java并发编程笔记（六）——AQS

使用了Node实现FIFO(first in first out)队列，可以用于构建锁或者其他同步装置的基础框架

利用了一个int类型表示状态

使用方法是继承

子类通过继承并通过实现它的方法管理其状态（acquire和release）的方法操纵状态

可以同时实现排它锁和共享锁模式（独占、共享）

AQS同步组件

CountDownLatch
Semaphore
CyclicBarrier
ReentrantLock
Condition
FutureTask

CountDownLatch

public class CountDownLatchExample1 {

    private final static int threadCount = 200;

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

        ExecutorService exec = Executors.newCachedThreadPool();

        final CountDownLatch countDownLatch = new CountDownLatch(threadCount);

        for (int i = 0; i < threadCount; i++) {
            final int threadNum = i;
            exec.execute(() -> {
                try {
                    test(threadNum);
                } catch (Exception e) {
                    log.error("exception", e);
                } finally {
                    countDownLatch.countDown();
                }
            });
        }
        countDownLatch.await();
        #countDownLatch.await(10, TimeUnit.MILLISECONDS);  //这种方式可以设置超时时间，如果指定时间未完成，就结束等待
        log.info("finish");
        exec.shutdown();
    }

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

Semaphore

信号量，控制某个资源同时被访问的个数

1、适用于仅能提供有限资源访问的场景，如：数据库连接

//基本使用
public class SemaphoreExample1 {

    private final static int threadCount = 20;

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

        ExecutorService exec = Executors.newCachedThreadPool();

        final Semaphore semaphore = new Semaphore(3);

        for (int i = 0; i < threadCount; i++) {
            final int threadNum = i;
            exec.execute(() -> {
                try {
                    semaphore.acquire(); // 获取一个许可
                    //semaphore.acquire(3); // 获取多个许可
                    test(threadNum);
                    semaphore.release(); // 释放一个许可
                    //semaphore.release(); // 释放多个许可
                } catch (Exception e) {
                    log.error("exception", e);
                }
            });
        }
        exec.shutdown();
    }

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

//尝试获取许可
public class SemaphoreExample3 {

    private final static int threadCount = 20;

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

        ExecutorService exec = Executors.newCachedThreadPool();

        final Semaphore semaphore = new Semaphore(3);

        for (int i = 0; i < threadCount; i++) {
            final int threadNum = i;
            exec.execute(() -> {
                try {
                    if (semaphore.tryAcquire()) { // 尝试获取一个许可
                        test(threadNum);
                        semaphore.release(); // 释放一个许可
                    }
                } catch (Exception e) {
                    log.error("exception", e);
                }
            });
        }
        exec.shutdown();
    }

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

//在等待的时间内，尝试获取许可
public class SemaphoreExample4 {

    private final static int threadCount = 20;

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

        ExecutorService exec = Executors.newCachedThreadPool();

        final Semaphore semaphore = new Semaphore(3);

        for (int i = 0; i < threadCount; i++) {
            final int threadNum = i;
            exec.execute(() -> {
                try {
                    if (semaphore.tryAcquire(5000, TimeUnit.MILLISECONDS)) { // 尝试获取一个许可
                        test(threadNum);
                        semaphore.release(); // 释放一个许可
                    }
                } catch (Exception e) {
                    log.error("exception", e);
                }
            });
        }
        exec.shutdown();
    }

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

CyclicBarrier

允许一组线程相互等待，直到到达某个公共的屏障点。

可以完成多个线程相互等待，只有当每个线程都准备就绪后，才能各自继续往下执行后面的操作。

与CountDownLatch很相似，但存在几个差异点：

1、CountDownLatch是一次性的，用完就销毁了，CyclicBarrier可以通过reset()方法重置，重复使用。

2、CountDownLatch主要是实现一个或N个线程需要等待其他线程完成某项操作之后，才能继续往下执行；而CyclicBarrier主要是实现了多个线程之间相互等待，直到所有的线程都满足了条件之后才能继续后续的操作，它描述的是各个线程内部相互等待的关系。比如我们设置了初始值是5，只有当5个线程都达到某个条件了，才能继续往下执行。

//基本使用
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) {
                    log.error("exception", e);
                }
            });
        }
        executor.shutdown();
    }

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

public class CyclicBarrierExample2 {

    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) {
                    log.error("exception", e);
                }
            });
        }
        executor.shutdown();
    }

    private static void race(int threadNum) throws Exception {
        Thread.sleep(1000);
        log.info("{} is ready", threadNum);
        try {
            barrier.await(2000, TimeUnit.MILLISECONDS);   //设置等待超时时间
        } catch (Exception e) {
            log.warn("BarrierException", e);
        }
        log.info("{} continue", threadNum);
    }
}

public class CyclicBarrierExample3 {

    private static CyclicBarrier barrier = new CyclicBarrier(5, () -> {
        log.info("callback is running");
    });  /线程达到屏障点的的时候，优先执行这个方法

    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) {
                    log.error("exception", e);
                }
            });
        }
        executor.shutdown();
    }

    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

锁的种类：

synchronized
J.U.C中提供的锁，核心锁就是ReentrantLock

ReentrantLock（可重入锁）和synchronized区别

可重入性：前者是可重入锁，后者也是可重入锁，两者相差不大
锁的实现：后者是依赖JVM实现的，前者是JDK实现的
性能的区别：自从后者引入了轻量锁，偏向锁，自选锁后其性能与前者相差不大，由于后者使用方便，可读性高，建议使用后者。
功能的区别：
- 后者比前者便利，后者是通过编译器去控制加锁和释放锁的，而后者需要调用者手动去加锁和释放锁。
- 前者锁的细粒度比后者好。

ReentrantLock独有的功能

可以指定是公平锁还是非公平锁;
提供了一个Condition类，可以分组唤醒需要唤醒的线程
提供能够中断等待锁的线程的机制，lock.lockInterruptibly()

public class LockExample2 {

    // 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 Lock lock = new ReentrantLock();

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

java并发编程笔记（六）——AQS

java并发编程笔记（六）——AQS

AQS同步组件

CountDownLatch

Semaphore

CyclicBarrier

ReentrantLock与锁

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