J.U.C-AQS框架同步组件之栅栏锁CyclicBarrier详解

其他 2020-04-12 11:11:55 阅读次数: 0

CyclicBarrier是什么？

CyclicBarrier也叫同步屏障，在JDK1.5被引入，可以让一组线程达到一个屏障时被阻塞，直到最后一个线程达到屏障时，屏障才会开门，所有被阻塞的线程才会继续执行。
他的主要用途是控制多个线程互相等待，只有当多个线程都到达时，这些线程才会继续执行。
CyclicBarrier好比一扇门，默认情况下关闭状态，堵住了线程执行的道路，直到所有线程都就位，门才打开，让所有线程一起通过。

CyclicBarrier如何使用和工作？

API

CyclicBarrier有两个构造函数

public CyclicBarrier(int parties)
public CyclicBarrier(int parties, Runnable barrierAction)

第一个参数，其参数表示屏障拦截的线程数量，每个线程调用await方法告诉CyclicBarrier已经到达屏障位置，线程被阻塞。
第二个参数，表示线程都处于barrier时，一起执行之前，其中barrierAction任务会在所有线程到达屏障后执行。

让线程处于barrier状态的方法await()
public int await()
public int await(long timeout, TimeUnit unit)

第一个默认方法，表示要等到所有的线程都处于barrier状态，才一起执行
第二个方法，指定了等待的时间，当所有线程没有都处于barrier状态，又到了指定的时间，所在的线程就继续执行了。
其它的一些方法

获取当前有多少个线程阻塞等待在临界点上
int getNumberWaiting()
用于查询阻塞等待的线程是否被中断
boolean isBroken()

CyclicBarrier是通过维护计数器来实现的。线程执行 await() 方法之后计数器会减 1(而不是很多人认为的加1)
，并进行等待，直到计数器为 0，所有调用 await() 方法而在等待的线程才能继续执行。

CyclicBarrier的底层原理

CyclicBarrier类是concurrent并发包下的一工具类。CyclicBarrier实现主要基于ReentrantLock。
线程间同步阻塞是使用的是ReentrantLock，可重入锁
线程间通信使用的是Condition，Condition 将 Object 监视器方法（wait、notify 和 notifyAll）分解成截然不同的对象，以便通过将这些对象与任意 Lock 实现组合使用。

public class CyclicBarrier {
    private static class Generation {
        boolean broken = false;
    }
    /** The lock for guarding barrier entry */
    private final ReentrantLock lock = new ReentrantLock();
    /** Condition to wait on until tripped */
    private final Condition trip = lock.newCondition();
    /** The number of parties */
    private final int parties;
    /* The command to run when tripped */
    private final Runnable barrierCommand;
    /** The current generation */
    private Generation generation = new Generation();
    ...省略后面代码
}

其中Generation用来控制屏障的循环使用，如果generation.broken为true的话，说明这个屏障已经损坏，当某个线程await的时候，直接抛出异常.

await实现

private int dowait(boolean timed, long nanos)
    throws InterruptedException, BrokenBarrierException,
           TimeoutException {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        final Generation g = generation;
        if (g.broken)
            throw new BrokenBarrierException();
        if (Thread.interrupted()) {
            breakBarrier();
            throw new InterruptedException();
        }
        int index = --count;
        if (index == 0) {  // tripped
            boolean ranAction = false;
            try {
                final Runnable command = barrierCommand;
                if (command != null)
                    command.run();
                ranAction = true;
                nextGeneration();
                return 0;
            } finally {
                if (!ranAction)
                    breakBarrier();
            }
        }

