J.U.C 之AQS

作者: 磊_5d71 | 来源:发表于2018-11-03 22:14 被阅读0次

图片.png

CountDownLatch

CountDownLatchExample1

package com.alan.concurrency.example.aqs;


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 InterruptedException {

        ExecutorService exec = Executors.newCachedThreadPool();

        final CountDownLatch countDownLatch = new CountDownLatch(threadCount);

        for (int i = 0; i < threadCount; i++) {
            {
                int threadNum = i;
                exec.execute(() -> {
                    try {
                        test(threadNum);
                    } catch (InterruptedException e) {
                        log.error("InterruptedException", e);
                    } finally {
                        countDownLatch.countDown();
                    }
                });
            }
        }

        //通过countDown()和await()能保证所有线程执行完成后，再调用log.info("finish")
        countDownLatch.await();
        log.info("finish");
        exec.shutdown();

    }

    public static void test(int threadNum) throws InterruptedException {
        Thread.sleep(100);
        log.info("{}",threadNum);
    }

}

CountDownLatchExample2 限制指定时间完成

package com.alan.concurrency.example.aqs;


import lombok.extern.slf4j.Slf4j;

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 InterruptedException {

        ExecutorService exec = Executors.newCachedThreadPool();

        final CountDownLatch countDownLatch = new CountDownLatch(threadCount);

        for (int i = 0; i < threadCount; i++) {
            {
                int threadNum = i;
                exec.execute(() -> {
                    try {
                        test(threadNum);
                    } catch (InterruptedException e) {
                        log.error("InterruptedException", e);
                    } finally {
                        countDownLatch.countDown();
                    }
                });
            }
        }

        //通过countDown()和await()能保证所有线程执行完成后，再调用log.info("finish")
        //设置超时时间10毫秒
        countDownLatch.await(10,TimeUnit.MILLISECONDS);
        log.info("finish");
        //是先让当前线程任务都执行完成后，才进行shutdown操作
        exec.shutdown();

    }

    public static void test(int threadNum) throws InterruptedException {
        Thread.sleep(100);
        log.info("{}",threadNum);
    }

}

Semaphore 同步组件-信号量

Semaphore是一种在多线程环境下使用的设施，该设施负责协调各个线程，以保证它们能够正确、合理的使用公共资源的设施，也是操作系统中用于控制进程同步互斥的量。
以一个停车场是运作为例。为了简单起见，假设停车场只有三个车位，一开始三个车位都是空的。这时如果同时来了五辆车，看门人允许其中三辆不受阻碍的进入，然后放下车拦，剩下的车则必须在入口等待，此后来的车也都不得不在入口处等待。这时，有一辆车离开停车场，看门人得知后，打开车拦，放入一辆，如果又离开两辆，则又可以放入两辆，如此往复。
在这个停车场系统中，车位是公共资源，每辆车好比一个线程，看门人起的就是信号量的作用。
更进一步，信号量的特性如下：信号量是一个非负整数（车位数），所有通过它的线程（车辆）都会将该整数减一（通过它当然是为了使用资源），当该整数值为零时，所有试图通过它的线程都将处于等待状态。在信号量上我们定义两种操作： Wait（等待）和 Release（释放）。当一个线程调用Wait（等待）操作时，它要么通过然后将信号量减一，要么一直等下去，直到信号量大于一或超时。Release（释放）实际上是在信号量上执行加操作，对应于车辆离开停车场，该操作之所以叫做“释放”是因为加操作实际上是释放了由信号量守护的资源。
应用场景：只能访问有限的资源
1、设置数据库的连接数
2、设置数为1，将相当于单线程运行了。
单一许可

package com.alan.concurrency.example.aqs;


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 = 200;

    //设置允许的并发数为20
    private final static Semaphore semaphore = new Semaphore(20);

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

        ExecutorService exec = Executors.newCachedThreadPool();


        for (int i = 0; i < threadCount; i++) {
            {
                int threadNum = i;
                exec.execute(() -> {
                    try {
                        semaphore.acquire();  //获取一个许可
                        test(threadNum);
                        semaphore.release();  //释放一个许可
                    } catch (InterruptedException e) {
                        log.error("InterruptedException", e);
                    }
                });
            }
        }

        exec.shutdown();

    }

    public static void test(int threadNum) throws InterruptedException {
        log.info("{}",threadNum);
        Thread.sleep(1000);
    }

