BlockingQueue and releasing threads from waiting

Question

Suppose I have BlockingQueue and some of the threads are called take() but the queue is empty at the moment. And suppose I somehow know that in the future new elements will not appear in the queue. How can I release those threads that are called take() from waiting? Thanks!

  public void run() {
        //noinspection InfiniteLoopStatement
        while (true) {
            try {
                Thread.sleep(millisecondsToSleep);
                if (!kitchen.processedOrdersEmpty()) {
                    Order processedOrder = kitchen.getFromProcessedOrders();
                    kitchen.printMessage("took order#" + processedOrder.getOrderNumber());
                    Thread.sleep(millisecondsToServe);
                    kitchen.printMessage("served order#" + processedOrder.getOrderNumber());
                } else {
                    int currentRandom = getNextRandom();
                    if (currentRandom <= 10) {
                        Order newOrder = new Order(kitchen.getLastOrderNumber());
                        kitchen.puIntoUnprocessedOrders(newOrder);
                        kitchen.printMessage("generated new order#" + newOrder.getOrderNumber());
                    } else {
                        Thread.sleep(millisecondsToSleep);
                    }
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

where kitchen.getFromProcessedOrders() equals:

public Order getFromProcessedOrders() throws InterruptedException {
    return processedOrders.take();
}

and processedOrders is a BlockingQueue

http://stackoverflow.com/questions/5378391/closing-a-blocking-queue — Edmondo, Apr 09 '13 at 09:58

Evgeniy Dorofeev · Answer 1 · 2013-04-09T11:01:05.937

2

1) You can interrupt the thread where take() is running

2) You can put a special object to the queue as a signal of the end. If a thread gets a special object (END) it puts it back and exits, so other waiting threads will get END too.

edited Apr 09 '13 at 11:01

answered Apr 09 '13 at 09:57

Evgeniy Dorofeev

133,369
30
199
275

3) Set a volatile boolean indicating that the thread should exit. – jontro Apr 09 '13 at 10:18
@jontro That doesnt help if three threads try to take from a queue with size of 2. The third thread will wait indefinitely – John Vint Apr 09 '13 at 10:51
@JohnVint yes. Depends on your strategy. My comment wouldnt apply here as I look at the code in the question. – jontro Apr 09 '13 at 11:52

score 2 · Accepted Answer · answered Apr 09 '13 at 10:02

According to java docs-

A BlockingQueue does not intrinsically support any kind of "close" or "shutdown" operation to indicate that no more items will be added. The needs and usage of such features tend to be implementation-dependent. For example, a common tactic is for producers to insert special end-of-stream or poison objects, that are interpreted accordingly when taken by consumers.

You should interrupt the thread where it is blocked -

public void run() {
        //noinspection InfiniteLoopStatement
        while (true) {
            try {
                Thread.sleep(millisecondsToSleep);
                if (!kitchen.processedOrdersEmpty()) {
                    Order processedOrder = kitchen.getFromProcessedOrders();
                    kitchen.printMessage("took order#" + processedOrder.getOrderNumber());
                    Thread.sleep(millisecondsToServe);
                    kitchen.printMessage("served order#" + processedOrder.getOrderNumber());
                } else {
                    int currentRandom = getNextRandom();
                    if (currentRandom <= 10) {
                        Order newOrder = new Order(kitchen.getLastOrderNumber());
                        kitchen.puIntoUnprocessedOrders(newOrder);
                        kitchen.printMessage("generated new order#" + newOrder.getOrderNumber());
                    } else {
                        Thread.sleep(millisecondsToSleep);
                    }
                }
            } catch (InterruptedException ex) { 
                //... handle ...
            }catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

score 0 · Answer 3 · answered Apr 09 '13 at 10:01

0

You can add some dummy record like new String("##EOQ##") to signify End of Queue and stop consumer threads.

answered Apr 09 '13 at 10:01

Jaydeep Rajput

3,605
17
35

BlockingQueue and releasing threads from waiting

3 Answers3