java一些常用并发工具示例

最近把《java并发编程实战》-Java Consurrency in Practice 重温了一遍，把书中提到的一些常用工具记录于此：

一、闭锁（门栓）- CountDownLatch

适用场景：多线程测试时，通常为了精确计时，要求所有线程都ready后，才开始执行，防止有线程先起跑，造成不公平，类似的，所有线程执行完，整个程序才算运行完成。

    /**
     * 闭锁测试（菩提树下的杨过 http://yjmyzz.cnblogs.com/）
     *
     * @throws InterruptedException
     */
    @Test
    public void countdownLatch() throws InterruptedException {
        CountDownLatch startLatch = new CountDownLatch(1); //类似发令枪
        CountDownLatch endLatch = new CountDownLatch(10);//这里的数量，要与线程数相同

        for (int i = 0; i < 10; i++) {
            Thread t = new Thread(() -> {
                try {
                    startLatch.await(); //先等着，直到发令枪响，防止有线程先run
                    System.out.println(Thread.currentThread().getName() + " is running...");
                    Thread.sleep(10);
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                } finally {
                    endLatch.countDown(); //每个线程执行完成后，计数
                }
            });
            t.setName("线程-" + i);
            t.start();
        }
        long start = System.currentTimeMillis();
        startLatch.countDown();//发令枪响，所有线程『开跑』
        endLatch.await();//等所有线程都完成
        long end = System.currentTimeMillis();
        System.out.println("done! exec time => " + (end - start) + " ms");
    }　　

执行结果：

线程-1 is running...
线程-5 is running...
线程-8 is running...
线程-4 is running...
线程-3 is running...
线程-0 is running...
线程-2 is running...
线程-9 is running...
线程-7 is running...
线程-6 is running...
done! exec time => 13 ms

注：大家可以把第14行注释掉，再看看运行结果有什么不同。

二、信号量（Semaphore）

适用场景：用于资源数有限制的并发访问场景。

   public class BoundedHashSet<T> {
        private final Set<T> set;
        private final Semaphore semaphore;

        public BoundedHashSet(int bound) {
            this.set = Collections.synchronizedSet(new HashSet<T>());
            this.semaphore = new Semaphore(bound);
        }

        public boolean add(T t) throws InterruptedException {
            if (!semaphore.tryAcquire(5, TimeUnit.SECONDS)) {
                return false;
            }
            ;
            boolean added = false;
            try {
                added = set.add(t);
                return added;
            } finally {
                if (!added) {
                    semaphore.release();
                }
            }
        }

        public boolean remove(Object o) {
            boolean removed = set.remove(o);
            if (removed) {
                semaphore.release();
            }
            return removed;
        }
    }

    @Test
    public void semaphoreTest() throws InterruptedException {

        BoundedHashSet<String> set = new BoundedHashSet<>(5);
        for (int i = 0; i < 6; i++) {
            if (set.add(i + "")) {
                System.out.println(i + " added !");
            } else {
                System.out.println(i + " not add to Set!");
            }
        }
    }

上面的示例将一个普通的Set变成了有界容器。执行结果如下：

0 added !
1 added !
2 added !
3 added !
4 added !
5 not add to Set!

三、栅栏CyclicBarrier 

这个跟闭锁类似，可以通过代码设置一个『屏障』点，其它线程到达该点后才能继续，常用于约束其它线程都到达某一状态后，才允许做后面的事情。

    public class Worker extends Thread {

        private CyclicBarrier cyclicBarrier;

        public Worker(CyclicBarrier cyclicBarrier) {
            this.cyclicBarrier = cyclicBarrier;
        }

        private void step1() {
            System.out.println(this.getName() + " step 1 ...");
        }

        private void step2() {
            System.out.println(this.getName() + " step 2 ...");
        }

        public void run() {
            step1();
            try {
                cyclicBarrier.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (BrokenBarrierException e) {
                e.printStackTrace();
            }
            step2();
        }
    }

    @Test
    public void cyclicBarrierTest() throws InterruptedException, BrokenBarrierException {
        CyclicBarrier cyclicBarrier = new CyclicBarrier(11);
        for (int i = 0; i < 10; i++) {
            Worker w = new Worker(cyclicBarrier);
            w.start();
        }
        cyclicBarrier.await();

    }

这里我们假设有一个worder线程，里面有2步操作，要求所有线程完成step1后，才能继续step2.　执行结果如下：

Thread-0 step 1 ...
Thread-1 step 1 ...
Thread-2 step 1 ...
Thread-3 step 1 ...
Thread-4 step 1 ...
Thread-5 step 1 ...
Thread-6 step 1 ...
Thread-7 step 1 ...
Thread-8 step 1 ...
Thread-9 step 1 ...
Thread-9 step 2 ...
Thread-0 step 2 ...
Thread-3 step 2 ...
Thread-4 step 2 ...
Thread-6 step 2 ...
Thread-2 step 2 ...
Thread-1 step 2 ...
Thread-8 step 2 ...
Thread-7 step 2 ...
Thread-5 step 2 ...

