CountDownLatch概述
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。
有一点要说明的是CountDownLatch初始化后计数器值递减到0的时候，不能再复原的，这一点区别于Semaphore，Semaphore是可以通过release操作恢复信号量的。
CountDownLatch使用原理
使用原理
创建CountDownLatch并设置计数器值。
启动多线程并且调用CountDownLatch实例的countDown()方法。
主线程调用 await() 方法，这样主线程的操作就会在这个方法上阻塞，直到其他线程完成各自的任务，count值为0，停止阻塞，主线程继续执行。
使用模板
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();
}
运行结果：
main thread do something-1
pool-1-thread-1 do something!
pool-1-thread-2 do something!
pool-1-thread-3 do something!
pool-1-thread-5 do something!
pool-1-thread-6 do something!
pool-1-thread-7 do something!
pool-1-thread-8 do something!
pool-1-thread-4 do something!
pool-1-thread-9 do something!
pool-1-thread-10 do something!
countDownLatch: java.util.concurrent.CountDownLatch@76fb509a[Count = 0]
main thread do something-2
CountDownLatch常用方法
在这里插入图片描述
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。
在这里插入图片描述
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，具体参考源码。
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。
CountDownLatch源码
构造方法源码
/**
* 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);
}
toString()方法源码
/*
* 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
*/
public String toString() {
return super.toString() + "[Count = " + sync.getCount() + "]";
}
CountDownLatch示例
作为线程启动信号
代码
public class CountDownLatchTest {
/**
* a start signal that prevents any worker from proceeding
* until the driver is ready for them to proceed;
* @param args
* @throws InterruptedException
*/
public static void main(String[] args) 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 {
startSignal.await();
doWork();
doneSignal.countDown();
} catch (InterruptedException ex) {
ex.printStackTrace();
}
}
void doWork() {
System.out.println("do work!");
}
}
}
运行结果
do something else 1
do something else 2
do work!
do work!
do work!
do work!
do work!
do work!
do work!
do work!
do work!
do work!
wait for all to finsh
从运行结果可以看出：
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
代码
public class CountDownLatchTest2 {
/**
* a completion signal that allows the driver to wait
* until all workers have completed.
* @param args
* @throws InterruptedException
*/
public static void main(String[] args) 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 Worker(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 Worker implements Runnable{
private final CountDownLatch doneSignal;
private final int i;
Worker(CountDownLatch doneSignal, int i) {
this.doneSignal = doneSignal;
this.i = i;
}
@Override
public void run() {
try {
doWork();
doneSignal.countDown();
System.out.println("i = " + i + ", " + doneSignal.toString());
} catch (Exception ex) {
ex.printStackTrace();
}
}
void doWork() {
System.out.println("do work!");
}
}
}
运行结果
do something else 1
do work!
i = 0, java.util.concurrent.CountDownLatch@128abd43[Count = 4]
do work!
i = 1, java.util.concurrent.CountDownLatch@128abd43[Count = 3]
do work!
i = 2, java.util.concurrent.CountDownLatch@128abd43[Count = 2]
do work!
i = 3, java.util.concurrent.CountDownLatch@128abd43[Count = 1]
do work!
i = 4, java.util.concurrent.CountDownLatch@128abd43[Count = 0]
===========================count: 0
do something else 2
do work!
i = 5, java.util.concurrent.CountDownLatch@128abd43[Count = 0]
do work!
i = 6, java.util.concurrent.CountDownLatch@128abd43[Count = 0]
do work!
i = 7, java.util.concurrent.CountDownLatch@128abd43[Count = 0]
do work!
i = 8, java.util.concurrent.CountDownLatch@128abd43[Count = 0]
do work!
i = 9, java.util.concurrent.CountDownLatch@128abd43[Count = 0]
从运行结果可以看出，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。