J.U.C-三剑客[semaphore\CyclicBarrier\CountDownLatch]

一.semaphore信号量，底层也是基于AQS

使用：

/**

* 可以理解为控制某个资源最多有多少个线程同时执行，(比如洗手间，并行与排队)

* 如果满了只能等待直到其它资源释放（可以理解为并发量控制）

* @author Binglong

* @date 2018-11-12

*/

public class SemaphoreUtils {

    public static void main(String[] args) {

        final int SH_SIZE = 10;

        Semaphore semaphore = new Semaphore(SH_SIZE);

        final int TH_NUM = 20;

        for (int i = 0; i < TH_NUM; i++) {

            ThreadPoolUtils.getSingle().threadPoolDo(new TaskSemaphore(semaphore));

        }

    }

}

class TaskSemaphore implements Runnable {

    private Semaphore semaphore;

    TaskSemaphore(Semaphore semaphore) {

        this.semaphore = semaphore;

    }

    public void run() {

        String threadName = Thread.currentThread().getName();

        try {

            this.semaphore.acquire();

            System.out.println(threadName + ":occupy...");

            Thread.sleep(new Random().nextInt(10000));

            System.out.println(threadName + ":over...");

        } catch (InterruptedException e) {

            e.printStackTrace();

        } finally {

　　　 //注意一点要放到finally

            semaphore.release();

        }

    }

}

源码

package java.util.concurrent;

import java.util.*;

import java.util.concurrent.locks.*;

import java.util.concurrent.atomic.*;

public class Semaphore implements java.io.Serializable {

    private static final long serialVersionUID = -3222578661600680210L;

    //定义一个内部类

    private final Sync sync;

    abstract static class Sync extends AbstractQueuedSynchronizer {

        private static final long serialVersionUID = 1192457210091910933L;

        Sync(int permits) {

            setState(permits);

        }

        final int getPermits() {

            return getState();

        }

        final int nonfairTryAcquireShared(int acquires) {

            for (;;) {

                int available = getState();

                int remaining = available - acquires;

                if (remaining < 0 ||

                    compareAndSetState(available, remaining))

                    return remaining;

            }

        }

        protected final boolean tryReleaseShared(int releases) {

            for (;;) {

                int current = getState();

                int next = current + releases;

                if (next < current) // overflow

                    throw new Error("Maximum permit count exceeded");

                if (compareAndSetState(current, next))

                    return true;

            }

        }

        final void reducePermits(int reductions) {

            for (;;) {

                int current = getState();

                int next = current - reductions;

                if (next > current) // underflow

                    throw new Error("Permit count underflow");

                if (compareAndSetState(current, next))

                    return;

            }

        }

        final int drainPermits() {

            for (;;) {

                int current = getState();

                if (current == 0 || compareAndSetState(current, 0))

                    return current;

            }

        }

    }

    static final class NonfairSync extends Sync {

        private static final long serialVersionUID = -2694183684443567898L;

        NonfairSync(int permits) {

            super(permits);

        }

        protected int tryAcquireShared(int acquires) {

            return nonfairTryAcquireShared(acquires);

        }

    }

    static final class FairSync extends Sync {

        private static final long serialVersionUID = 2014338818796000944L;

        FairSync(int permits) {

            super(permits);

        }

        protected int tryAcquireShared(int acquires) {

            for (;;) {

                if (hasQueuedPredecessors())

                    return -1;

                int available = getState();

                int remaining = available - acquires;

                if (remaining < 0 ||

                    compareAndSetState(available, remaining))

                    return remaining;

            }

        }

    }

    public Semaphore(int permits) {

        sync = new NonfairSync(permits);

    }

    public Semaphore(int permits, boolean fair) {

        sync = fair ? new FairSync(permits) : new NonfairSync(permits);

    }

    public void acquire() throws InterruptedException {

        sync.acquireSharedInterruptibly(1);

    }

    public void acquireUninterruptibly() {

        sync.acquireShared(1);

    }

    public boolean tryAcquire() {

        return sync.nonfairTryAcquireShared(1) >= 0;

    }

    public boolean tryAcquire(long timeout, TimeUnit unit)

        throws InterruptedException {

        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));

    }

    public void release() {

        sync.releaseShared(1);

    }

    public void acquire(int permits) throws InterruptedException {

        if (permits < 0) throw new IllegalArgumentException();

        sync.acquireSharedInterruptibly(permits);

    }

    public void acquireUninterruptibly(int permits) {

        if (permits < 0) throw new IllegalArgumentException();

        sync.acquireShared(permits);

