我正在尝试在java中创建一个多生产者多个消费者问题的解决方案（即监视器）。
我决定使用Semaphore类并按如下方式进行同步。
import java.util.concurrent.Semaphore;
public class PC implements Runnable {
Semaphore s;
Object lock;
boolean isProducer;
final int ProcessCount = 4;
final int producerDelay = 1500;
final int consumerDelay = 1000;
public static void main(String[] args) {
new PC();
}
PC() {
this.s = new Semaphore(10);
this.lock = new Object();
this.isProducer = true;
// create few consumers and producers
for (int i = 0; i < ProcessCount; i++) {
new Thread(this).start();
}
}
public void run() { //alternately create producers and consumers
if (isProducer) {
isProducer = false;
producer();
} else {
isProducer = true;
consumer();
}
}
void producer() {
try {
while (true) {
synchronized (this.lock) {
s.release(); // produce an resource
}
System.out.println("produced:\tAvailable now:\t" + s.availablePermits());
Thread.sleep(producerDelay);
}
} catch (Exception ex) {
ex.printStackTrace();
}
}
void consumer() {
try {
while (true) {
synchronized (this.lock) {
s.acquire(); // consume resource
}
System.out.println("consumed:\tAvailable now:\t" + s.availablePermits());
Thread.sleep(consumerDelay);
}
} catch (Exception ex) {
ex.printStackTrace();
}
}
}
但输出并不像预期的那样。当生产者延迟小于消费者延迟时，信号量可用资源的数量超过10.另一方面，当生产者延迟更大时，当资源变为0时，程序似乎陷入死锁（而我希望消费者线程暂停，直到资源线程产生一些东西）
示例输出：
消费者延迟较低时：
consumed: Available now: 10
consumed: Available now: 10
produced: Available now: 11
produced: Available now: 10
consumed: Available now: 8
consumed: Available now: 8
produced: Available now: 10
produced: Available now: 10
consumed: Available now: 9
consumed: Available now: 8
produced: Available now: 9
consumed: Available now: 8
produced: Available now: 9
consumed: Available now: 8
consumed: Available now: 7
consumed: Available now: 6
produced: Available now: 7
produced: Available now: 8
consumed: Available now: 7
consumed: Available now: 6
produced: Available now: 7
produced: Available now: 8
consumed: Available now: 7
consumed: Available now: 6
consumed: Available now: 5
consumed: Available now: 4
produced: Available now: 5
produced: Available now: 6
consumed: Available now: 5
consumed: Available now: 4
produced: Available now: 5
produced: Available now: 6
consumed: Available now: 5
consumed: Available now: 4
consumed: Available now: 3
consumed: Available now: 2
produced: Available now: 3
produced: Available now: 4
consumed: Available now: 3
consumed: Available now: 2
produced: Available now: 3
produced: Available now: 4
consumed: Available now: 3
consumed: Available now: 2
consumed: Available now: 0
consumed: Available now: 0
produced: Available now: 2
produced: Available now: 2
consumed: Available now: 1
consumed: Available now: 0
produced: Available now: 1
produced: Available now: 2
consumed: Available now: 1
consumed: Available now: 0
：
produced: Available now: 11
consumed: Available now: 10
produced: Available now: 11
consumed: Available now: 10
produced: Available now: 11
produced: Available now: 12
consumed: Available now: 11
consumed: Available now: 10
produced: Available now: 11
produced: Available now: 12
produced: Available now: 13
produced: Available now: 12
consumed: Available now: 12
consumed: Available now: 11
produced: Available now: 13
produced: Available now: 14
consumed: Available now: 13
consumed: Available now: 12
produced: Available now: 13
produced: Available now: 14
produced: Available now: 15
produced: Available now: 16
consumed: Available now: 15
consumed: Available now: 14
produced: Available now: 15
produced: Available now: 16
consumed: Available now: 15
consumed: Available now: 14
produced: Available now: 15
produced: Available now: 16
produced: Available now: 17
consumed: Available now: 16
consumed: Available now: 17
produced: Available now: 18
produced: Available now: 17
produced: Available now: 18
consumed: Available now: 17
consumed: Available now: 16
produced: Available now: 17
produced: Available now: 18
consumed: Available now: 16
consumed: Available now: 17
produced: Available now: 17
produced: Available now: 18
produced: Available now: 19
produced: Available now: 20
consumed: Available now: 19
consumed: Available now: 18
produced: Available now: 19
produced: Available now: 20
consumed: Available now: 18
produced: Available now: 19
consumed: Available now: 18
produced: Available now: 20
produced: Available now: 21
produced: Available now: 22
consumed: Available now: 21
consumed: Available now: 20
produced: Available now: 21
produced: Available now: 22
consumed: Available now: 21
consumed: Available now: 20
produced: Available now: 21
produced: Available now: 22
produced: Available now: 23
produced: Available now: 24
consumed: Available now: 23
consumed: Available now: 22
produced: Available now: 23
produced: Available now: 24
该计划有什么问题？
答案 0 :(得分：1)
首先，按照here的说明创建一个包含10个初始许可的信号量。这不是上限，因此release()可以使可用许可数超过10。
另外，请看下面的代码：
synchronized (this.lock) {
s.acquire(); // consume resource
}
你获得了锁定。如果没有可用资源，则线程暂停，但不释放锁。因此，下面的代码无法获取锁并且无法执行。消费者等待许可证，生产者等待锁定，导致死锁。
synchronized (this.lock) {
s.release(); // produce an resource
}
您可以通过不同步s.acquire()和s.release()来解决此问题。
答案 1 :(得分：-1)
这将会发生什么
void producer() {
try {
while (true) {
//***All producers are stuck here because "first consumer" is having the lock.
synchronized (this.lock) {
s.release(); // produce an resource
}
System.out.println("produced:\tAvailable now:\t" + s.availablePermits());
Thread.sleep(producerDelay);
}
} catch (Exception ex) {
ex.printStackTrace();
}
}
void consumer() {
try {
while (true) {
//***All other consumers are stuck here because of line below.
synchronized (this.lock) {
//***"First consumer" is stuck here waiting s.release()
s.acquire(); // consume resource
}
System.out.println("consumed:\tAvailable now:\t" + s.availablePermits());
Thread.sleep(consumerDelay);
}
} catch (Exception ex) {
ex.printStackTrace();
}
}