编写优质的并发代码是一件难度极高的事情。Java语言从第一版本开始内置了对多线程的支持，这一点在当年是非常了不起的，但是当我们对并发编程有了更深刻的认识和更多的实践后，实现并发编程就有了更多的方案和更好的选择。本文是对并发编程的一点总结和思考，同时也分享了Java 5以后的版本中如何编写并发代码的一点点经验。

/**
 ID序列生成器
*/
public class IdGenerator {
    private final AtomicLong sequenceNumber = new AtomicLong(0);

    public long next() {
        return sequenceNumber.getAndIncrement(); 
    }
}

import java.util.concurrent.CountDownLatch;

/**
 * 工人类
 * @author 骆昊
 *
 */
class Worker {
    private String name;        // 名字
    private long workDuration;  // 工作持续时间

    /**
     * 构造器
     */
    public Worker(String name, long workDuration) {
        this.name = name;
        this.workDuration = workDuration;
    }

    /**
     * 完成工作
     */
    public void doWork() {
        System.out.println(name + " begins to work...");
        try {
            Thread.sleep(workDuration); // 用休眠模拟工作执行的时间
        } catch(InterruptedException ex) {
            ex.printStackTrace();
        }
        System.out.println(name + " has finished the job...");
    }
}

/**
 * 测试线程
 * @author 骆昊
 *
 */
class WorkerTestThread implements Runnable {
    private Worker worker;
    private CountDownLatch cdLatch;

    public WorkerTestThread(Worker worker, CountDownLatch cdLatch) {
        this.worker = worker;
        this.cdLatch = cdLatch;
    }

    @Override
    public void run() {
        worker.doWork();        // 让工人开始工作
        cdLatch.countDown();    // 工作完成后倒计时次数减1
    }
}

class CountDownLatchTest {

    private static final int MAX_WORK_DURATION = 5000;  // 最大工作时间
    private static final int MIN_WORK_DURATION = 1000;  // 最小工作时间

    // 产生随机的工作时间
    private static long getRandomWorkDuration(long min, long max) {
        return (long) (Math.random() * (max - min) + min);
    }

    public static void main(String[] args) {
        CountDownLatch latch = new CountDownLatch(2);   // 创建倒计时闩并指定倒计时次数为2
        Worker w1 = new Worker("骆昊", getRandomWorkDuration(MIN_WORK_DURATION, MAX_WORK_DURATION));
        Worker w2 = new Worker("王大锤", getRandomWorkDuration(MIN_WORK_DURATION, MAX_WORK_DURATION));

        new Thread(new WorkerTestThread(w1, latch)).start();
        new Thread(new WorkerTestThread(w2, latch)).start();

        try {
            latch.await();  // 等待倒计时闩减到0
            System.out.println("All jobs have been finished!");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

class AddThread implements Runnable {
    private List<Double> list;

    public AddThread(List<Double> list) {
        this.list = list;
    }

    @Override
    public void run() {
        for(int i = 0; i < 10000; ++i) {
            list.add(Math.random());
        }
    }
}

public class Test05 {
    private static final int THREAD_POOL_SIZE = 2;

    public static void main(String[] args) {
        List<Double> list = new ArrayList<>();
        ExecutorService es = Executors.newFixedThreadPool(THREAD_POOL_SIZE);
        es.execute(new AddThread(list));
        es.execute(new AddThread(list));
        es.shutdown();
    }
}

List<Double> list = new CopyOnWriteArrayList<>();

import java.util.ArrayList;
import java.util.List;
import java.util.UUID;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * 公共常量
 * @author 骆昊
 *
 */
class Constants {
    public static final int MAX_BUFFER_SIZE = 10;
    public static final int NUM_OF_PRODUCER = 2;
    public static final int NUM_OF_CONSUMER = 3;
}

/**
 * 工作任务
 * @author 骆昊
 *
 */
class Task {
    private String id;  // 任务的编号

    public Task() {
        id = UUID.randomUUID().toString();
    }

    @Override
    public String toString() {
        return "Task[" + id + "]";
    }
}

/**
 * 消费者
 * @author 骆昊
 *
 */
class Consumer implements Runnable {
    private List<Task> buffer;

    public Consumer(List<Task> buffer) {
        this.buffer = buffer;
    }

    @Override
    public void run() {
        while(true) {
            synchronized(buffer) {
                while(buffer.isEmpty()) {
                    try {
                        buffer.wait();
                    } catch(InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                Task task = buffer.remove(0);
                buffer.notifyAll();
                System.out.println("Consumer[" + Thread.currentThread().getName() + "] got " + task);
            }
        }
    }
}

