使用信号量解决并发问题

使用信号量解决并发问题

本文主要讨论的是java的信号量在并发过程中的应用。信号量Semaphore的具体实现如下：

package java.util.concurrent;
import java.util.Collection;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;

public class Semaphore implements java.io.Serializable {
    private static final long serialVersionUID = -3222578661600680210L;
    /** 同步机制基于AQS实现 **/
    private final Sync sync;

    /**
     * 信号量的同步器实现，使用AQS的状态作为许可，子类提供了公平和非公平版本。
     */
    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;
            }
        }
    }

    /**
     * 非公平版本
     */
    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);
        }
    }

    /**
     * 公平版本
     */
    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;
            }
        }
    }

    /**
     * 使用给定数量的许可创建一个非公平的信号量对象
     */
    public Semaphore(int permits) {
        sync = new NonfairSync(permits);
    }

    /**
     * 使用指定的许可创建一个可选公平或非公平版本的信号量对象
     */
    public Semaphore(int permits, boolean fair) {
        sync = fair ? new FairSync(permits) : new NonfairSync(permits);
    }

    /**     
     * 从信号量对象中获取一个许可，会阻塞等待许可可用或者线程被中断，同时可用的许可数量减一。
     */
    public void acquire() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }

    /**
     * 从信号量对象中获取一个许可，会阻塞等待许可可用，同时可用的许可数量减一。              
     */
    public void acquireUninterruptibly() {
        sync.acquireShared(1);
    }

    /**
     * 从信号量对象中获取一个许可，当且仅当调用时可用则立即返回true，否则直接给返回false。
     */
    public boolean tryAcquire() {
        return sync.nonfairTryAcquireShared(1) >= 0;
    }

    /**
     * 从信号量对象中获取一个许可，如果许可在给定等待时间内变得可用并且当前线程未被中断。     
     */
    public boolean tryAcquire(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    }

    /**
     * 释放一个许可，返回给信号量。     
     */
    public void release() {
        sync.releaseShared(1);
    }

    /**
     * 从信号量中获取给定数量的许可，阻塞等待许可可用。
     */
    public void acquire(int permits) throws InterruptedException {
        if (permits < 0) throw new IllegalArgumentException();
        sync.acquireSharedInterruptibly(permits);
    }

    /**
     * 从信号量中获取给定数量的许可，阻塞等待所有的许可可用。
     */
    public void acquireUninterruptibly(int permits) {
        if (permits < 0) throw new IllegalArgumentException();
        sync.acquireShared(permits);
    }

    /**
     * 从信号量中获取给定数量的许可，当且仅当本次访问时所有许可可用。
     */
    public boolean tryAcquire(int permits) {
        if (permits < 0) throw new IllegalArgumentException();
        return sync.nonfairTryAcquireShared(permits) >= 0;
    }

    /**
     * 从信号量中获取给定数量的许可，如果许可在给定等待时间内变得可用并且当前线程未被中断。  
     */
    public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
        throws InterruptedException {
        if (permits < 0) throw new IllegalArgumentException();
        return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
    }

    /**
     * 释放给定数量的许可，返回信号量。    
     */
    public void release(int permits) {
        if (permits < 0) throw new IllegalArgumentException();
        sync.releaseShared(permits);
    }

    /**
     * 获取当前信号量中可用的许可
     */
    public int availablePermits() {
        return sync.getPermits();
    }

    /**
     * 获取立即可用的许可
     */
    public int drainPermits() {
        return sync.drainPermits();
    }

    /**
     * 通过指示的减少减少可用许可证的数量。
     */
    protected void reducePermits(int reduction) {
        if (reduction < 0) throw new IllegalArgumentException();
        sync.reducePermits(reduction);
    }

    /**
     * 如果此信号量的公平性设置为true，则返回true
     */
    public boolean isFair() {
        return sync instanceof FairSync;
    }

    /**
     * 查询是否有线程在等待获取
     */
    public final boolean hasQueuedThreads() {
        return sync.hasQueuedThreads();
    }

    /**
	 * 获取等待此锁的估计线程数
     */
    public final int getQueueLength() {
        return sync.getQueueLength();
    }

    /**
     * 获取包含等待获取许可的线程集合
     */
    protected Collection<Thread> getQueuedThreads() {
        return sync.getQueuedThreads();
    }

    /**
     * 获取标识信号量的字符串
     */
    public String toString() {
        return super.toString() + "[Permits = " + sync.getPermits() + "]";
    }
}

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semaphore示例：

package org.dsg.jdk.locks.aqs;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.Semaphore;

@Slf4j(topic = "c.semaphoreInstance")
public class SemaphoreInstance {

    public static void main(String[] args) {
        //1.创建信号量
        Semaphore semaphore = new Semaphore(3);
        //2.创建线程使用信号量
        for(int i=0;i<10;i++){
            new Thread(()->{
                try {
                    //运行前需要获得许可
                    semaphore.acquire();
                    log.info("running...");
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    //使用完毕释放许可
                    semaphore.release();
                }
                log.info("end...");
            }).start();
        }
    }

}

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