countDownLatch是在java1.5被引入,跟它一起被引入的工具类还有CyclicBarrier、Semaphore、concurrentHashMap和BlockingQueue。
存在于java.util.cucurrent包下
countDownLatch类中只提供了一个构造器:
//参数count为计数值
public CountDownLatch(int count) { };
类中有三个方法是最重要的:
//调用await()方法的线程会被挂起,它会等待直到count值为0才继续执行
public void await() throws InterruptedException { };
//和await()类似,只不过等待一定的时间后count值还没变为0的话就会继续执行
public boolean await(long timeout, TimeUnit unit) throws InterruptedException { };
//将count值减1
public void countDown() { };
public class CountDownLatchTest {
public static void main(String[] args) {
final CountDownLatch latch = new CountDownLatch(2);
System.out.println("主线程开始执行…… ……");
//第一个子线程执行
ExecutorService es1 = Executors.newSingleThreadExecutor();
es1.execute(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(3000);
System.out.println("子线程:"+Thread.currentThread().getName()+"执行");
} catch (InterruptedException e) {
e.printStackTrace();
}
latch.countDown();
}
});
es1.shutdown();
//第二个子线程执行
ExecutorService es2 = Executors.newSingleThreadExecutor();
es2.execute(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("子线程:"+Thread.currentThread().getName()+"执行");
latch.countDown();
}
});
es2.shutdown();
System.out.println("等待两个线程执行完毕…… ……");
try {
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("两个子线程都执行完毕,继续执行主线程");
}
}
结果集:
主线程开始执行…… ……
等待两个线程执行完毕…… ……
子线程:pool-1-thread-1执行
子线程:pool-2-thread-1执行
两个子线程都执行完毕,继续执行主线程
public class Parallellimit {
public static void main(String[] args) {
ExecutorService pool = Executors.newCachedThreadPool();
CountDownLatch cdl = new CountDownLatch(100);
for (int i = 0; i < 100; i++) {
CountRunnable runnable = new CountRunnable(cdl);
pool.execute(runnable);
}
}
}
class CountRunnable implements Runnable {
private CountDownLatch countDownLatch;
public CountRunnable(CountDownLatch countDownLatch) {
this.countDownLatch = countDownLatch;
}
@Override
public void run() {
try {
synchronized (countDownLatch) {
/*** 每次减少一个容量*/
countDownLatch.countDown();
System.out.println("thread name:"+Thread.currentThread().getName()+",thread counts = " + (countDownLatch.getCount()));
}
countDownLatch.await();
System.out.println("thread name:"+Thread.currentThread().getName()+",concurrency counts = " + (100 - countDownLatch.getCount()));
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
结果集:
thread name:pool-1-thread-1,thread counts = 99
thread name:pool-1-thread-5,thread counts = 98
thread name:pool-1-thread-4,thread counts = 97
thread name:pool-1-thread-6,thread counts = 96
thread name:pool-1-thread-3,thread counts = 95
thread name:pool-1-thread-7,thread counts = 94
thread name:pool-1-thread-2,thread counts = 93
thread name:pool-1-thread-8,thread counts = 92
thread name:pool-1-thread-9,thread counts = 91
thread name:pool-1-thread-10,thread counts = 90
thread name:pool-1-thread-11,thread counts = 89
thread name:pool-1-thread-12,thread counts = 88
thread name:pool-1-thread-13,thread counts = 87
...
thread name:pool-1-thread-98,thread counts = 2
thread name:pool-1-thread-99,thread counts = 1
thread name:pool-1-thread-100,thread counts = 0
thread name:pool-1-thread-100,concurrency counts = 100
thread name:pool-1-thread-1,concurrency counts = 100
thread name:pool-1-thread-4,concurrency counts = 100
thread name:pool-1-thread-5,concurrency counts = 100
...
1.countDownLatch是一个计数器,线程完成一个记录一个,计数器递减,只能只用一次
2.CyclicBarrier的计数器更像一个阀门,需要所有线程都到达,然后继续执行,计数器递增,提供reset功能,可以多次使用
// 定义工作线程
class ThreadDown implements Runnable {
private CountDownLatch latch = null;
public ThreadDown(CountDownLatch latch) {
this.latch = latch;
}
@Override
public void run() {
for (int index = 0; index < 10; index++) {
System.out.println(Thread.currentThread().getName() + " : " + index);
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
this.latch.countDown();
}
}
@Test
public void testCountDown() throws InterruptedException {
//--1 定义锁
CountDownLatch countDownLatch = new CountDownLatch(10);
//--2 创建线程
for (int index = 0; index < 10; index++) {
Thread thread = new Thread(new ThreadDown(countDownLatch));
thread.start();
}
//--3 等待所有线程结束
countDownLatch.await();
//--4 继续执行主线程
System.out.println("主线程:"+Thread.currentThread().getName());
}