等待/通知所有未按预期工作

Question

I have a class with two methods, one sends messages, the other confirm the message has been recevied/processed

   public void send(OTAHotelAvailRS otaHotelAvailRS) throws Exception {
        MessageAvailRs messageAvailRs = new MessageAvailRs();
        messageMap.put(messageAvailRs.getMessageId(), messageAvailRs);
        synchronized (messageAvailRs) {
            messagesSend++;
            messageAvailRs.wait();
            messageWake++;
        }
    }



 public void confirmMessage(String messageId) {
        logger.debug("Confirmed message: " + messageId);
        MessageAvailRs messageAvailRs = messageMap.remove(messageId);
        if (messageAvailRs != null) {
            synchronized (messageAvailRs) {
                messageAvailRs.notifyAll();
                messagesReceived++;
            }
        }
    }

When running in a multihread envoirnement (3 thread x 100 req) there are some messages don't wake up from the notify..

For example, once all message has been sent

messageSend = 100
messageRec = 100
messageWake = 98

And the size of the map is 0, There are no repeated messageIds.

I have shorten the case. It's more complex.

I have a service that each time a request is received the method send is called. This method (It's not in the code) send the message to a JMS Queue, other serivice receives this messages, process them and send a response to another JMSQueue, then a JMSConsumer reads the responses and calls the confirm method.

What is wrong ?

Answer 1

You are using wait() wrongly.

The wait method should always in a while loop like this:

synchronized (obj) {
     while (<condition does not hold>)
         obj.wait();
     ... // Perform action appropriate to condition
 }

Suppose Thread1 and Thread2 are waiting. Thread3 calls notifyAll . Thread1 wake up first, and consumes this condition. When it comes to Thread2 , Thread2 is supposed to check this condition again, if it is not satisfied, Thread2 should fall into another wait .

Answer 2

Thanks to james large

The problem is that notifyAll() is called before the wait(). The solution is to use a boolean to no wait() if response has been received.

 public void send(OTAHotelAvailRS otaHotelAvailRS) throws Exception {
        MessageAvailRs messageAvailRs = new MessageAvailRs();
        messageMap.put(messageAvailRs.getMessageId(), messageAvailRs);
        synchronized (messageAvailRs) {
            messagesSend++;
            if  (!messageAvailRs.isConfirmed()) {
                messageAvailRs.wait();
            }
            messageWake++;
        }
    }

public void confirmMessage(String messageId) {
        logger.debug("Confirmed message: " + messageId);
        MessageAvailRs messageAvailRs = messageMap.remove(messageId);
        if (messageAvailRs != null) {
            meesageAvailRs.confirm();
            synchronized (messageAvailRs) {               
                messageAvailRs.notifyAll();
                messagesReceived++;
            }
        }
    }

public synchronized boolean isConfirmed() {
   return confimed;
}

public synchronized confirm() {
   cofirmed = true;
}