Using wait() and notify() to Implement a Blocking Queue
Introduction
In multithreaded programming, wait() and notify() are used for thread synchronization. This article explains how to use wait() and notify() to implement a blocking queue, a data structure that allows threads to block until an item is available or space becomes available.
Implementing a Blocking Queue with wait() and notify()
Conditions for Blocking:
- put() method: Blocks until there is free space in the queue.
- take() method: Blocks until there is an element available in the queue.
Java Code:
public class BlockingQueue<t> { private Queue<t> queue = new LinkedList(); private int capacity; public BlockingQueue(int capacity) { this.capacity = capacity; } public synchronized void put(T element) throws InterruptedException { while (queue.size() == capacity) { wait(); } queue.add(element); notify(); // notifyAll() for multiple producer/consumer threads } public synchronized T take() throws InterruptedException { while (queue.isEmpty()) { wait(); } T item = queue.remove(); notify(); // notifyAll() for multiple producer/consumer threads return item; } }</t></t>
Considerations When Using wait() and notify()
- Synchronized Code: Call wait() and notify() within a synchronized method or block.
- While Loops: Use while loops instead of if statements to check conditions due to spurious wake-ups.
Java 1.5 Concurrency Library
Java 1.5 introduced a concurrency library that provides higher-level abstractions:
Modified Blocking Queue Implementation:
public class BlockingQueue<t> { private Queue<t> queue = new LinkedList(); private int capacity; private Lock lock = new ReentrantLock(); private Condition notFull = lock.newCondition(); private Condition notEmpty = lock.newCondition(); public BlockingQueue(int capacity) { this.capacity = capacity; } public void put(T element) throws InterruptedException { lock.lock(); try { while (queue.size() == capacity) { notFull.await(); } queue.add(element); notEmpty.signal(); } finally { lock.unlock(); } } public T take() throws InterruptedException { lock.lock(); try { while (queue.isEmpty()) { notEmpty.await(); } T item = queue.remove(); notFull.signal(); return item; } finally { lock.unlock(); } } }</t></t>
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