    }

    public boolean tryAcquire(int permits) {

        if (permits < 0) throw new IllegalArgumentException();

        return sync.nonfairTryAcquireShared(permits) >= 0;

    }

    public boolean tryAcquire(int permits, long timeout, TimeUnit unit)

        throws InterruptedException {

        if (permits < 0) throw new IllegalArgumentException();

        return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));

    }

    public void release(int permits) {

        if (permits < 0) throw new IllegalArgumentException();

        sync.releaseShared(permits);

    }

    public int availablePermits() {

        return sync.getPermits();

    }

    public int drainPermits() {

        return sync.drainPermits();

    }

    protected void reducePermits(int reduction) {

        if (reduction < 0) throw new IllegalArgumentException();

        sync.reducePermits(reduction);

    }

    public boolean isFair() {

        return sync instanceof FairSync;

    }

    public final boolean hasQueuedThreads() {

        return sync.hasQueuedThreads();

    }

    public final int getQueueLength() {

        return sync.getQueueLength();

    }

    protected Collection<Thread> getQueuedThreads() {

        return sync.getQueuedThreads();

    }

    public String toString() {

        return super.toString() + "[Permits = " + sync.getPermits() + "]";

    }

}

二、CyclicBarrier

使用：

public static void main(String[] args) {
　　 //创建一个CyclicBarrier并在主线程上new一个任务

    final int N = 5;

    final CyclicBarrier cyclic = new CyclicBarrier(N, new Runnable() {

        public void run() {
　　　　　　　 //主线程任务（等最后一个线程做完）

            try {

                System.out.println("汇总计算开始");

                Thread.sleep(Math.abs(10));

                System.out.println("汇总计算完成");

            } catch (Exception e) {

                e.printStackTrace();

            }

        }

    });

    for (int i = 0; i < N; i++) {

        final int t = i;

        new Thread(new Runnable() {

            public void run() {
　　　　　　　　　//每个线程任务做完等待（主线程做完才继续往下走）

                try {

                    System.out.println(t + "中心数据已计算开始");

                    Thread.sleep(Math.abs(new Random().nextInt() % 10000));

                    System.out.println(t + "中心数据已计算结束");

                    cyclic.await();

                    System.out.println(t + "中心数据退出");

                } catch (Exception e) {

                    e.printStackTrace();

                }

            }

        }).start();

    }

}

0中心数据已计算开始

3中心数据已计算开始

4中心数据已计算开始

2中心数据已计算开始

4中心数据已计算结束

1中心数据已计算开始

1中心数据已计算结束

3中心数据已计算结束

2中心数据已计算结束

0中心数据已计算结束

汇总计算开始

汇总计算完成

0中心数据退出

1中心数据退出

4中心数据退出

2中心数据退出

3中心数据退出

源码：

1.构造方法

//只是做等待parties个线程（没有主线程任务）

public CyclicBarrier(int parties) {

    this(parties, null);

}

//等待parties个线程后，先完成barrierAction的run方法，其它线程继续执行

public CyclicBarrier(int parties, Runnable barrierAction) {

    if (parties <= 0) throw new IllegalArgumentException();

    this.parties = parties;

    this.count = parties;

    this.barrierCommand = barrierAction;

}

package java.util.concurrent;

import java.util.concurrent.locks.*;

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();

    private int count;

    private void nextGeneration() {

        // signal completion of last generation

        trip.signalAll();

        // set up next generation

        count = parties;

        generation = new Generation();

    }

    private void breakBarrier() {

        generation.broken = true;

        count = parties;

        trip.signalAll();

    }

    private int dowait(boolean timed, long nanos) throws InterruptedException, BrokenBarrierException,

               TimeoutException {

        final ReentrantLock lock = this.lock;
　　　　 //获取锁

        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();

        }

    }

    public CyclicBarrier(int parties, Runnable barrierAction) {

        if (parties <= 0) throw new IllegalArgumentException();

        this.parties = parties;

        this.count = parties;

        this.barrierCommand = barrierAction;

    }

    public CyclicBarrier(int parties) {

        this(parties, null);

    }

    public int getParties() {

        return parties;

    }

    public int await() throws InterruptedException, BrokenBarrierException {

        try {

            return dowait(false, 0L);

        } catch (TimeoutException toe) {

            throw new Error(toe); // cannot happen;

        }

    }

    public int await(long timeout, TimeUnit unit)

        throws InterruptedException,

               BrokenBarrierException,

               TimeoutException {

        return dowait(true, unit.toNanos(timeout));

    }

    public boolean isBroken() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return generation.broken;

        } finally {

            lock.unlock();

        }

    }

    public void reset() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            breakBarrier();   // break the current generation

            nextGeneration(); // start a new generation

        } finally {

            lock.unlock();