/**
 * 生产者
 * @author 骆昊
 *
 */
class Producer implements Runnable {
    private List<Task> buffer;

    public Producer(List<Task> buffer) {
        this.buffer = buffer;
    }

    @Override
    public void run() {
        while(true) {
            synchronized (buffer) {
                while(buffer.size() >= Constants.MAX_BUFFER_SIZE) {
                    try {
                        buffer.wait();
                    } catch(InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                Task task = new Task();
                buffer.add(task);
                buffer.notifyAll();
                System.out.println("Producer[" + Thread.currentThread().getName() + "] put " + task);
            }
        }
    }

}

public class Test06 {

    public static void main(String[] args) {
        List<Task> buffer = new ArrayList<>(Constants.MAX_BUFFER_SIZE);
        ExecutorService es = Executors.newFixedThreadPool(Constants.NUM_OF_CONSUMER + Constants.NUM_OF_PRODUCER);
        for(int i = 1; i <= Constants.NUM_OF_PRODUCER; ++i) {
            es.execute(new Producer(buffer));
        }
        for(int i = 1; i <= Constants.NUM_OF_CONSUMER; ++i) {
            es.execute(new Consumer(buffer));
        }
    }
}

import java.util.UUID;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.LinkedBlockingQueue;

/**
 * 公共常量
 * @author 骆昊
 *
 */
class Constants {
    public static final int MAX_BUFFER_SIZE = 10;
    public static final int NUM_OF_PRODUCER = 2;
    public static final int NUM_OF_CONSUMER = 3;
}

/**
 * 工作任务
 * @author 骆昊
 *
 */
class Task {
    private String id;  // 任务的编号

    public Task() {
        id = UUID.randomUUID().toString();
    }

    @Override
    public String toString() {
        return "Task[" + id + "]";
    }
}

/**
 * 消费者
 * @author 骆昊
 *
 */
class Consumer implements Runnable {
    private BlockingQueue<Task> buffer;

    public Consumer(BlockingQueue<Task> buffer) {
        this.buffer = buffer;
    }

    @Override
    public void run() {
        while(true) {
            try {
                Task task = buffer.take();
                System.out.println("Consumer[" + Thread.currentThread().getName() + "] got " + task);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

/**
 * 生产者
 * @author 骆昊
 *
 */
class Producer implements Runnable {
    private BlockingQueue<Task> buffer;

    public Producer(BlockingQueue<Task> buffer) {
        this.buffer = buffer;
    }

    @Override
    public void run() {
        while(true) {
            try {
                Task task = new Task();
                buffer.put(task);
                System.out.println("Producer[" + Thread.currentThread().getName() + "] put " + task);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

        }
    }

}

public class Test07 {

    public static void main(String[] args) {
        BlockingQueue<Task> buffer = new LinkedBlockingQueue<>(Constants.MAX_BUFFER_SIZE);
        ExecutorService es = Executors.newFixedThreadPool(Constants.NUM_OF_CONSUMER + Constants.NUM_OF_PRODUCER);
        for(int i = 1; i <= Constants.NUM_OF_PRODUCER; ++i) {
            es.execute(new Producer(buffer));
        }
        for(int i = 1; i <= Constants.NUM_OF_CONSUMER; ++i) {
            es.execute(new Consumer(buffer));
        }
    }
}

//import java.util.concurrent.ExecutorService;
//import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

/**
 * 存放线程共享信号量的上下问
 * @author 骆昊
 *
 */
class AppContext {
    public static final int NUM_OF_FORKS = 5;   // 叉子数量(资源)
    public static final int NUM_OF_PHILO = 5;   // 哲学家数量(线程)

    public static Semaphore[] forks;    // 叉子的信号量
    public static Semaphore counter;    // 哲学家的信号量

    static {
        forks = new Semaphore[NUM_OF_FORKS];

        for (int i = 0, len = forks.length; i < len; ++i) {
            forks[i] = new Semaphore(1);    // 每个叉子的信号量为1
        }

        counter = new Semaphore(NUM_OF_PHILO - 1);  // 如果有N个哲学家，最多只允许N-1人同时取叉子
    }

    /**
     * 取得叉子
     * @param index 第几个哲学家
     * @param leftFirst 是否先取得左边的叉子
     * @throws InterruptedException
     */
    public static void putOnFork(int index, boolean leftFirst) throws InterruptedException {
        if(leftFirst) {
            forks[index].acquire();
            forks[(index + 1) % NUM_OF_PHILO].acquire();
        }
        else {
            forks[(index + 1) % NUM_OF_PHILO].acquire();
            forks[index].acquire();
        }
    }