}

多个许可

package com.alan.concurrency.example.aqs;


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 = 200;

    //设置允许的并发数为20
    private final static Semaphore semaphore = new Semaphore(20);

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

        ExecutorService exec = Executors.newCachedThreadPool();


        for (int i = 0; i < threadCount; i++) {
            {
                int threadNum = i;
                exec.execute(() -> {
                    try {
                        semaphore.acquire(20);
                        test(threadNum);
                        semaphore.release(20);
                    } catch (InterruptedException e) {
                        log.error("InterruptedException", e);
                    }
                });
            }
        }

        exec.shutdown();

    }

    public static void test(int threadNum) throws InterruptedException {
        log.info("{}",threadNum);
        Thread.sleep(1000);
    }

}

CyclicBarrier

CyclicBarrier是一个同步工具类，它允许一组线程互相等待，直到到达某个公共屏障点。与CountDownLatch不同的是该barrier在释放等待线程后可以重用，所以称它为循环（Cyclic）的屏障（Barrier）。
CyclicBarrier支持一个可选的Runnable命令，在一组线程中的最后一个线程到达之后（但在释放所有线程之前），该命令只在每个屏障点运行一次。若在继续所有参与线程之前更新共享状态，此屏障操作很有用。

package com.alan.concurrency.example.aqs;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;



@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) {
                    e.printStackTrace();
                }
            });
        }
    }


    private static  void race(int threadNum) throws Exception{
        Thread.sleep(1000);
        log.info("{} is ready",threadNum);
        barrier.await();
        log.info("{} continue",threadNum);

    }

}

ReentrantLock 与锁

可重入性：
从名字上理解，ReenTrantLock的字面意思就是再进入的锁，其实synchronized关键字所使用的锁也是可重入的，两者关于这个的区别不大。两者都是同一个线程没进入一次，锁的计数器都自增1，所以要等到锁的计数器下降为0时才能释放锁。
锁的实现：
Synchronized是依赖于JVM实现的，而ReenTrantLock是JDK实现的，有什么区别，说白了就类似于操作系统来控制实现和用户自己敲代码实现的区别。前者的实现是比较难见到的，后者有直接的源码可供阅读。
性能的区别：
在Synchronized优化以前，synchronized的性能是比ReenTrantLock差很多的，但是自从Synchronized引入了偏向锁，轻量级锁（自旋锁）后，两者的性能就差不多了，在两种方法都可用的情况下，官方甚至建议使用synchronized，其实synchronized的优化我感觉就借鉴了ReenTrantLock中的CAS技术。都是试图在用户态就把加锁问题解决，避免进入内核态的线程阻塞。
功能区别：
便利性：很明显Synchronized的使用比较方便简洁，并且由编译器去保证锁的加锁和释放，而ReenTrantLock需要手工声明来加锁和释放锁，为了避免忘记手工释放锁造成死锁，所以最好在finally中声明释放锁。

锁的细粒度和灵活度：很明显ReenTrantLock优于Synchronized

ReenTrantLock独有的能力：
1、ReenTrantLock可以指定是公平锁还是非公平锁。而synchronized只能是非公平锁。所谓的公平锁就是先等待的线程先获得锁。
2、ReenTrantLock提供了一个Condition（条件）类，用来实现分组唤醒需要唤醒的线程们，而不是像synchronized要么随机唤醒一个线程要么唤醒全部线程。
3、ReenTrantLock提供了一种能够中断等待锁的线程的机制，通过lock.lockInterruptibly()来实现这个机制。

package com.alan.concurrency.example.lock;


import com.alan.concurrency.annoations.ThreadSafe;
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;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

@Slf4j
@ThreadSafe
public class LockExample2 {


    //请求数1000
    public static int clientTotal = 5000;
    //同时并发执行的线程数
    public static int threadTotal = 200;

    public static int count = 0;


    //通过Lock接口实现
    private static Lock lock = new ReentrantLock();


    private  static void add(){

        lock.lock();
        try {
            count++;
        } finally {
            lock.unlock();
        }
    }

