How Do Java\'s `wait()` and `notify()` Methods Implement a Blocking Queue?

wait() and notify() in Java: A Simple Scenario with Queue

In Java, the wait() and notify() methods provide a mechanism for thread synchronization. Let's explore a simple scenario where these methods can be used to implement a blocking queue.

Blocking Queue Implementation

A blocking queue is a queue data structure that blocks threads when trying to perform certain operations if specific conditions are not met. In our implementation, we will implement the put() and take() methods, which will block if the queue is full or empty, respectively.

public class BlockingQueue<T> {

    private Queue<T> queue = new LinkedList<>();
    private int capacity;

    public BlockingQueue(int capacity) {
        this.capacity = capacity;
    }

    // Blocks if the queue is full (no space to insert)
    public synchronized void put(T element) throws InterruptedException {
        while (queue.size() == capacity) {
            wait();
        }

        queue.add(element);
        notifyAll();
    }

    // Blocks if the queue is empty (nothing to remove)
    public synchronized T take() throws InterruptedException {
        while (queue.isEmpty()) {
            wait();
        }

        T item = queue.remove();
        notifyAll();
        return item;
    }
}

Usage

Now, let's see how to use this blocking queue.

BlockingQueue<Integer> queue = new BlockingQueue<>(10);

// Producer thread: adds elements to the queue
new Thread(() -> {
    for (int i = 0; i < 15; i++) {
        try {
            queue.put(i);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}).start();

// Consumer thread: retrieves elements from the queue
new Thread(() -> {
    for (int i = 0; i < 15; i++) {
        try {
            System.out.println(queue.take());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}).start();

In this example, the producer thread will add elements to the queue, blocking when it reaches the capacity limit. The consumer thread will retrieve elements, blocking when the queue is empty.

Key Considerations

• Use synchronized blocks: wait() and notify() must be used within synchronized blocks to ensure thread safety and prevent missed signals.
• Use a while loop: Use a while loop to check conditions because of spurious wake-ups (when a thread is re-activated without being notified).
• Consider Java 1.5 concurrency library: Java 1.5 introduced a concurrency library with higher-level abstractions (e.g., Lock and Condition). It offers a cleaner and more versatile approach.

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