Java并發之CountDownLatch、Semaphore和CyclicBarrier

JAVA并發包中有三個類用于同步一批線程的行為，分別是CountDownLatch、Semaphore和CyclicBarrier。

CountDownLatch

CountDownLatch是一個計數器閉鎖，通過它可以完成類似于阻塞當前線程的功能，即：一個線程或多個線程一直等待，直到其他線程執行的操作完成。CountDownLatch用一個給定的計數器來初始化，該計數器的操作是原子操作，即同時只能有一個線程去操作該計數器。調用該類await方法的線程會一直處于阻塞狀態，直到其他線程調用countDown方法使當前計數器的值變為零，每次調用countDown計數器的值減1。當計數器值減至零時，所有因調用await()方法而處于等待狀態的線程就會繼續往下執行。這種現象只會出現一次，因為計數器不能被重置，如果業務上需要一個可以重置計數次數的版本，可以考慮使用CycliBarrier。

在某些業務場景中，程序執行需要等待某個條件完成后才能繼續執行后續的操作；典型的應用如并行計算，當某個處理的運算量很大時，可以將該運算任務拆分成多個子任務，等待所有的子任務都完成之后，父任務再拿到所有子任務的運算結果進行匯總。

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);
    }
}

結果：

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);
    }
}

結果：超過指定時間跳過等待

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

Semaphore與CountDownLatch相似，不同的地方在于Semaphore的值被獲取到后是可以釋放的，并不像CountDownLatch那樣一直減到底。它也被更多地用來限制流量，類似閥門的功能。如果限定某些資源最多有N個線程可以訪問，那么超過N個主不允許再有線程來訪問，同時當現有線程結束后，就會釋放，然后允許新的線程進來。有點類似于鎖的lock與 unlock過程。相對來說他也有兩個主要的方法：

用于獲取權限的acquire(),其底層實現與CountDownLatch.countdown()類似;
用于釋放權限的release()，其底層實現與acquire()是一個互逆的過程。

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();
       // 每次最多三個線程獲取許可
        final Semaphore semaphore = new Semaphore(3);

        for (int i = 0; i < threadCount; i++) {
            final int threadNum = i;
            exec.execute(() -> {
                try {
                    semaphore.acquire(); // 獲取一個許可
                    test(threadNum);
                    semaphore.release(); // 釋放一個許可
                } 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); // 獲取多個許可
                    test(threadNum);
                    semaphore.release(3); // 釋放多個許可
                } 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()) { // 嘗試獲取一個許可
                        test(threadNum);
                        semaphore.release(); // 釋放一個許可
                    }
                } 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)) { // 嘗試獲取一個許可
                        test(threadNum);
                        semaphore.release(); // 釋放一個許可
                    }
                } catch (Exception e) {
                    log.error("exception", e);
                }
            });
        }
        exec.shutdown();
    }

    private static void test(int threadNum) throws Exception {
        log.info("{}", threadNum);
        Thread.sleep(1000);
    }
}

CyclicBarrier

CyclicBarrier也是一個同步輔助類，它允許一組線程相互等待，直到到達某個公共屏障點（common barrier point）。通過它可以完成多個線程之間相互等待，只有當每個線程都準備就緒后，才能各自繼續往下執行后面的操作。類似于CountDownLatch，它也是通過計數器來實現的。當某個線程調用await方法時，該線程進入等待狀態，且計數器加1，當計數器的值達到設置的初始值時，所有因調用await進入等待狀態的線程被喚醒，繼續執行后續操作。因為CycliBarrier在釋放等待線程后可以重用，所以稱為循環barrier。CycliBarrier支持一個可選的Runnable，在計數器的值到達設定值后（但在釋放所有線程之前），該Runnable運行一次，注，Runnable在每個屏障點只運行一個。

使用場景類似于CountDownLatch與CountDownLatch的區別

CountDownLatch主要是實現了1個或N個線程需要等待其他線程完成某項操作之后才能繼續往下執行操作，描述的是1個線程或N個線程等待其他線程的關系。CyclicBarrier主要是實現了多個線程之間相互等待，直到所有的線程都滿足了條件之后各自才能繼續執行后續的操作，描述的多個線程內部相互等待的關系。
CountDownLatch是一次性的，而CyclicBarrier則可以被重置而重復使用。

@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);
    }
}

結果： 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);
    }
}

結果：

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 continue

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