生产者、消费者模式是多线程中的经典问题。通过中间的缓冲队列，使得生产者和消费者的速度可以相互调节。
对于比较常见的单生产者、多消费者的情况，主要有以下两种策略：
- 通过volatile boolean producerDone ＝false 来标示是否完成。生产者结束后标示为true, 消费者轮询这个变量来决定自己是否退出。 这种方式对producerDone产生比较大的争用，实现起来也有诸多问题需要考虑。
- 比较经典的“毒丸”策略，生产者结束后，把一个特别的对象：“毒丸”对象放入队列。消费者从队列中拿到对象后，判断是否是毒丸对象。如果是普通非毒丸对象，则正常消费。如果是毒丸对象，则放回队列（杀死其他消费者），然后结束自己。这种方式不会对结束状态产生争用，是比较好的方式。
由于“毒丸”策略是在单生产者多消费者情况下的。对于多生产者的情况，需要对之进行一些修改。我的想法是这样的。用Countdownlatch作为生产者计数器。所有生产者结束后，由协调者放入毒丸对象，消费者退出过程是一样的。上代码：
Coordinator: 启动生产者消费者，提供队列、计数器。生产者全部结束后，放入毒丸。
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.CountDownLatch;
public class Coordinator {
public static final Object POISON_PILL = new Object();//special object to kill consumers
private int productCount = 3;
private int consumerCount = 5;
public void startAll() throws Exception{
BlockingQueue<Object> queue = new ArrayBlockingQueue<Object>(20);
CountDownLatch noMoreToProduce = new CountDownLatch(productCount);
//start consumers;
for(int i = 0; i < consumerCount; i++){
new Thread(new Consumer("consumer " + i, queue)).start();
}
//start producers;
for(int i = 0; i < productCount; i++){
new Thread(new Producer("producer " + i, queue, noMoreToProduce)).start();
}
//wait until all producer down
noMoreToProduce.await();
System.out.println("All producer finished, putting POISON_PILL to the queue to stop consumers!");
//put poison pill
queue.put(POISON_PILL);
}
public static void main(String[] args) throws Exception{
new Coordinator().startAll();
}
}
Producer: 随机生产和结束，结束前使countdownlatch + 1
import java.util.Random;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.CountDownLatch;
public class Producer implements Runnable {
private String name;
private CountDownLatch noMoreToProduce;
private BlockingQueue<Object> queue;
private Random random = new Random();
public Producer(String name, BlockingQueue<Object> queue, CountDownLatch noMoreToProduce){
this.name = name;
this.queue = queue;
this.noMoreToProduce = noMoreToProduce;
}
@Override
public void run() {
System.out.println(name + " started.");
try {
while (true) {
Object item = randomProduce();
if (item == null) {
break; //break if no more item
}
queue.put(item);
System.out.println(name + " produced one.");
}
} catch (InterruptedException e) {
//log
} finally{
System.out.println(name + " finished.");
noMoreToProduce.countDown();//count down to signal "I finished."
}
}
private Object randomProduce() {
if (random.nextBoolean()) {
return new Object();
}
return null;
}
}
Consumer: 判断毒丸对象。如果是毒丸，放回队列（杀死其他消费者），然后自己退出。
import java.util.concurrent.BlockingQueue;
public class Consumer implements Runnable {
private String name;
private BlockingQueue<Object> queue;
public Consumer(String name, BlockingQueue<Object> queue){
this.name = name;
this.queue = queue;
}
@Override
public void run() {
try {
System.out.println(name + " started.");
while (true) {
Object item = queue.take();
//poison pill processing
if (item == Coordinator.POISON_PILL) {
queue.put(item);//put back to kill others
System.out.println(name + " finished");
break;
}
item = null;//pretend to consume the item;
System.out.println(name + " consumed one");
}
} catch (InterruptedException e) {
}
}
}
执行结果：
consumer 0 started.
consumer 4 started.
consumer 3 started.
consumer 2 started.
consumer 1 started.
producer 0 started.
producer 1 started.
producer 0 finished.
producer 1 produced one.
producer 2 started.
producer 1 produced one.
producer 1 finished.
consumer 3 consumed one
consumer 4 consumed one
consumer 0 consumed one
producer 2 produced one.
producer 2 produced one.
producer 2 produced one.
consumer 1 consumed one
consumer 2 consumed one
producer 2 finished.
All producer finished, putting POISON_PILL to the queue to stop consumers!
consumer 3 finished
consumer 4 finished
consumer 0 finished
consumer 2 finished
consumer 1 finished