    /**
     * 放回叉子
     * @param index 第几个哲学家
     * @param leftFirst 是否先放回左边的叉子
     * @throws InterruptedException
     */
    public static void putDownFork(int index, boolean leftFirst) throws InterruptedException {
        if(leftFirst) {
            forks[index].release();
            forks[(index + 1) % NUM_OF_PHILO].release();
        }
        else {
            forks[(index + 1) % NUM_OF_PHILO].release();
            forks[index].release();
        }
    }
}

/**
 * 哲学家
 * @author 骆昊
 *
 */
class Philosopher implements Runnable {
    private int index;      // 编号
    private String name;    // 名字

    public Philosopher(int index, String name) {
        this.index = index;
        this.name = name;
    }

    @Override
    public void run() {
        while(true) {
            try {
                AppContext.counter.acquire();
                boolean leftFirst = index % 2 == 0;
                AppContext.putOnFork(index, leftFirst);
                System.out.println(name + "正在吃意大利面（通心粉）...");   // 取到两个叉子就可以进食
                AppContext.putDownFork(index, leftFirst);
                AppContext.counter.release();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

public class Test04 {

    public static void main(String[] args) {
        String[] names = { "骆昊", "王大锤", "张三丰", "杨过", "李莫愁" };   // 5位哲学家的名字
//      ExecutorService es = Executors.newFixedThreadPool(AppContext.NUM_OF_PHILO); // 创建固定大小的线程池
//      for(int i = 0, len = names.length; i < len; ++i) {
//          es.execute(new Philosopher(i, names[i]));   // 启动线程
//      }
//      es.shutdown();
        for(int i = 0, len = names.length; i < len; ++i) {
            new Thread(new Philosopher(i, names[i])).start();
        }
    }

}

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;


public class Test07 {
    private static final int POOL_SIZE = 10;

    static class CalcThread implements Callable<Double> {
        private List<Double> dataList = new ArrayList<>();

        public CalcThread() {
            for(int i = 0; i < 10000; ++i) {
                dataList.add(Math.random());
            }
        }

        @Override
        public Double call() throws Exception {
            double total = 0;
            for(Double d : dataList) {
                total += d;
            }
            return total / dataList.size();
        }

    }

    public static void main(String[] args) {
        List<Future<Double>> fList = new ArrayList<>();
        ExecutorService es = Executors.newFixedThreadPool(POOL_SIZE);
        for(int i = 0; i < POOL_SIZE; ++i) {
            fList.add(es.submit(new CalcThread()));
        }

        for(Future<Double> f : fList) {
            try {
                System.out.println(f.get());
            } catch (Exception e) {
                e.printStackTrace();
            }
        }

        es.shutdown();
    }
}

import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.Future;
import java.util.concurrent.RecursiveTask;

class Calculator extends RecursiveTask<Integer> {
    private static final long serialVersionUID = 7333472779649130114L;

    private static final int THRESHOLD = 10;
    private int start;
    private int end;

    public Calculator(int start, int end) {
        this.start = start;
        this.end = end;
    }

    @Override
    public Integer compute() {
        int sum = 0;
        if ((end - start) < THRESHOLD) {    // 当问题分解到可求解程度时直接计算结果
            for (int i = start; i <= end; i++) {
                sum += i;
            }
        } else {
            int middle = (start + end) >>> 1;
            // 将任务一分为二
            Calculator left = new Calculator(start, middle);
            Calculator right = new Calculator(middle + 1, end);
            left.fork();
            right.fork();
            // 注意：由于此处是递归式的任务分解，也就意味着接下来会二分为四，四分为八...

            sum = left.join() + right.join();   // 合并两个子任务的结果
        }
        return sum;
    }

}

public class Test08 {

    public static void main(String[] args) throws Exception {
        ForkJoinPool forkJoinPool = new ForkJoinPool();
        Future<Integer> result = forkJoinPool.submit(new Calculator(1, 10000));
        System.out.println(result.get());
    }
}