这篇文章主要介绍“CountDownLatch、Semaphore、CyclicBarrier的原理和作用是什么”,在日常操作中,相信很多人在CountDownLatch、Semaphore、CyclicBarrier的原理和作用是什么问题上存在疑惑,小编查阅了各式资料,整理出简单好用的操作方法,希望对大家解答”CountDownLatch、Semaphore、CyclicBarrier的原理和作用是什么”的疑惑有所帮助!接下来,请跟着小编一起来学习吧!
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CountDownLatch是一个计数器闭锁,通过它可以完成类似于阻塞当前线程的功能,即:一个线程或多个线程一直等待,直到其他线程执行的操作完成。CountDownLatch用一个给定的计数器来初始化,该计数器的操作是原子操作,即同时只能有一个线程去操作该计数器。调用该类await方法的线程会一直处于阻塞状态,直到其他线程调用countDown方法使当前计数器的值变为零,每次调用countDown计数器的值减1。当计数器值减至零时,所有因调用await()方法而处于等待状态的线程就会继续往下执行。这种现象只会出现一次,因为计数器不能被重置,如果业务上需要一个可以重置计数次数的版本,可以考虑使用CycliBarrier。
在某些业务场景中,程序执行需要等待某个条件完成后才能继续执行后续的操作;典型的应用如并行计算,当某个处理的运算量很大时,可以将该运算任务拆分成多个子任务,等待所有的子任务都完成之后,父任务再拿到所有子任务的运算结果进行汇总。
import lombok.extern.slf4j.Slf4j; import java.util.concurrent.CountDownLatch; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; @Slf4j public class CountDownLatchExample1 { private final static int threadCount = 200; public static void main(String[] args) throws Exception { ExecutorService exec = Executors.newCachedThreadPool(); final CountDownLatch countDownLatch = new CountDownLatch(threadCount); for (int i = 0; i < threadCount; i++) { final int threadNum = i; exec.execute(() -> { try { test(threadNum); } catch (Exception e) { log.error("exception", e); } finally { countDownLatch.countDown(); } }); } countDownLatch.await(); log.info("finish"); exec.shutdown(); } private static void test(int threadNum) throws Exception { Thread.sleep(100); log.info("{}", threadNum); Thread.sleep(100); } }
结果:
20:18:32.917 [pool-1-thread-7] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 6 20:18:32.917 [pool-1-thread-6] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 5 20:18:32.919 [pool-1-thread-5] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 4 20:18:32.918 [pool-1-thread-1] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 0 20:18:32.918 [pool-1-thread-3] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 2 20:18:32.916 [pool-1-thread-9] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 8 20:18:32.918 [pool-1-thread-4] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 3 20:18:32.916 [pool-1-thread-10] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 9 20:18:32.916 [pool-1-thread-8] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 7 20:18:32.917 [pool-1-thread-2] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - 1 20:18:33.032 [main] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample1 - finish
import java.util.concurrent.CountDownLatch; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.TimeUnit; @Slf4j public class CountDownLatchExample2 { private final static int threadCount = 200; public static void main(String[] args) throws Exception { ExecutorService exec = Executors.newCachedThreadPool(); final CountDownLatch countDownLatch = new CountDownLatch(threadCount); for (int i = 0; i < threadCount; i++) { final int threadNum = i; exec.execute(() -> { try { test(threadNum); } catch (Exception e) { log.error("exception", e); } finally { countDownLatch.countDown(); } }); } countDownLatch.await(10, TimeUnit.MILLISECONDS); log.info("finish"); exec.shutdown(); } private static void test(int threadNum) throws Exception { Thread.sleep(100); log.info("{}", threadNum); } }
结果: 超过指定时间跳过等待
20:19:34.878 [main] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - finish 20:19:34.964 [pool-1-thread-3] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 2 20:19:34.965 [pool-1-thread-10] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 9 20:19:34.964 [pool-1-thread-1] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 0 20:19:34.965 [pool-1-thread-8] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 7 20:19:34.964 [pool-1-thread-2] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 1 20:19:34.965 [pool-1-thread-5] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 4 20:19:34.965 [pool-1-thread-7] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 6 20:19:34.964 [pool-1-thread-4] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 3 20:19:34.965 [pool-1-thread-9] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 8 20:19:34.965 [pool-1-thread-6] INFO com.mmall.concurrency.example.aqs.CountDownLatchExample2 - 5
Semaphore与CountDownLatch相似,不同的地方在于Semaphore的值被获取到后是可以释放的,并不像CountDownLatch那样一直减到底。它也被更多地用来限制流量,类似阀门的 功能。如果限定某些资源最多有N个线程可以访问,那么超过N个主不允许再有线程来访问,同时当现有线程结束后,就会释放,然后允许新的线程进来。有点类似于锁的lock与 unlock过程。相对来说他也有两个主要的方法:
用于获取权限的acquire(),其底层实现与CountDownLatch.countdown()类似;
用于释放权限的release(),其底层实现与acquire()是一个互逆的过程。
import lombok.extern.slf4j.Slf4j; import java.util.concurrent.CountDownLatch; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Semaphore; @Slf4j public class SemaphoreExample1 { private final static int threadCount = 20; public static void main(String[] args) throws Exception { ExecutorService exec = Executors.newCachedThreadPool(); // 每次最多三个线程获取许可 final Semaphore semaphore = new Semaphore(3); for (int i = 0; i < threadCount; i++) { final int threadNum = i; exec.execute(() -> { try { semaphore.acquire(); // 获取一个许可 test(threadNum); semaphore.release(); // 释放一个许可 } catch (Exception e) { log.error("exception", e); } }); } exec.shutdown(); } private static void test(int threadNum) throws Exception { log.info("{}", threadNum); Thread.sleep(1000); } }
