My question is purely conceptual. And merely for a deeper understanding of communication between threads.
In a Producer Consumer problem,
- There is one producer thread and one consumer thread.
- The producer thread calls the method produce and the Consumer thread calls the method consume.
The example code is taken from here
package ProducerConsumer;
import java.util.LinkedList;
import java.util.Queue;
public class ClassicProducerConsumerExample {
public static void main(String[] args) throws InterruptedException {
Buffer buffer = new Buffer(2);
Thread producerThread = new Thread(new Runnable() {
@Override
public void run() {
try {
buffer.produce();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
Thread consumerThread = new Thread(new Runnable() {
@Override
public void run() {
try {
buffer.consume();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
producerThread.start();
consumerThread.start();
producerThread.join();
consumerThread.join();
}
static class Buffer {
private Queue<Integer> list;
private int size;
public Buffer(int size) {
this.list = new LinkedList<>();
this.size = size;
}
public void produce() throws InterruptedException {
int value = 0;
while (true) {
synchronized (this) {
while (list.size() >= size) {
// wait for the consumer
wait();
}
list.add(value);
System.out.println("Produced " + value);
value++;
// notify the consumer
notify();
Thread.sleep(1000);
}
}
}
public void consume() throws InterruptedException {
while (true) {
synchronized (this) {
while (list.size() == 0) {
// wait for the producer
wait();
}
int value = list.poll();
System.out.println("Consume " + value);
// notify the producer
notify();
Thread.sleep(1000);
}
}
}
}
}
I have read that wait and notify should be inside the synchronized block to avoid race condition.
I don't understand why I should enclose the wait() and notify() in synchronized block when both the threads are calling different methods. Consumer thread is not going to call produce() so if I do not enclose the wait call inside the produce method with synchronized keyword then it should still behave the same because produce() is only being called by the Producer thread. Is that right?