        }

    }

    public int getNumberWaiting() {

        final ReentrantLock lock = this.lock;

        lock.lock();

        try {

            return parties - count;

        } finally {

            lock.unlock();

        }

    }

}

2.重要方法

a.wait（）方法，当调用wait()方法的线程数量，达到CyclicBarrier构造方法的N时，（CyclicBarrier在构造方法的Runnable barrierAction，方法完成后，当前线程继续执行）

在CyclicBarrier上等待的线程数量达到parties，则所有线程被释放，继续执行。

当前线程被中断，则抛出InterruptedException异常，并停止等待，继续执行。

当前线程等待超时，则抛出TimeoutException异常，并停止等待，继续执行。

其他等待的线程被中断，则当前线程抛出BrokenBarrierException异常，并停止等待，继续执行。

其他等待的线程超时，则当前线程抛出BrokenBarrierException异常，并停止等待，继续执行。

其他线程调用CyclicBarrier.reset()方法，则当前线程抛出BrokenBarrierException异常，并停止等待，继续执行

public int await(long timeout, TimeUnit unit) throws InterruptedException, BrokenBarrierException,TimeoutException {

    return dowait(true, unit.toNanos(timeout));

}

public int await() throws InterruptedException, BrokenBarrierException {

    try {

        return dowait(false, 0L);

    } catch (TimeoutException toe) {

        throw new Error(toe); // cannot happen

    }

}

b.getParties()获取CyclicBarrier等待的线程数，也就是CyclicBarrier构造方法参数parties的值

c.getNumberWaiting() how many thread wait now

d.rest()

如果有正在等待的线程，则会抛出BrokenBarrierException异常，且这些线程停止等待，继续执行。

将是否破损标志位broken置为false。

三、CountDownLatch

使用：

/**

* countDownLatch.countDown()调用一次减一，到0时，其它await方法继续往下执行

* 可以做并发开关(把SIZE设置为1，通过主线程来countDown(),其它线程都调用await()方法)

* @author Binglong

* @date 2018-11-12

*/

public class CountDownLatchUtils {

    public static void main(String[] args) throws InterruptedException {

        final int SIZE = 20;

        CountDownLatch countDownLatch = new CountDownLatch(SIZE);

        for (int i = 0; i < SIZE; i++) {

            ThreadPoolUtils.getSingle().threadPoolDo(new TaskCountDownLatch(countDownLatch));

        }

        System.out.println("waiting.....");

//        Thread.sleep(10000);

//        countDownLatch.countDown();

    }

}

class TaskCountDownLatch implements Runnable {

    private CountDownLatch countDownLatch;

    TaskCountDownLatch(CountDownLatch countDownLatch) {

        this.countDownLatch = countDownLatch;

    }

    public void run() {

        String name = Thread.currentThread().getName();

        try {

            System.out.println(name + ":waiting.."+countDownLatch.getCount());

            //等待一定数量任务继续执行

            Thread.sleep(new Random().nextInt(10000));

            countDownLatch.countDown();

            System.out.println(name + ":over...");

        } catch (Exception e) {

            e.printStackTrace();

        }

    }

}

1.使用

三个方法

CountDownLatch(int count)：构造器中的计数值（count）。

void await() ：会一直阻塞当前线程，直到计时器的值为0

void countDown()：计数减一

2.原理

CountDownLatch源代码是有内部类Sync实现，而Sync是继承AQS(抽象队列同步器)

private static final class Sync extends AbstractQueuedSynchronizer {

    private static final long serialVersionUID = 4982264981922014374L;

    Sync(int count) {

        setState(count);

    }

    int getCount() {

        return getState();

    }

    protected int tryAcquireShared(int acquires) {

        return (getState() == 0) ? 1 : -1;

    }

    protected boolean tryReleaseShared(int releases) {

        for (;;) {

            int c = getState();

            if (c == 0)

                return false;

            int nextc = c-1;

            if (compareAndSetState(c, nextc))

                return nextc == 0;

        }

    }

}

//构造器

public CountDownLatch(int count) {

    if (count < 0) throw new IllegalArgumentException("count < 0");

    this.sync = new Sync(count);

}

//countDown方法

public void countDown() {

    //releaseShared方法是抽象队列同步器的方法

    sync.releaseShared(1);

}

//await方法

public void await() throws InterruptedException {

    //acquireSharedInterruptibly方法是抽象队列同步器的方法

    sync.acquireSharedInterruptibly(1);

}

巴特西

J.U.C-三剑客[semaphore\CyclicBarrier\CountDownLatch]

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