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

        //定义线程池ExecutorService接口
        ExecutorService executorService = Executors.newCachedThreadPool();
        //定义信号量,传入并发线程数 final修饰不允许重新赋值
        final Semaphore semaphore = new Semaphore(threadTotal);
        //定义计数器闭锁。传入请求总数
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);

        for (int i = 0; i < clientTotal; i++) {
            //通过匿名内部类方式
            executorService.execute(new Runnable() {
                @Override
                public void run() {
                    try {
                        //semaphore控制并发数量
                        semaphore.acquire();
                        add();
                        semaphore.release();
                    } catch (InterruptedException e) {
                        log.error("exception",e);
                    }
                    //每次执行计数器减掉一个
                    countDownLatch.countDown();
                }

            });

        }
        countDownLatch.await();
        executorService.shutdown();
        log.info("count:{}",count);
    }
}

ReentrantReadWriteLock

package com.alan.concurrency.example.lock;


import com.alan.concurrency.annoations.ThreadSafe;
import lombok.Data;
import lombok.extern.slf4j.Slf4j;

import java.util.Date;
import java.util.Map;
import java.util.Set;
import java.util.TreeMap;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

@Slf4j
public class LockExample3 {


    private final Map<String, Data> map = new TreeMap<>();

    private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    //分别定义读锁和写锁

    private final Lock readLock = lock.readLock();

    private final Lock writeLock = lock.writeLock();

    public Data get(String key) {
        readLock.lock();
        try {
            return  map.get(key);
        } finally {
            readLock.unlock();
        }
    }

    public Set<String> getAllKeys(){
        readLock.lock();
        try {
            return  map.keySet();
        } finally {
            readLock.unlock();
        }
    }

    public Data put(String key, Data value){
        writeLock.lock();
        try {
            return  map.put(key,value);
        } finally {
            writeLock.unlock();
        }
    }
}

StampedLock


package com.alan.concurrency.example.lock;

import java.util.concurrent.locks.StampedLock;

public class LockExample4 {

    class Point {
        private double x, y;
        private final StampedLock sl = new StampedLock();

        void move(double deltaX, double deltaY) { // an exclusively locked method
            long stamp = sl.writeLock();
            try {
                x += deltaX;
                y += deltaY;
            } finally {
                sl.unlockWrite(stamp);
            }
        }

        //下面看看乐观读锁案例
        double distanceFromOrigin() { // A read-only method
            long stamp = sl.tryOptimisticRead(); //获得一个乐观读锁
            double currentX = x, currentY = y;  //将两个字段读入本地局部变量
            if (!sl.validate(stamp)) { //检查发出乐观读锁后同时是否有其他写锁发生？
                stamp = sl.readLock();  //如果没有，我们再次获得一个读悲观锁
                try {
                    currentX = x; // 将两个字段读入本地局部变量
                    currentY = y; // 将两个字段读入本地局部变量
                } finally {
                    sl.unlockRead(stamp);
                }
            }
            return Math.sqrt(currentX * currentX + currentY * currentY);
        }

        //下面是悲观读锁案例
        void moveIfAtOrigin(double newX, double newY) { // upgrade
            // Could instead start with optimistic, not read mode
            long stamp = sl.readLock();
            try {
                while (x == 0.0 && y == 0.0) { //循环，检查当前状态是否符合
                    long ws = sl.tryConvertToWriteLock(stamp); //将读锁转为写锁
                    if (ws != 0L) { //这是确认转为写锁是否成功
                        stamp = ws; //如果成功 替换票据
                        x = newX; //进行状态改变
                        y = newY;  //进行状态改变
                        break;
                    } else { //如果不能成功转换为写锁
                        sl.unlockRead(stamp);  //我们显式释放读锁
                        stamp = sl.writeLock();  //显式直接进行写锁 然后再通过循环再试
                    }
                }
            } finally {
                sl.unlock(stamp); //释放读锁或写锁
            }
        }
    }
}

package com.alan.concurrency.example.lock;

import com.alan.concurrency.annoations.ThreadSafe;
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;
import java.util.concurrent.locks.StampedLock;

@Slf4j
@ThreadSafe
public class LockExample5 {

    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    public static int count = 0;

    private final static StampedLock lock = new StampedLock();

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    add();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
        log.info("count:{}", count);
    }

    private static void add() {
        long stamp = lock.writeLock();
        try {
            count++;
        } finally {
            lock.unlock(stamp);
        }
    }
}

J.U.C 之AQS

CountDownLatch

Semaphore 同步组件-信号量

CyclicBarrier

ReentrantLock 与锁

相关文章

网友评论

延伸阅读

深度阅读

栏目导航

热点阅读