四、Exchanger

如果2个线程需要交换数据，Exchanger就能派上用场了，见下面的示例：

    @Test
    public void exchangerTest() {
        Exchanger<String> exchanger = new Exchanger<>();

        Thread t1 = new Thread(() -> {
            String temp = "AAAAAA";
            System.out.println("thread 1 交换前：" + temp);
            try {
                temp = exchanger.exchange(temp);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("thread 1 交换后：" + temp);
        });

        Thread t2 = new Thread(() -> {
            String temp = "BBBBBB";
            System.out.println("thread 2 交换前：" + temp);
            try {
                temp = exchanger.exchange(temp);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("thread 2 交换后：" + temp);
        });

        t1.start();
        t2.start();
    }

　执行结果：

thread 1 交换前：AAAAAA
thread 2 交换前：BBBBBB
thread 2 交换后：AAAAAA
thread 1 交换后：BBBBBB

五、FutureTask/Future

一些很耗时的操作，可以用Future转化成异步，不阻塞后续的处理，直到真正需要返回结果时调用get拿到结果

    @Test
    public void futureTaskTest() throws ExecutionException, InterruptedException, TimeoutException {

        Callable<String> callable = () -> {
            System.out.println("很耗时的操作处理中。。。");
            Thread.sleep(5000);
            return "done";
        };

        FutureTask<String> futureTask = new FutureTask<>(callable);

        System.out.println("就绪。。。");
        new Thread(futureTask).start();
        System.out.println("主线程其它处理。。。");
        System.out.println(futureTask.get());
        System.out.println("处理完成！");

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

        System.out.println("executor 就绪。。。");
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        Future<String> future = executorService.submit(callable);
        System.out.println(future.get(10, TimeUnit.SECONDS));
    }

　执行结果：

就绪。。。
主线程其它处理。。。
很耗时的操作处理中。。。
done
处理完成！
-----------------
executor 就绪。。。
很耗时的操作处理中。。。
done

六、阻塞队列BlockingQueue

阻塞队列可以在线程间实现生产者-消费者模式。比如下面的示例：线程producer模拟快速生产数据，而线程consumer模拟慢速消费数据，当达到队列的上限时（即：生产者产生的数据，已经放不下了），队列就堵塞住了。

@Test
    public void blockingQueueTest() throws InterruptedException {
        final BlockingQueue<String> blockingDeque = new ArrayBlockingQueue<>(5);

        Thread producer = new Thread() {
            public void run() {
                Random rnd = new Random();
                while (true) {
                    try {
                        int i = rnd.nextInt(10000);
                        blockingDeque.put(i + "");
                        System.out.println(this.getName() + " 产生了一个数字：" + i);
                        Thread.sleep(rnd.nextInt(50));//模拟生产者快速生产
                    } catch (InterruptedException e) {
                        Thread.currentThread().interrupt();
                    }
                }
            }
        };
        producer.setName("producer 1");


        Thread consumer = new Thread() {
            public void run() {
                while (true) {
                    Random rnd = new Random();
                    try {

                        String i = blockingDeque.take();
                        System.out.println(this.getName() + " 消费了一个数字：" + i);
                        Thread.sleep(rnd.nextInt(10000));//消费者模拟慢速消费
                    } catch (InterruptedException e) {
                        Thread.currentThread().interrupt();
                    }
                }
            }
        };
        consumer.setName("consumer 1");

        producer.start();
        consumer.start();

        while (true) {
            Thread.sleep(100);
        }
    }

执行结果：

producer 1 产生了一个数字：6773
consumer 1 消费了一个数字：6773
producer 1 产生了一个数字：4456
producer 1 产生了一个数字：8572
producer 1 产生了一个数字：5764
producer 1 产生了一个数字：2874
producer 1 产生了一个数字：780 # 注意这里就已经堵住了，直到有消费者消费一条数据，才能继续生产
consumer 1 消费了一个数字：4456
producer 1 产生了一个数字：4193