BlockingQueue (producer and consumer)

Definition

A BlockingQueue passes work between threads safely. put waits when the queue is full and take waits when it is empty, so a producer and a consumer can run at their own speed without any manual locking.

Example code

One producer, one consumer

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;

class Demo {
    public static void main(String[] args) throws InterruptedException {
        BlockingQueue<Integer> queue = new ArrayBlockingQueue<>(2);        // holds at most 2 items
        int[] sum = {0};

        Thread producer = new Thread(() -> {
            try {
                for (int i = 1; i <= 5; i++) queue.put(i);                  // waits if the queue is full
                queue.put(-1);                                              // a "stop" marker
            } catch (InterruptedException ignored) { }
        });

        Thread consumer = new Thread(() -> {
            try {
                for (int item = queue.take(); item != -1; item = queue.take()) sum[0] += item;   // waits if empty
            } catch (InterruptedException ignored) { }
        });

        producer.start();
        consumer.start();
        producer.join();
        consumer.join();
        System.out.println("sum of consumed items: " + sum[0]);
    }
}

// Output:
// sum of consumed items: 15