Question

I am trying to write a mail utility that places mails in a queue, and it is later consumed by a consumer thread.

I am trying to implement a typical producer-consumer pattern, but something is going wrong.

I just wrote a skeleton , and the skeleton is not working as expected.

MailProducer.java

public class MailProducer implements Callable<Void>
 {

@Override
public Void call() throws Exception
{
    System.out.println("inside mail Producer");
    System.out.println("Thread executing = " +
                           Thread.currentThread().getName());
    return null;
}

}

MailConsumer.java

public class MailConsumer implements Callable<Void>
{

@Override
public Void call() throws Exception
{
    System.out.println("inside mail consumer");
    System.out.println("Thread executing = " + 
                        Thread.currentThread().getName());
    return null;
}

 }

and finally the Executor

MailExecutor.java

  public class MailExecutor
  {

private static final int NTHREADS = 25;
private static final ExecutorService exec = 
                Executors.newFixedThreadPool(NTHREADS);

public static void main(String[] args)
{
    exec.submit(new MailConsumer());
    exec.submit(new MailProducer());

    System.out.println("inside main");

}

  }

Now when I run the program, I expect it to go back and forth the producer and consumer to keep printing what is written in the respective classes. But instead , the program hangs/does nothing after printing the below lines . What is going wrong ? Am I missing something?

Output …(Output is not what I had expected. What is going wrong ?)

   inside mail consumer
   inside main
   Thread executing = pool-1-thread-1
   inside mail Producer
   Thread executing = pool-1-thread-2

Answer 1

You are missing the shared queue. Without the queue, you have nothing.

Producers put work onto the queue. Consumers take work off the queue. Use a BlockingQueue, whose put() and take() methods are blocking calls. Running producers and consumers in separate threads allows them to safely block while calling these methods.

Neither the producers nor the consumers need to be Callable; Runnable will do. Using an Executor to tie it all together is a good idea.