        // loop until tripped, broken, interrupted, or timed out
        for (;;) {
            try {
                if (!timed)
                    trip.await();
                else if (nanos > 0L)
                    nanos = trip.awaitNanos(nanos);
            } catch (InterruptedException ie) {
                if (g == generation && ! g.broken) {
                    breakBarrier();
                    throw ie;
                } else {
                    // We're about to finish waiting even if we had not
                    // been interrupted, so this interrupt is deemed to
                    // "belong" to subsequent execution.
                    Thread.currentThread().interrupt();
                }
            }
            if (g.broken)
                throw new BrokenBarrierException();
            if (g != generation)
                return index;
            if (timed && nanos <= 0L) {
                breakBarrier();
                throw new TimeoutException();
            }
        }
    } finally {
        lock.unlock();
    }
}

  1. 每当线程执行await，内部变量count减1，如果count！= 0，说明有线程还未到屏障处，则在锁条件变量trip上等待。
    当count == 0时，说明所有线程都已经到屏障处，执行条件变量的signalAll方法唤醒等待的线程。

  2. 其中 nextGeneration方法可以实现屏障的循环使用：

    • 重新生成Generation对象
    • 恢复count值

CountDownLatch与CyclicBarrier的比较

CountDownLatch与CyclicBarrier都是用于控制并发的工具类，都可以理解成维护的就是一个计数器，但是这两者还是各有不同侧重点的：

  1. 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；CountDownLatch强调一个线程等多个线程完成某件事情。CyclicBarrier是多个线程互等，等大家都完成，再携手共进。

  2. 调用CountDownLatch的countDown()方法后，当前线程并不会阻塞，会继续往下执行；而调用CyclicBarrier的await()方法，会阻塞当前线程，直到CyclicBarrier指定的线程全部都到达了指定点的时候，才能继续往下执行；

  3. CountDownLatch方法比较少，操作比较简单，而CyclicBarrier提供的方法更多，比如能够通过getNumberWaiting()，isBroken()这些方法获取当前多个线程的状态，并且CyclicBarrier的构造方法可以传入barrierAction，指定当所有线程都到达时执行的业务功能；

  4. CountDownLatch是不能复用的，而CyclicLatch是可以复用的。

  5. 和 CountdownLatch 相似，都是通过维护计数器来实现的。线程执行 await() 方法之后计数器会减 1，并进行等待，直到计数器为 0，所有调用 await() 方法而在等待的线程才能继续执行。

  6. CyclicBarrier 和 CountdownLatch 的一个区别是，CyclicBarrier 的计数器通过调用 reset() 方法可以循环使用，所以它才叫做循环屏障。

CyclicBarrier示例程序

package com.bestksl.example.aqs;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.*;

@Slf4j
public class CyclicBarrierExample3 {

    static final int clientTotal = 500;
    static final int threadTotal = 10;

    static CyclicBarrier cyclicBarrier = new CyclicBarrier(5, () -> {  // 可以选择传入一个Runnable 对象 完成更复杂的逻辑
        log.info("runnable");
    });

    public static void main(String[] args) throws InterruptedException {
        ExecutorService executorService = Executors.newCachedThreadPool();
        for (int i = 0; i < 10; i++) {
            final int n = i;
            Thread.sleep(1000);
            executorService.execute(() -> {
                try {
                    p(n);
                } catch (InterruptedException e) {
                    log.info(e.toString());
                }
            });
        }
        executorService.shutdown();
    }

    private static void p(int n) throws InterruptedException {
        log.info(n + "ready..");
        try {
            cyclicBarrier.await(20000, TimeUnit.MILLISECONDS);  // 如果超出等待时间(可选参数), 则会抛出BrokenBarrierException & TimeoutException
        } catch (BrokenBarrierException | TimeoutException e) {
            log.info("warn!! {}", e.toString());
        }
        log.info(n + "resume");
    }
}

