请简要说明一下JAVA中cyclicbarrier和countdownlatch的区别分别是什么?
CountDownLatch
/**
* CountDownLatch
* A synchronization aid that allows one or more threads to wait until
* a set of operations being performed in other threads completes.
* 支持一个或多个线程等待直到其他线程中的一系列操作完成为止的一个同步解决方案
**/
解释:其实就是一个计数器可以保证线程之间的顺序执行把线程从并发状态调整为串行状态保证了线程的执行顺序
具体实例:
// CountDownLatch
// CyclicBarrier
public class CountAndCyclic {
CountDownLatch countDownLatch = new CountDownLatch(1);
public static void main(String[] args) {
CountAndCyclic countAndCyclic = new CountAndCyclic();
System.out.println("主线程初始化完成开始执行...");
new Thread(()->{
System.out.println("subClassA start working...");
try {
TimeUnit.SECONDS.sleep(2);
// 此线程执行完毕计数-1
countAndCyclic.countDownLatch.countDown();
System.out.println("subClassA end working...");
} catch (InterruptedException e) {
e.printStackTrace();
}
}).start();
try {
// 阻塞主线程等待子线程执行完毕
countAndCyclic.countDownLatch.await();
System.out.println("主线程继续执行.");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
// class subClassA implements Runnable{
// @Override
// public void run() {
//
// }
// }
}
运行结果:
CyclicBarrier
/**
* A synchronization aid that allows a set of threads to all wait for
* each other to reach a common barrier point. CyclicBarriers are
* useful in programs involving a fixed sized party of threads that
* must occasionally wait for each other. The barrier is called
* <em>cyclic</em> because it can be re-used after the waiting threads
* are released.
* 支持一系列线程全部等待直至彼此都到达一个公共的屏障的一个同步辅助工具。
*/
示例代码
/**
* @author Satsuki
* @time 2019/10/21 17:54
* @description:
*/
public class CyclicTest {
public static void main(String[] args) {
ExecutorService executorService = Executors.newCachedThreadPool();
final CyclicBarrier barrier = new CyclicBarrier(4, new Runnable() {
@Override
public void run() {
System.out.println("好了,大家可以去吃饭了……" );
}
});
// 尝试替换成CountDownLatch 可以替换为CountDownLatch
CountDownLatch countDownLatch = new CountDownLatch(4);
System.out.println("要吃饭,必须所有人都到终点,oK?");
System.out.println("不放弃不抛弃!");
for (int i = 0; i < 4; i++) {
executorService.execute(new Runnable() {
@Override
public void run() {
System.out.println(Thread.currentThread().getName() + ":Go");
try {
TimeUnit.MILLISECONDS.sleep((long)(2000*Math.random()));
// countDownLatch.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+ ":我到终点了");
try {
// countDownLatch.await();
barrier.await();
} catch (Exception e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+ ":终于可以吃饭啦!");
}
});
}
}
}
执行结果:
cyclicbarrier和countdownlatch的区别
-
cyclicbarrier支持重用
-
countdownlatch不支持重用
-
countdownlatch用于保持多个线程之间的执行顺序
-
cyclicbarrier用于一组线程互相等待对方执行一些操作后再一起继续执行
-
用的侧重点不同,但是countdownlatch在某些时候是可以和cyclicbarrier互换的
-
如果要用countdownlatch代替cyclicbarrier那么必须在每个线程的操作完成之后调用countDown方法
-
CountDownLatch一般用于某个线程A等待若干个其他线程执行完任务之后,它才执行;
-
而CyclicBarrier一般用于一组线程互相等待至某个状态,然后这一组线程再同时执行;
上述来自https://www.nowcoder.com牛客网
-
countdownlatch是计数器,线程完成一个就记一个,就像报数,只不过是递减的
-
而CyclicBarrier更像一个水闸,线程执行就想水流动,但是会在水闸处停止,直至水满(线程都运行完毕)才开始泄流
上述两句话的原文链接:https://blog.youkuaiyun.com/paul342/article/details/49661991 部分代码也参照了此链接