUTES);
} catch (InterruptedException e) {
System.out.println("Exception: await interrupted exception");
} finally {
System.out.println("countDownLatch: " + countDownLatch.toString());
}
System.out.println("main thread do something-2");
//若需要停止线程池可关闭;
// cachedThreadPool.shutdown();
}结果输出：
public void await() throws InterruptedException：调用await()方法的线程会被挂起，等待直到count值为0再继续执行。public boolean await(long timeout, TimeUnit unit) throws InterruptedException：同await()，若等待timeout时长后，count值还是没有变为0，不再等待，继续执行。时间单位如下常用的毫秒、天、小时、微秒、分钟、纳秒、秒。public void countDown()： count值递减1。public long getCount()：获取当前count值。public String toString()：重写了toString()方法，多打印了count值。
CountDownLatch使用场景
一个程序中有N个任务在执行，我们可以创建值为N的CountDownLatch，当每个任务完成后，调用一下countDown()方法进行递减count值，再在主线程中使用await()方法等待任务执行完成，主线程继续执行。
****作为线程启动信号。
java
@Test
void test2() throws InterruptedException {
CountDownLatch startSignal = new CountDownLatch(1);
CountDownLatch doneSignal = new CountDownLatch(10);
for (int i = 0; i < 10; i++) {
// create and start threads
new Thread(new Worker(startSignal, doneSignal)).start();
}
// don't let run yet
System.out.println("do something else 1");
// let all threads proceed
startSignal.countDown();
System.out.println("do something else 2");
// wait for all to finish
doneSignal.await();
System.out.println("wait for all to finsh");
}
static class Worker implements Runnable {
private final CountDownLatch startSignal;
private final CountDownLatch doneSignal;
Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
this.startSignal = startSignal;
this.doneSignal = doneSignal;
}
@Override
public void run() {
try {
this.startSignal.await();
doWork();
this.doneSignal.countDown();
} catch (InterruptedException ex) {
ex.printStackTrace();
}
}
void doWork() {
System.out.println("do work!");
}
}结果输出
java
do something else 1
do work!
do work!
do work!
do work!
do work!
do work!
do something else 2
do work!
do work!
do work!
do work!
wait for all to finsh
- 主线程先打印
do something else 1和do something else 2。因为startSignal.countDown();完后，count才为0，子线程才能打印。- 因为
startSignal.await();是在子线程内，所有子线程都等待startSignal.countDown()执行后才能打印do work!。doneSignal.await();等待所有子线程执行后，每次都doneSignal.countDown()，最后count为0，主线程才执行打印wait for all to finsh。
****作为线程等待完成信号。
java
@Test
void test3() throws InterruptedException {
CountDownLatch doneSignal = new CountDownLatch(5);
ExecutorService cachedThreadPool = Executors.newCachedThreadPool();
for (int i = 0; i < 10; i++) {
// create and start threads
cachedThreadPool.execute(new Work(doneSignal, i));
}
// don't let run yet
System.out.println("do something else 1");
// wait for all to finish
doneSignal.await();
System.out.println("===========================count: " + doneSignal.getCount());
System.out.println("do something else 2");
cachedThreadPool.shutdown();
}
static class Work implements Runnable {
private final CountDownLatch doneSignal;
private final int i;
Work(CountDownLatch doneSignal, int i) {
this.doneSignal = doneSignal;
this.i = i;
}
@Override
public void run() {
try {
doWork();
this.doneSignal.countDown();
System.out.println("i = " + this.i + ", " + this.doneSignal.toString());
} catch (Exception ex) {
ex.printStackTrace();
}
}
void doWork() {
System.out.println("do work!");
}
}结果输出
java
do work!
do work!
do work!
do work!
do work!
do work!
i = 3, java.util.concurrent.CountDownLatch@707128b6[Count = 1]
i = 5, java.util.concurrent.CountDownLatch@707128b6[Count = 0]
i = 4, java.util.concurrent.CountDownLatch@707128b6[Count = 0]
i = 2, java.util.concurrent.CountDownLatch@707128b6[Count = 2]
do work!
i = 6, java.util.concurrent.CountDownLatch@707128b6[Count = 0]
i = 0, java.util.concurrent.CountDownLatch@707128b6[Count = 3]
i = 1, java.util.concurrent.CountDownLatch@707128b6[Count = 3]
do work!
i = 7, java.util.concurrent.CountDownLatch@707128b6[Count = 0]
do work!
i = 8, java.util.concurrent.CountDownLatch@707128b6[Count = 0]
do something else 1
do work!
i = 9, java.util.concurrent.CountDownLatch@707128b6[Count = 0]
===========================count: 0
do something else 2主线程是等待其他线程运行了5次结束后就打印了
do something else 2信息，因为CountDownLatch数值为5。