执行结果

/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/bin/java "-javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=56252:/Applications/IntelliJ IDEA.app/Contents/bin" -Dfile.encoding=UTF-8 -classpath /Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_241.jdk/Contents/Home/lib/tools.jar:/Users/haoxuanli/Documents/GitHub/Concurrency/target/classes:/Users/haoxuanli/.m2/repository/org/springframework/boot/spring-boot-starter-web/2.2.5.RELEASE/spring-boot-starter-web-2.2.5.RELEASE.jar:/Users/haoxuanli/.m2/repository/org/springframework/boot/spring-boot-starter/2.2.5.RELEASE/spring-boot-starter-2.2.5.RELEASE.jar:/Users/haoxuanli/.m2/repository/org/springframework/boot/spring-boot/2.2.5.RELEASE/spring-boot-2.2.5.RELEASE.jar:/Users/haoxuanli/.m2/repository/org/springframework/boot/spring-boot-autoconfigure/2.2.5.RELEASE/spring-boot-autoconfigure-2.2.5.RELEASE.jar:/Users/haoxuanli/.m2/repository/org/springframework/boot/spring-boot-starter-logging/2.2.5.RELEASE/spring-boot-starter-logging-2.2.5.RELEASE.jar:/Users/haoxuanli/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar:/Users/haoxuanli/.m2/repository/ch/qos/logback/logback-core/1.2.3/logback-core-1.2.3.jar:/Users/haoxuanli/.m2/repository/org/slf4j/slf4j-api/1.7.25/slf4j-api-1.7.25.jar:/Users/haoxuanli/.m2/repository/org/apache/logging/log4j/log4j-to-slf4j/2.12.1/log4j-to-slf4j-2.12.1.jar:/Users/haoxuanli/.m2/repository/org/apache/logging/log4j/log4j-api/2.12.1/log4j-api-2.12.1.jar:/Users/haoxuanli/.m2/repository/org/slf4j/jul-to-slf4j/1.7.30/jul-to-slf4j-1.7.30.jar:/Users/haoxuanli/.m2/repository/jakarta/annotation/jakarta.annotation-api/1.3.5/jakarta.annotation-api-1.3.5.jar:/Users/haoxuanli/.m2/repository/org/springframework/spring-core/5.2.4.RELEASE/spring-core-5.2.4.RELEASE.jar:/Users/haoxuanli/.m2/repository/org/springframework/spring-jcl/5.2.4.RELEASE/spring-jcl-5.2.4.RELEASE.jar:/Users/haoxuanli/.m2/repository/org/yaml/snakeyaml/1.25/snakeyaml-1.25.jar:/Users/haoxuanli/.m2/repository/org/springframework/boot/spring-boot-starter-json/2.2.5.RELEASE/spring-boot-starter-json-2.2.5.RELEASE.jar:/Users/haoxuanli/.m2/repository/com/fasterxml/jackson/core/jackson-databind/2.10.2/jackson-databind-2.10.2.jar:/Users/haoxuanli/.m2/repository/com/fasterxml/jackson/core/jackson-annotations/2.10.2/jackson-annotations-2.10.2.jar:/Users/haoxuanli/.m2/repository/com/fasterxml/jackson/core/jackson-core/2.10.2/jackson-core-2.10.2.jar:/Users/haoxuanli/.m2/repository/com/fasterxml/jackson/datatype/jackson-datatype-jdk8/2.10.2/jackson-datatype-jdk8-2.10.2.jar:/Users/haoxuanli/.m2/repository/com/fasterxml/jackson/datatype/jackson-datatype-jsr310/2.10.2/jackson-datatype-jsr310-2.10.2.jar:/Users/haoxuanli/.m2/repository/com/fasterxml/jackson/module/jackson-module-parameter-names/2.10.2/jackson-module-parameter-names-2.10.2.jar:/Users/haoxuanli/.m2/repository/org/springframework/boot/spring-boot-starter-tomcat/2.2.5.RELEASE/spring-boot-starter-tomcat-2.2.5.RELEASE.jar:/Users/haoxuanli/.m2/repository/org/apache/tomcat/embed/tomcat-embed-core/9.0.31/tomcat-embed-core-9.0.31.jar:/Users/haoxuanli/.m2/repository/org/apache/tomcat/embed/tomcat-embed-el/9.0.31/tomcat-embed-el-9.0.31.jar:/Users/haoxuanli/.m2/repository/org/apache/tomcat/embed/tomcat-embed-websocket/9.0.31/tomcat-embed-websocket-9.0.31.jar:/Users/haoxuanli/.m2/repository/org/springframework/boot/spring-boot-starter-validation/2.2.5.RELEASE/spring-boot-starter-validation-2.2.