import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Semaphore; @Slf4j public class SemaphoreExample2 { private final static int threadCount = 20; public static void main(String[] args) throws Exception { ExecutorService exec = Executors.newCachedThreadPool(); final Semaphore semaphore = new Semaphore(3); for (int i = 0; i < threadCount; i++) { final int threadNum = i; exec.execute(() -> { try { semaphore.acquire(3); // 获取多个许可 test(threadNum); semaphore.release(3); // 释放多个许可 } catch (Exception e) { log.error("exception", e); } }); } exec.shutdown(); } private static void test(int threadNum) throws Exception { log.info("{}", threadNum); Thread.sleep(1000); } }
import lombok.extern.slf4j.Slf4j; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Semaphore; import java.util.concurrent.TimeUnit; @Slf4j public class SemaphoreExample3 { private final static int threadCount = 20; public static void main(String[] args) throws Exception { ExecutorService exec = Executors.newCachedThreadPool(); final Semaphore semaphore = new Semaphore(3); for (int i = 0; i < threadCount; i++) { final int threadNum = i; exec.execute(() -> { try { if (semaphore.tryAcquire()) { // 尝试获取一个许可 test(threadNum); semaphore.release(); // 释放一个许可 } } catch (Exception e) { log.error("exception", e); } }); } exec.shutdown(); } private static void test(int threadNum) throws Exception { log.info("{}", threadNum); Thread.sleep(1000); } }
import lombok.extern.slf4j.Slf4j; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Semaphore; import java.util.concurrent.TimeUnit; @Slf4j public class SemaphoreExample4 { private final static int threadCount = 20; public static void main(String[] args) throws Exception { ExecutorService exec = Executors.newCachedThreadPool(); final Semaphore semaphore = new Semaphore(3); for (int i = 0; i < threadCount; i++) { final int threadNum = i; exec.execute(() -> { try { if (semaphore.tryAcquire(5000, TimeUnit.MILLISECONDS)) { // 尝试获取一个许可 test(threadNum); semaphore.release(); // 释放一个许可 } } catch (Exception e) { log.error("exception", e); } }); } exec.shutdown(); } private static void test(int threadNum) throws Exception { log.info("{}", threadNum); Thread.sleep(1000); } }
CyclicBarrier也是一个同步辅助类,它允许一组线程相互等待,直到到达某个公共屏障点(common barrier point)。通过它可以完成多个线程之间相互等待,只有当每个线程都准备就绪后,才能各自继续往下执行后面的操作。类似于CountDownLatch,它也是通过计数器来实现的。当某个线程调用await方法时,该线程进入等待状态,且计数器加1,当计数器的值达到设置的初始值时,所有因调用await进入等待状态的线程被唤醒,继续执行后续操作。因为CycliBarrier在释放等待线程后可以重用,所以称为循环barrier。CycliBarrier支持一个可选的Runnable,在计数器的值到达设定值后(但在释放所有线程之前),该Runnable运行一次,注,Runnable在每个屏障点只运行一个。
使用场景类似于CountDownLatch与CountDownLatch的区别
CountDownLatch主要是实现了1个或N个线程需要等待其他线程完成某项操作之后才能继续往下执行操作,描述的是1个线程或N个线程等待其他线程的关系。CyclicBarrier主要是实现了多个线程之间相互等待,直到所有的线程都满足了条件之后各自才能继续执行后续的操作,描述的多个线程内部相互等待的关系。
CountDownLatch是一次性的,而CyclicBarrier则可以被重置而重复使用。
@Slf4j public class CyclicBarrierExample1 { private static CyclicBarrier barrier = new CyclicBarrier(5); public static void main(String[] args) throws Exception { ExecutorService executor = Executors.newCachedThreadPool(); for (int i = 0; i < 10; i++) { final int threadNum = i; Thread.sleep(1000); executor.execute(() -> { try { race(threadNum); } catch (Exception e) { log.error("exception", e); } }); } executor.shutdown(); } private static void race(int threadNum) throws Exception { Thread.sleep(1000); log.info("{} is ready", threadNum); barrier.await(); log.info("{} continue", threadNum); } }
结果: ready ready .. go
20:24:34.616 [pool-1-thread-1] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 0 is ready 20:24:35.610 [pool-1-thread-2] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 1 is ready 20:24:36.610 [pool-1-thread-3] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 2 is ready 20:24:37.611 [pool-1-thread-4] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 3 is ready 20:24:38.612 [pool-1-thread-5] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 4 is ready 20:24:38.612 [pool-1-thread-1] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 0 continue 20:24:38.612 [pool-1-thread-2] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 1 continue 20:24:38.612 [pool-1-thread-5] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 4 continue 20:24:38.612 [pool-1-thread-4] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 3 continue 20:24:38.612 [pool-1-thread-3] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 2 continue 20:24:39.614 [pool-1-thread-6] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 5 is ready 20:24:40.613 [pool-1-thread-5] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 6 is ready 