生产者和消费者

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Java生产者消费者是最基础的线程同步问题，java岗面试中还是很容易遇到的，之前没写过多线程的代码，面试中被问到很尬啊，面完回来恶补下。在网上查到大概有5种生产者消费者的写法，分别如下。

  1. 用synchronized对存储加锁，然后用object原生的wait() 和 notify()做同步。
  2. 用concurrent.locks.Lock，然后用condition的await() 和signal()做同步。
  3. 直接使用concurrent.BlockingQueue。
  4. 使用PipedInputStream/PipedOutputStream。
  5. 使用信号量semaphore。

我的理解，生产者消费者模式，其实只要保证在存储端同一时刻只有一个线程读或写就不会有问题，然后再去考虑线程同步。方法1 2 5都比较类似，都是加锁来限制同一时刻只能有一个读或写。而方法3 4其实是在存储内部去保证读和写的唯一的，最低层肯定还是通过锁机制来实现的，java底层代码都封装好了而已。
我自己尝试写了下前三种，代码如下：

synchronized版本

import java.util.LinkedList;
import java.util.Queue;

public class ProducerAndConsumer {
    private final int MAX_LEN = 10;
    private Queue<Integer> queue = new LinkedList<Integer>();
    class Producer extends Thread {
        @Override
        public void run() {
            producer();
        }
        private void producer() {
            while(true) {
                synchronized (queue) {
                    while (queue.size() == MAX_LEN) {
                        queue.notify();
                        System.out.println("当前队列满");
                        try {
                            queue.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    queue.add(1);
                    queue.notify();
                    System.out.println("生产者生产一条任务，当前队列长度为" + queue.size());
                    try {
                        Thread.sleep(500);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    }
    class Consumer extends Thread {
        @Override
        public void run() {
            consumer();
        }
        private void consumer() {
            while (true) {
                synchronized (queue) {
                    while (queue.size() == 0) {
                        queue.notify();
                        System.out.println("当前队列为空");
                        try {
                            queue.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    queue.poll();
                    queue.notify();
                    System.out.println("消费者消费一条任务，当前队列长度为" + queue.size());
                    try {
                        Thread.sleep(500);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    }
    public static void main(String[] args) {
        ProducerAndConsumer pc = new ProducerAndConsumer();
        Producer producer = pc.new Producer();
        Consumer consumer = pc.new Consumer();
        producer.start();
        consumer.start();
    }
}

lock版实现，使用了condition做线程之间的同步。

import java.util.LinkedList;
import java.util.Queue;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * version 1 doesn't use synchronized to improve performance
 */
public class ProducerAndConsumer1 {
    private final int MAX_LEN = 10;
    private Queue<Integer> queue = new LinkedList<Integer>();
    private final Lock lock = new ReentrantLock();
    private final Condition condition = lock.newCondition();
    class Producer extends Thread {
        @Override
        public void run() {
            producer();
        }
        private void producer() {
            while(true) {
                lock.lock();
                try {
                    while (queue.size() == MAX_LEN) {
                        System.out.println("当前队列满");
                        try {
                            condition.await();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    queue.add(1);
                    condition.signal();
                    System.out.println("生产者生产一条任务，当前队列长度为" + queue.size());
                    try {
                        Thread.sleep(500);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                } finally {
                    lock.unlock();
                }
            }
        }
    }
    class Consumer extends Thread {
        @Override
        public void run() {
            consumer();
        }
        private void consumer() {
            while (true) {
                lock.lock();
                try {
                    while (queue.size() == 0) {
                        System.out.println("当前队列为空");
                        try {
                            condition.await();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    queue.poll();
                    condition.signal();
                    System.out.println("消费者消费一条任务，当前队列长度为" + queue.size());
                    try {
                        Thread.sleep(500);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                } finally {
                    lock.unlock();
                }
            }
        }
    }
    public static void main(String[] args) {
        ProducerAndConsumer pc = new ProducerAndConsumer();
        Producer producer = pc.new Producer();
        Consumer consumer = pc.new Consumer();
        producer.start();
        consumer.start();
    }
}

BlockingQueue版实现

import java.util.Random;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;


public class ProducerAndConsumer {
    private BlockingQueue<Integer> queue = new LinkedBlockingQueue<Integer>(10);
    class Producer extends Thread {
        @Override
        public void run() {
            producer();
        }
        private void producer() {
            while(true) {
                try {
                    queue.put(1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("生产者生产一条任务，当前队列长度为" + queue.size());
                try {
                    Thread.sleep(new Random().nextInt(1000)+500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
    class Consumer extends Thread {
        @Override
        public void run() {
            consumer();
        }
        private void consumer() {
            while (true) {
                try {
                    queue.take();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("消费者消费一条任务，当前队列长度为" + queue.size());
                try {
                    Thread.sleep(new Random().nextInt(1000)+500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
    public static void main(String[] args) {
        ProducerAndConsumer pc = new ProducerAndConsumer();
        Producer producer = pc.new Producer();
        Consumer consumer = pc.new Consumer();
        producer.start();
        consumer.start();
    }
}