How do I block on a FIFO queue if its empty

In this Q&A we'll go over how to add and poll ( retrieve ) items from a queue, while also blocking if the queue is empty.

Java collections framework provides a BlockingQueue interface and multiple implementations.  As the name indicates its a blocking queue class.  We'll go over how to use LinkedBlockingQueue.

BlockingQueue offers three basic blocking methods

- offer()  timed blocking add element method

- poll() timed blocking get element method

- take() () blocking get element method

Example below uses the poll() method.

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import java.util.concurrent.LinkedBlockingQueue;

import java.util.concurrent.TimeUnit;

public class BlockingQueueTest {

    private static LinkedBlockingQueue<Integer> lbq = new LinkedBlockingQueue<>();

    public static void queueAdd() {

        for (int i = 0; i < 1000; ++i)

            lbq.add(i);

    }

    public static void queueReceive() {

        Integer i  = -1;

        try {

            while (i != null) {

                System.out.println(i);

                // blocking call

                i = lbq.poll(1, TimeUnit.SECONDS);

            }

        } catch(InterruptedException ie ) {

            throw new RuntimeException(ie.getMessage());

        }

    }

    public static void main(String[] args ) throws InterruptedException {

        ExecutorService prod = Executors.newSingleThreadExecutor();

        ExecutorService consumer = Executors.newSingleThreadExecutor();

        prod.submit( () -> queueAdd() );

        consumer.submit( () -> queueReceive());

        prod.awaitTermination(1, TimeUnit.SECONDS);

        consumer.awaitTermination(1, TimeUnit.SECONDS);

        prod.shutdown();

        consumer.shutdown();

    }

}