CountDownLatch、Semaphore、CyclicBarrier的原理和作用是什么
這篇文章主要介紹“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問(wèn)題上存在疑惑，小編查閱了各式資料，整理出簡(jiǎn)單好用的操作方法，希望對(duì)大家解答”CountDownLatch、Semaphore、CyclicBarrier的原理和作用是什么”的疑惑有所幫助！接下來(lái)，請(qǐng)跟著小編一起來(lái)學(xué)習(xí)吧！
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CountDownLatch
CountDownLatch是一個(gè)計(jì)數(shù)器閉鎖，通過(guò)它可以完成類似于阻塞當(dāng)前線程的功能，即：一個(gè)線程或多個(gè)線程一直等待，直到其他線程執(zhí)行的操作完成。CountDownLatch用一個(gè)給定的計(jì)數(shù)器來(lái)初始化，該計(jì)數(shù)器的操作是原子操作，即同時(shí)只能有一個(gè)線程去操作該計(jì)數(shù)器。調(diào)用該類await方法的線程會(huì)一直處于阻塞狀態(tài)，直到其他線程調(diào)用countDown方法使當(dāng)前計(jì)數(shù)器的值變?yōu)榱悖看握{(diào)用countDown計(jì)數(shù)器的值減1。當(dāng)計(jì)數(shù)器值減至零時(shí)，所有因調(diào)用await()方法而處于等待狀態(tài)的線程就會(huì)繼續(xù)往下執(zhí)行。這種現(xiàn)象只會(huì)出現(xiàn)一次，因?yàn)橛?jì)數(shù)器不能被重置，如果業(yè)務(wù)上需要一個(gè)可以重置計(jì)數(shù)次數(shù)的版本，可以考慮使用CycliBarrier。
在某些業(yè)務(wù)場(chǎng)景中，程序執(zhí)行需要等待某個(gè)條件完成后才能繼續(xù)執(zhí)行后續(xù)的操作；典型的應(yīng)用如并行計(jì)算，當(dāng)某個(gè)處理的運(yùn)算量很大時(shí)，可以將該運(yùn)算任務(wù)拆分成多個(gè)子任務(wù)，等待所有的子任務(wù)都完成之后，父任務(wù)再拿到所有子任務(wù)的運(yùn)算結(jié)果進(jìn)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(jié)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(jié)果： 超過(guò)指定時(shí)間跳過(guò)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(lái)限制流量，類似閥門(mén)的 功能。如果限定某些資源最多有N個(gè)線程可以訪問(wèn)，那么超過(guò)N個(gè)主不允許再有線程來(lái)訪問(wèn)，同時(shí)當(dāng)現(xiàn)有線程結(jié)束后，就會(huì)釋放，然后允許新的線程進(jìn)來(lái)。有點(diǎn)類似于鎖的lock與 unlock過(guò)程。相對(duì)來(lái)說(shuō)他也有兩個(gè)主要的方法：
用于獲取權(quán)限的acquire(),其底層實(shí)現(xiàn)與CountDownLatch.countdown()類似;
用于釋放權(quán)限的release()，其底層實(shí)現(xiàn)與acquire()是一個(gè)互逆的過(guò)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(gè)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(gè)許可
test(threadNum);
semaphore.release(); // 釋放一個(gè)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(gè)許可
test(threadNum);
semaphore.release(3); // 釋放多個(gè)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(gè)許可
test(threadNum);
semaphore.release(); // 釋放一個(gè)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(gè)許可
test(threadNum);
semaphore.release(); // 釋放一個(gè)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(gè)同步輔助類，它允許一組線程相互等待，直到到達(dá)某個(gè)公共屏障點(diǎn)（common barrier point）。通過(guò)它可以完成多個(gè)線程之間相互等待，只有當(dāng)每個(gè)線程都準(zhǔn)備就緒后，才能各自繼續(xù)往下執(zhí)行后面的操作。類似于CountDownLatch，它也是通過(guò)計(jì)數(shù)器來(lái)實(shí)現(xiàn)的。當(dāng)某個(gè)線程調(diào)用await方法時(shí)，該線程進(jìn)入等待狀態(tài)，且計(jì)數(shù)器加1，當(dāng)計(jì)數(shù)器的值達(dá)到設(shè)置的初始值時(shí)，所有因調(diào)用await進(jìn)入等待狀態(tài)的線程被喚醒，繼續(xù)執(zhí)行后續(xù)操作。因?yàn)镃ycliBarrier在釋放等待線程后可以重用，所以稱為循環(huán)barrier。CycliBarrier支持一個(gè)可選的Runnable，在計(jì)數(shù)器的值到達(dá)設(shè)定值后（但在釋放所有線程之前），該Runnable運(yùn)行一次，注，Runnable在每個(gè)屏障點(diǎn)只運(yùn)行一個(gè)。
使用場(chǎng)景類似于CountDownLatch與CountDownLatch的區(qū)別
CountDownLatch主要是實(shí)現(xiàn)了1個(gè)或N個(gè)線程需要等待其他線程完成某項(xiàng)操作之后才能繼續(xù)往下執(zhí)行操作，描述的是1個(gè)線程或N個(gè)線程等待其他線程的關(guān)系。CyclicBarrier主要是實(shí)現(xiàn)了多個(gè)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(xù)執(zhí)行后續(xù)的操作，描述的多個(gè)線程內(nèi)部相互等待的關(guān)系。
CountDownLatch是一次性的，而CyclicBarrier則可以被重置而重復(fù)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(jié)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(jié)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
底層實(shí)現(xiàn)
1.CountDownLatch底層使用的是共享鎖，它有個(gè)內(nèi)部類Sync，這個(gè)Sync繼承AQS，實(shí)現(xiàn)了共享鎖。
具體參考JUC系列回顧之-CountDownLatch底層原理和示例
簡(jiǎn)單畫(huà)了一下共享鎖的實(shí)現(xiàn)。
比如有4個(gè)線程在等待隊(duì)列里，并且節(jié)點(diǎn)類型都是共享鎖。 會(huì)喚醒head節(jié)點(diǎn)的下一節(jié)點(diǎn)中的線程Thread1。head節(jié)點(diǎn)就變成了之前head節(jié)點(diǎn)的下個(gè)節(jié)點(diǎn)，然后再做重復(fù)操作。 這個(gè)過(guò)程是一個(gè)傳播過(guò)程，會(huì)依次喚醒各個(gè)共享節(jié)點(diǎn)中的線程。
2.并發(fā)包下的另外一個(gè)工具類Semaphore底層也是使用共享鎖實(shí)現(xiàn)的。但是它跟CountDownLatch唯一的區(qū)別就是它不會(huì)喚醒所有的共享節(jié)點(diǎn)中的線程，而是喚醒它能喚醒的最大線程數(shù)(由信號(hào)量可用大小決定)。
3.CyclicBarrier底層使用的是ReentrantLock和這個(gè)lock的條件對(duì)象Condition。
到此，關(guān)于“CountDownLatch、Semaphore、CyclicBarrier的原理和作用是什么”的學(xué)習(xí)就結(jié)束了，希望能夠解決大家的疑惑。理論與實(shí)踐的搭配能更好的幫助大家學(xué)習(xí)，快去試試吧！若想繼續(xù)學(xué)習(xí)更多相關(guān)知識(shí)，請(qǐng)繼續(xù)關(guān)注創(chuàng)新互聯(lián)網(wǎng)站，小編會(huì)繼續(xù)努力為大家?guī)?lái)更多實(shí)用的文章！
文章名稱：CountDownLatch、Semaphore、CyclicBarrier的原理和作用是什么
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