生产者消费者模式中主要是两点：1、当没有资源时消费者阻塞 。2、当生产者生产资源后需要通知消费者。在Java中利用锁以及wait/notify操作可以简单实现该模式，代码如下：
public class CP {
private static ArrayList<String> cp = new ArrayList<>();
private static Object lock = new Object();
static class Consumer implements Runnable{
@Override
public void run() {
synchronized (lock) {
while (true) {
while (cp.isEmpty()){ //防止虚假唤醒
try {
lock.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("consuming"+cp.remove(0));
}
}
}
}
static class Producer implements Runnable{
@Override
public void run() {
for(int i = 0;i<5;i++) {
synchronized (lock) {
cp.add("hello");
System.out.println("add hello");
lock.notify();
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
}
public static void main(String[] args) {
Producer producer = new Producer();
Consumer consumer = new Consumer();
Thread pro = new Thread(producer);
Thread con = new Thread(consumer);
pro.start();
con.start();
}
}
除了内置锁与wait/notify实现以外，还可以使用显示锁与await/signal实现，在Java同步队列ArrayBlockingQueue中就是一个很好的实现例子，并且在ArrayBlockingQueue体现了多个Condition协调作用的优势。
ArrayBlockingQueue定义了三个与同步生产消费相关的域，通过notEmpty、notFull来控制生产与消费的阻塞与唤醒：
/** Main lock guarding all access */
final ReentrantLock lock;
/** Condition for waiting takes */
private final Condition notEmpty;
/** Condition for waiting puts */
private final Condition notFull;
public ArrayBlockingQueue(int capacity, boolean fair) {
if (capacity <= 0)
throw new IllegalArgumentException();
this.items = new Object[capacity];
lock = new ReentrantLock(fair);
notEmpty = lock.newCondition();
notFull = lock.newCondition();
}
队列有很多入队出队的方法，这里主要看put和take：
put：
队列满时put操作会阻塞，当有出队操作发生会调用signal方法唤醒阻塞的put。当put成功时同样会调用signal方法唤醒阻塞的take。
public void put(E e) throws InterruptedException {
Objects.requireNonNull(e);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == items.length)
notFull.await(); //队满阻塞
enqueue(e);
} finally {
lock.unlock();
}
}
private void enqueue(E e) {
// assert lock.isHeldByCurrentThread();
// assert lock.getHoldCount() == 1;
// assert items[putIndex] == null;
final Object[] items = this.items;
items[putIndex] = e;
if (++putIndex == items.length) putIndex = 0;
count++;
notEmpty.signal(); //唤醒因队空阻塞的线程
}
take：
队列空时，take操作会阻塞，当有put操作完成并调用signal之后，take才会被唤醒。
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == 0)
notEmpty.await();
return dequeue();
} finally {
lock.unlock();
}
}
private E dequeue() {
// assert lock.isHeldByCurrentThread();
// assert lock.getHoldCount() == 1;
// assert items[takeIndex] != null;
final Object[] items = this.items;
@SuppressWarnings("unchecked")
E e = (E) items[takeIndex];
items[takeIndex] = null;
if (++takeIndex == items.length) takeIndex = 0;
count--;
if (itrs != null)
itrs.elementDequeued();
notFull.signal();
return e;
}
与ArrayBlockingQueue类似的还有LinkedBlockingQueue，它使用两个锁来并行控制put与take的同步(在ArrayBlockingQueue中因为是使用同一个lock，所以put和take不能同时被执行)，所以效率高一点：
transient LinkedBlockingQueue.Node<E> head;
private transient LinkedBlockingQueue.Node<E> last;
private final ReentrantLock takeLock;
private final Condition notEmpty;
private final ReentrantLock putLock;
private final Condition notFull;