20:24:41.614 [pool-1-thread-2] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 7 is ready 20:24:42.615 [pool-1-thread-4] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 8 is ready 20:24:43.615 [pool-1-thread-3] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 9 is ready 20:24:43.615 [pool-1-thread-3] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 9 continue 20:24:43.615 [pool-1-thread-6] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 5 continue 20:24:43.615 [pool-1-thread-5] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 6 continue 20:24:43.615 [pool-1-thread-2] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 7 continue 20:24:43.615 [pool-1-thread-4] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample1 - 8 continue
import lombok.extern.slf4j.Slf4j; import java.util.concurrent.CyclicBarrier; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.TimeUnit; @Slf4j public class CyclicBarrierExample2 { private static CyclicBarrier barrier = new CyclicBarrier(5); public static void main(String[] args) throws Exception { ExecutorService executor = Executors.newCachedThreadPool(); for (int i = 0; i < 10; i++) { final int threadNum = i; Thread.sleep(1000); executor.execute(() -> { try { race(threadNum); } catch (Exception e) { log.error("exception", e); } }); } executor.shutdown(); } private static void race(int threadNum) throws Exception { Thread.sleep(1000); log.info("{} is ready", threadNum); try { barrier.await(2000, TimeUnit.MILLISECONDS); } catch (Exception e) { log.warn("BarrierException", e); } log.info("{} continue", threadNum); } }
import lombok.extern.slf4j.Slf4j; import java.util.concurrent.CyclicBarrier; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; @Slf4j public class CyclicBarrierExample3 { private static CyclicBarrier barrier = new CyclicBarrier(5, () -> { log.info("callback is running"); }); public static void main(String[] args) throws Exception { ExecutorService executor = Executors.newCachedThreadPool(); for (int i = 0; i < 10; i++) { final int threadNum = i; Thread.sleep(1000); executor.execute(() -> { try { race(threadNum); } catch (Exception e) { log.error("exception", e); } }); } executor.shutdown(); } private static void race(int threadNum) throws Exception { Thread.sleep(1000); log.info("{} is ready", threadNum); barrier.await(); log.info("{} continue", threadNum); } }
结果:
20:28:32.790 [pool-1-thread-1] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 0 is ready 20:28:33.785 [pool-1-thread-2] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 1 is ready 20:28:34.786 [pool-1-thread-3] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 2 is ready 20:28:35.787 [pool-1-thread-4] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 3 is ready 20:28:36.787 [pool-1-thread-5] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 4 is ready 20:28:36.787 [pool-1-thread-5] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - callback is running 20:28:36.787 [pool-1-thread-5] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 4 continue 20:28:36.788 [pool-1-thread-1] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 0 continue 20:28:36.788 [pool-1-thread-2] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 1 continue 20:28:36.788 [pool-1-thread-3] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 2 continue 20:28:36.788 [pool-1-thread-4] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 3 continue 20:28:37.788 [pool-1-thread-6] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 5 is ready 20:28:38.789 [pool-1-thread-4] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 6 is ready 20:28:39.789 [pool-1-thread-5] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 7 is ready 20:28:40.790 [pool-1-thread-2] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 8 is ready 20:28:41.791 [pool-1-thread-3] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 9 is ready 20:28:41.791 [pool-1-thread-3] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - callback is running 20:28:41.791 [pool-1-thread-3] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 9 continue 20:28:41.791 [pool-1-thread-6] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 5 continue 20:28:41.791 [pool-1-thread-4] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 6 continue 20:28:41.818 [pool-1-thread-2] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 8 continue 20:28:41.818 [pool-1-thread-5] INFO com.mmall.concurrency.example.aqs.CyclicBarrierExample3 - 7 c
1.CountDownLatch底层使用的是共享锁,它有个内部类Sync,这个Sync继承AQS,实现了共享锁。
具体参考JUC系列回顾之-CountDownLatch底层原理和示例
简单画了一下共享锁的实现。
比如有4个线程在等待队列里,并且节点类型都是共享锁。 会唤醒head节点的下一节点中的线程Thread1。head节点就变成了之前head节点的下个节点,然后再做重复操作。 这个过程是一个传播过程,会依次唤醒各个共享节点中的线程。
2.并发包下的另外一个工具类Semaphore底层也是使用共享锁实现的。但是它跟CountDownLatch唯一的区别就是它不会唤醒所有的共享节点中的线程,而是唤醒它能唤醒的最大线程数(由信号量可用大小决定)。
3.CyclicBarrier底层使用的是ReentrantLock和这个lock的条件对象Condition。
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