5.RELEASE.jar:/Users/haoxuanli/.m2/repository/jakarta/validation/jakarta.validation-api/2.0.2/jakarta.validation-api-2.0.2.jar:/Users/haoxuanli/.m2/repository/org/hibernate/validator/hibernate-validator/6.0.18.Final/hibernate-validator-6.0.18.Final.jar:/Users/haoxuanli/.m2/repository/org/jboss/logging/jboss-logging/3.3.2.Final/jboss-logging-3.3.2.Final.jar:/Users/haoxuanli/.m2/repository/com/fasterxml/classmate/1.3.4/classmate-1.3.4.jar:/Users/haoxuanli/.m2/repository/org/springframework/spring-web/5.2.4.RELEASE/spring-web-5.2.4.RELEASE.jar:/Users/haoxuanli/.m2/repository/org/springframework/spring-beans/5.2.4.RELEASE/spring-beans-5.2.4.RELEASE.jar:/Users/haoxuanli/.m2/repository/org/springframework/spring-webmvc/5.2.4.RELEASE/spring-webmvc-5.2.4.RELEASE.jar:/Users/haoxuanli/.m2/repository/org/springframework/spring-aop/5.2.4.RELEASE/spring-aop-5.2.4.RELEASE.jar:/Users/haoxuanli/.m2/repository/org/springframework/spring-context/5.2.4.RELEASE/spring-context-5.2.4.RELEASE.jar:/Users/haoxuanli/.m2/repository/org/springframework/spring-expression/5.2.4.RELEASE/spring-expression-5.2.4.RELEASE.jar:/Users/haoxuanli/.m2/repository/org/projectlombok/lombok/1.18.0/lombok-1.18.0.jar:/Users/haoxuanli/.m2/repository/joda-time/joda-time/2.10.5/joda-time-2.10.5.jar com.bestksl.example.aqs.CyclicBarrierExample3
20:28:26.711 [pool-1-thread-1] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 0ready..
20:28:27.715 [pool-1-thread-2] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 1ready..
20:28:28.716 [pool-1-thread-3] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 2ready..
20:28:29.719 [pool-1-thread-4] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 3ready..
20:28:30.724 [pool-1-thread-5] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 4ready..
20:28:30.724 [pool-1-thread-5] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - runnable
20:28:30.724 [pool-1-thread-5] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 4resume
20:28:30.725 [pool-1-thread-3] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 2resume
20:28:30.725 [pool-1-thread-4] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 3resume
20:28:30.725 [pool-1-thread-1] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 0resume
20:28:30.725 [pool-1-thread-2] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 1resume
20:28:31.725 [pool-1-thread-2] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 5ready..
20:28:32.726 [pool-1-thread-1] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 6ready..
20:28:33.729 [pool-1-thread-4] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 7ready..
20:28:34.734 [pool-1-thread-3] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 8ready..
20:28:35.738 [pool-1-thread-5] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 9ready..
20:28:35.738 [pool-1-thread-5] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - runnable
20:28:35.738 [pool-1-thread-5] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 9resume
20:28:35.738 [pool-1-thread-2] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 5resume
20:28:35.738 [pool-1-thread-1] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 6resume
20:28:35.738 [pool-1-thread-4] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 7resume
20:28:35.738 [pool-1-thread-3] INFO com.bestksl.example.aqs.CyclicBarrierExample3 - 8resume

Process finished with exit code 0
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