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);
}